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Hemp Husbandry
Robert A. NELSON
Chapter 3
Hemp Husbandry
Robert A. NELSON
Chapter 3
1. Retting
2. Hurds
3. Decortication
4. Fiber Technology
5. Hemp Paper
6. References
3.1 ~ RettingHemp bast fiber must be separated from the woody core by mechanical means (decortication) or by the process of "retting" (rotting). Natural retting is considered to be impractical for modern industrial purposes, but this low technology will always be appropriate somewhere, and never completely obsolete.
When hemp begins to rot, dark flecks of fungal colonies appear on the bark, continuing until the surface turns to a steel-gray color. The most frequently occurring fungi belong to the genera Alternaria, Hormodendrum, Fusarium, Cephalosporium, Phoma, and Trichothecium roseum, which decompose the pectin and polyuronide hemicellulose in the stalk. Cephalosporium attacks cellulose to a slightly greater extent than other fungi. Pseudomonas fluorescens and Clostridium felsineum are dominant during retting. Enzyme sprays have been developed to facilitate the retting process in a controlled manner. (1)
The ripples in hemp stalks cause the retting action to be irregular, beginning in the convex parts. The process starts earlier in females than in males, but pectin fermentation proceeds more rapidly in male plant. The difference, however, is balanced by the end of the process.
Dew-retting (field-retting) is accomplished by spreading stalks on the ground to be exposed to rain and dew. Hemp that is to be field-retted should be cut as close to the ground as possible. If the stalks are laid out on tall stubble, they will dry out quickly when the wind blows under the swath. Retting requires about 30% moisture content. Field-drying will decrease the humidity to below 20% within a week, or sooner if the crop is harvested with a mower crusher. The stalks should be wetted with a sprinkler if necessary. The farmer's control of the process is otherwise largely limited to turning the hemp at least once to promote uniform retting. It must be picked up at the right time to prevent over-retting. Turning of the stalks is done with poles pushed under the stalks near the head end. The stalks are turned over without moving the butts. The work is begun in the middle of the field, the first swath being turned over into the empty space in the center, and so forth.
If an early-maturing crop has been cultivated over 1000 feet above sea level, then altitude becomes an important consideration because the temperature may inhibit or prevent the processes of drying and retting. High altitudes tend to be sloped, and this too may cause problems. A high, sloped field must face south to allow stalks to dry sufficiently on the stubble.
Conditions of temperature and moisture usually are most favorable early in the fall. As winter sets in, low temperature limits bacterial activity even though adequate moisture is present. Hemp can winter up to 4 months without suffering serious damage, but often it will be over-retted. Alternate freezing and thawing spells gives best results in winter field-retting.
Retting at 20o C requires 7-8 days of retting. At 12o C, 15-17 days are required; at 7o C, 30-45 days are necessary. The shortest retting period is achieved in 3 days at 37o C. Retting does not occur at all at 5o C (bacteria are inactivated) or 40o C (destruction).
During the 1940s, Lyle Hessler (Kentucky Agric. Exp. Station) worked to develop improved methods of retting, summarized as follows:
"Retted hemp fiber is composed of about 80% cellulose and lignin, while the remaining 20% or partially soluble fraction is made up of N compounds, pectic substances, pentosans, ash, and other extractable substances. Unretted hemp fiber consists of about 30% of the partially soluble fraction. By retting, part of it is removed in order to free the fiber. When the fiber is exposed to microbial action, it is weakened and, as a result, the breaking strength varies inversely with the length of the retting period. Exposure to the sun shortens the time of retting. Damage to hemp fiber during retting may occur to a greater extent when the fresh green plants are retted than if the plants are allowed to cure by shocking; further, the younger top part of the plant may readily result in weaker fiber. Winter retting compared with fall retting usually has been observed in practice, and substantiated by these experiments, to result in a better color but weaker fiber. This condition is probably due to the slower, longer retting period. The chemical composition of winter-retted fiber was lower in the more soluble fractions, which indicates more retting...
"N fertilizers increase the growth of hemp and give greater yields of dew-retted hemp fiber, but at the same time the quality may be inferior because the fiber is coarser and weaker... The cellulose content of the fiber was significantly increased where the complete fertilizer was used, but during retting the more labile secondary constitutents tend to level off in concentration regardless of fertilizer treatment. Correlation coefficients show that encrusting material plays a part in increasing or decreasing strength of fiber. Fineness of fiber as measured by centimeters per gram gave a significant positive correlation coefficient of breaking strength. Correlation between fiber constituents and fineness was not significant. All encrustants showed a negative correlation with the primary fiber constituents, cellulose; lignin, protein, and ash gave highly significant correlation coefficients, while pentosans gave a significant r." (2, 3)
Hessler also studied the removal of encrustants from dew-retted hemp fiber in order to find a more uniform basis for testing:
"The formation of cellulose nitrate and the detection of degree of polymerization and percentage of N may be used as an index to encrustant removal and good fiber degradation and quality. NaOH and N2CO3 are good chemicals to use in removing encrustants. The former is more effective, but it causes some degradation of the cellulose chain. Increasing the concentration of Na2CO3 over the 1% level did not give better removal of encrustants, although it caused some cellulose degradation. The lower boiling alcohols such as methyl, ethyl, butyl, isobutyl and amyl are not very effective in removing encrustants; but they do open up the fiber and allow other mild reagents to act more effectively. Of the two high-boiling alcohols (ethylene glycol and glycerol), ethylene glycol was better in the removal of encrustants and caused slightly less degradation of cellulose. The higher boiling alcohols have a tendency to make the fiber more subject to oxidation in bleaching. Increasing the boiling time over one hour did not greatly increase the removal of encrustants, but, especially in the case of glycerol, it did cause a lowering of the degree of polymerization. These tests indicate that degumming of bast fibers can be undertaken with a minimum of cellulose degradation and that encrustant removal to a common cellulose base will result in more uniform physical testing." (4)
The end-point of retting is determined by simple tests:
1. Bend some dried stalks back and forth. The fibers should not break when the woody core breaks. The hurd fragments should fall free from the fibers when shaken. If retting is incomplete, some hurds will adhere to the loose fibers. To test the strength of fibers, break some strands. They should break with great difficulty and a snap; otherwise, the hemp has been over-retted.
2. Peel the fiber near the base of the stem. If it peels easily, it is adequately retted. If retting has been insufficient, the fiber will break after only a few inches of peeling.
3. Break several stems all at once in several adjacent areas. If the wood separates easily from the fiber, the retting is complete.
4. The reduction in the total uronic content in the stalks indicates the progress of retting better than any other means. Well-retted hemp has a total uronic content of 5%, compared to 10% in unretted bark. Over-retted hemp contains less than 5% urones.
The retted stalks may be picked up by hand and bundled on a sloped "buck" rack, but a pick-up binder is most efficient. It is important to keep the stalks well butted when bundling them. This minimizes problems with tangled stalks at the mill, and results in a higher yield with less waste.
Retting pools must be designed to accommodate the expected yield of stalks. S.S. Boyce gave very detailed instructions for the various methods of retting in his excellent treatise Hemp (1900):
For this [to accommodate 1-5 acres of harvest] a pool of a size to hold 2 to 10 or 12 tons of hemp will be required, although these pools are usually 4 or 5 feet deep, 10 to 12 feet long, and 5 to 8 feet wide. The sheaves of hemp are packed with the butts alternately one way and the other, until the pit is full, or all the hemp is used up. It is then weighted down by stones and the pit filled with water. The same water may be used over several times, until all the hemp is steeped. The method is wasteful, the steep-water not being utilized, while the stench ... is something unbearable. Nor is the product of much greater value than by the more primitive method of spreading the hemp on the ground.
"The best results are obtained when hemp is grown upon a large scale and the hemp retted by being steeped in running water. Quite often the hemp is placed in crates holding a ton or more of stalks, and then weights of stones placed upon them to hold the hemp under water for 5 to 8 days, according to the temperature of the water. Part of the more modern practice is to dig pools 5 to 7 feet deep, which will hold 10 to 25 tons of hemp, and into which, if the pits are so situated, a small stream of water may be conducted and the overflow allowed to run out upon the land as a fertilizer...
"A later practice is to place the hemp in the water for 4 to 5 days and then take it out and dry it, returning it again to the retting- or steeping-place for 4 to 6 days more. This gives a better fiber, of a creamy white color, and a more evenly retted product. Or, after first being in the water for 5 to 6 days the hemp is dried, and when afterwards [decorticated] the hemp is 'boiled off'... to completely remove the [lignin, etc.]... Another process of retting consists in placing the hemp in tanks of convenient size, holding 5 to 10 tons of stalks, which are filled with water first impregnated with acid, and then emptied and refilled with water containing alkaline preparations, or vice versa. In some instances the hemp is first broken or decorticated and the fibrous material only subjected to steeping. This requires much less space, and after steeping the fiber can be hung up to dry.
"One method of 'boiling off' the fiber before spinning consists in first passing the partly water-retted hemp through a softening machine consisting of 16 sets of fluted rollers... The fiber is then macerated in a nearly boiling solution of carbonate of soda and soap, then washed, first in cold water and then in water containing a small amount of muriatic acid, and again steeped in water containing soda without soap, to remove the acid; it is then placed in a solution of one part of acetic acid and one part of water and afterward in water alone, and dried and again softened. The process is too long, but is well rewarded in producing an exceedingly fine, soft, valuable fiber, highly adapted to the manufacture of fine linen, lawns and laces." (5)
Such extremely fine hemp thread nearly equals silk. It can be hand-spun to such fineness that 600 miles of lace thread can be produced from 2-1/2 pounds of fiber. Cotton and wool cannot exceed 350 miles per 2-1/2 pounds. Boyce continues:
" There are three methods of retting hemp practicable where hemp is grown upon a large scale in the United States. If not grown upon a scale of at least 300 to 500 acres by one planter, there should be arrangements for uniting several smaller growers, or that the hemp grown upon a smaller scale should be disposed of to the middle man prepared to ret the hemp and prepare the fiber and properly classify it. There is little economy in the small acreage system... where the working up is done by others. If there is sufficient profit in raising hemp with a yield of 3 to 5 tons of hemp straw or stalks per acre, and disposing of them to the middle man or manufacturer of fibers... then it may be done so; but it is a division of profits against the farmer, as he loses all fertilizing matters where the hemp stalks are carted from the farm.
"The first method is the ordinary water retting. For this method a system of square wooden tanks... is constructed.... To handle 500 acres of hemp, growing 15 feet high, requires preparations to handle 2,500 tons of stalks. If the work of retting goes on continuously from March to November... it will require the handling of at least 10 tons of stalks per day. If there is an interruption [for plowing, planting, and harvesting], the capacity should be sufficient to handle 15 or 20 tons per day --- that is, of emptying tanks holding 20 tons and putting the stalks out to dry and refilling the tanks, and also taking in 20 tons of dried retted stalks and putting them under cover to be broken at a later day... The breaking can be done from December to March...
"To handle 20 tons of hemp stalks per day will require 8 retting tanks 8 x 15 feet and 5 to 6 feet deep. These should be situated upon the more elevated portions of the ranch... or the tanks may be so constructed upon timbers as to be moved from place to place once a year as the ground around the tanks becomes fertilized by water and refuse from handling the hemp. The steep-water and the foliage and waste from the hemp are high in fertilizing elements...
"The stalks are held down firmly by cross-pieces and the tanks are filled with water... In 6 to 10 days, according to temperature, the bark of the hemp stalks will be found to readily slip off when the stalks are broken in the hands, and the hemp should then be taken from the tanks and dried and and put under cover to be broken, shaken from the woody matter and baled...
"The above method will produce a prime cordage hemp for use where a strong, serviceable fiber is desired. Another process is to take the hemp stalks from the retting vats in 5 days and dry them by standing out or spreading, and again returning them to the vats for 5 to 8 days longer. This produce a fiber corresponding to the best Italian hemps.. and is adapted for fine cordage, coarse threads, carpet warps, canvas and similar products.
"Another method is to place false end pieces across the tanks some 2 inches from each end of the retting tanks and reaching down to within 2 to 4 inches of the bottom. A half-inch stream of water is let flow into the tank upon the top. This carries all impurities downward and out under the ends of the false ends and up and out over the real end, made an inch the lowest, and thus maintains a circulation of water which produces a fiber of much lighter color, especially if the water used is slightly hard and impregnated with lime.
"After the hemp is retted in water in the tanks for 5 days it may be taken out, dried and broken, and will furnish an exceedingly strong fiber for many uses. After water retting and drying the stalks, they are put under cover to further ripen and mellow. In all the work there should be some 6 weeks between the time of harvesting the hemp before it is retted, and the same length of time between the retting and the breaking, so that there will of necessity have to be a storage room for at least a supply for the work of 6 weeks.
"In retting, the tanks are emptied one or more each day, the contents put out to dry and again filled, so the work goes on steadily. Rain and snow and frosts do not injure the hemp after it is retted; in fact, the washing from a rain is an advantage, while a sharp frost serves to disintegrate the fibers.
"Another process is to first break the hemp stalks by passing them through a breaking machine [to remove the hurds]... As it requires 5 to 6 tons of hemp stalks to yield a ton of fiber, it can readily be seen that first breaking the hemp and disposing of four-fifths of the weight and bulk leaves a much less amount to be handled and very much saves labor in the work; besides, a tank holding 5 tons of stalks would hold all the fiber from 25 tons of stalks. If the hemp is first broken the retting tanks may be of much less size, while it is much easier to handle the fiber alone than the stalks, and in retting the water attacks the fiber evenly on all sides alike, whereas with the stalks the water only comes in contact with the outside of the fiber. In drying the fiber after it is so retted 25 tons may be hanged may be hanged upon an acre of ground if placed upon bars, horses, or other frames, for support. After drying in some 4 days, the fiber is put under cover to be again run through the breaking machine, and is in much finer condition for market. In all this work, if the retting tanks are filled with 1 pound of potash lye to each hundred pounds of hemp stalks or fiber, the retting will be done in 4 days instead of 8. When this is done with the fiber alone, the fiber is afterwards put into a solution of muriatic acid, 1 pound to 100 gallons of water, and again rinsed in water...
"Instead of potash, some 2 to 4 pounds of neutral soap, free from resin, may be used and the hemp fiber retted without the use of the acid bath, the fiber being rinsed in soft water. Also the retting will be done in 2 or 3 days if the weather is warm, and there will be but little of the bad odor attending ordinary water retting. If this solution of soap and water is made hot, the retting will be done in 12 hours. If perforated steam pipes are inserted at the bottom of the tanks and live steam turned in for boiling, the retting will be complete in 1 to 3 hours, according to the strength of solution used and the degree of fineness required. If the hemp which has previously been water retted and broken is boiled for half an hour in such a saponaceous solution a nearly perfect fiber results. After boiling and rinsing and drying, and the hemp has lain 4 to 6 weeks to mellow, ripen and gain nature and quality, it is run through the breaking machine, softened and baled, as is done with cotton.
"The steam can be produced at no cost by using hurds as fuel. The ashes of hurds contain about 12% potash, which can be extracted with water to make soap, soften water, and ret the hemp as described above, besides serving as fertilizer."
The research conducted by W. Fuller and A. Norman in the 1940s investigated various controlled methods of retting, with these results:
"Retting under anaerobic conditions was more rapid than any of the aerobic methods, and produced a fiber with more desirable characteristics. Retting under anaerobic conditions was most rapid if the temperature was kept at 37o C, the acidity of the solution was kept to a minimum, the solution was undisturbed, and an enrichment culture was added initially as inoculum in an amount equal to 10% or more of the total tank solution... Acid accumulation and consequent retardation of the retting process was prevented by aeration, and later return, of a portion of the retting solution in a separate tank... Under controlled anaerobic conditions about 1/2 of the water-soluble constituents and the furfuraldehyde-yielding constituents were removed in the first 36 hours. Attack upon the cellulose was negligible until after 48 hours... The progress of retting was invariably accompanied by an attack on the polyuronide hemicelluloses proportionately as great as that upon the pectin... There was no indication that Trichothecium roseum was any more vigorous in cellulose decomposition than any other organisms normally present on hemp straw.
"The loss in weight of the hemp and production of volatile [acetic and butyric] acids may be used to a limited extent as indicating the rate and degree of retting on a laboratory scale, providing that uniform material of the same source and maturity is employed... The reduction in the total uronic content of the bark reflected the progress of retting better than that of any other constituent. Well-retted hemp bark had a total uronic content of about 5-6% as compared to 9-10% of the unretted bark." (6)
J. Kulas and L. Nowackiewicz also studied anaerobic retting:
"The weight-ratio of water during retting, the rinsing of straw after retting and the intensity of spraying during pressing were more important for fiber quality more than a change of the water during the retting. A weight-ratio of 1:20 during the retting of hemp straw gives the best results. Rinsing of the straw with 18 m3 water sprayed per ton improves the quality. A total quantity of 55 m3 of water per ton of straw is necessary." (7)
US Patent #1,448,391 describes an improved method of retting in which hemp stalks are stacked in superimposed layers in an inclined position; after they have been cured, they are subjected to moisture while still in stacks but without submersion, and they are aerated after retting.
US Patent #2,457,856 teaches a chemical retting process for hemp by immersion in a mixture of hydrogen peroxide (0.5%), ammonium phosphate (0.5%), and urea (1%) at 50o C; the bath is then heated to 100o C. The xyloid material is to be removed by decortication.
In some parts of Asia, hemp fiber is prepared by soaking the stalks in water for a day or two, then steaming them for 3 hours. Then the fiber is peeled off by hand or by scraping. The resulting product is a stiff ribbon which is not well suited for spinning. Steam retting results in a considerable loss of fiber, and the remaining fiber has weaker tensile strength than water-retted hemp. (8)
Edward Antil added a precaution about fresh-water retting in his Observations on the Raising and Dressing of Hemp (1777):
"If the Hemp be rotted in a brook or running water, the sheaves must be laid across the stream, for if they be laid down lengthways with the stream, the current of water will wash away the lint, and ruin the Hemp. It must be laid down heads and points, two, four, or six deep, according to the depth of the water and the quantity of the Hemp..."
The anonymous Farmer from Annapolis advised against the practice of fresh-water retting in his Essay on the Culture & Management of Hemp (1775):
"Before placing the Hemp in the water, it will be necessary to take care that the bundles have not been made too large, and that the Hemp is tolerably even at the roots, as well as near the top; care however must be had not to bind it too close, an error, here, being of more consequence than may be imagined, the watering never succeeding thoroughly when the bundles are hard tied, the fermentation going on unequally in the several parts, as they are more or less confined...
"We are apt to imagine that a fine clear stream would be fitter to accomplish this end, because, at the same time that it dissolved, it would also purge and wash it from that gum and filth, thereby leaving the hemp in a purer state, but experience, against which there is no reasoning, convinces us, that the properest places for watering Hemp, are deep ditches, or pits of stagnated water, such as mill-ponds, or deep pools, where the water is seldom or never changed. The more still and putrid the easier it ferments, and penetrates the hemp more quickly, as well as more effectually, and though that which is watered in a limpid stream will appear far whiter, at the brake, than that which is watered in stagnated water, yet, upon minute examination, the first will be found inferior in quality to this last, it appearing in a manner exhausted and dead as it were, and of a pale white, whilst the other appears with a fine lively gloss, with a bluish cast that never quits it; for the cloth made of this Hemp most readily attains a perfect degree of the purest white, whilst the cloth made of the river watered Hemp, notwithstanding all the efforts of art, will still retain a yellowish cast, and without great care, will increase as the linen is used...
"If possible, [the retting pit] ought to be in such a place... that a small quantity of soft water, free from any mineral, may, when clear, be let into the pit, near the surface, which, else, from the excessive power of the sun at this season, would be much warmer at the top, and consequently the Hemp there would be sooner watered than at the bottom. Where no fresh water can with convenience be admitted, the pit may be shaded by means of a few more green boughs...
"To fit these pools the better and sooner for the purpose, it will be necessary to have them dug two or three months before they are used, and to throw therein, to rot, succulent weeds or plants, which may be taken out immediately when the Hemp is ready to be laid in, by which means the water will be stirred up from the bottom, and mixed with that at the top...
"If it is perceived that the Hemp has been taken out too soon, it is only permitting to lie a few days longer where it is spread, and the dew or rain will not only compleat what the water has left undone, but it will also take off some of the harshness of the Hemp."
Lionel Slator made this recommendation in his Instructions for Cultivating Flax and Hemp (1775):
"Unless the Ponds be made in a gravelly soil the bottom of the Ponds will be apt to be muddy or foul, which may be injuring to the Hemp; therefore, to prevent such Mudd, the Bottom of the Ponds may be either flag'd or planked, where there is not a solid Bottom of Gravel."
"Hurds" are the pieces of the woody core of hemp. Hurds are a valuable commodity with many uses, especially such as paper and Isochanvre, a petrified form of hurds manufactured by Chenovette Habitat in France.
The value of hemp hurds was recognized by the anonymous Farmer from Annapolis:
"A most considerable advantage comes from the coarse tow, or hards, of the first hackling, which, by means of the second watering, becomes an object of great utility, being thereby excellently prepared to make the best sorts of coarse linens... and preferable in strength and quality to imported osnabrigs, being greatly superior to the coarse tow prepared in the mill."
Marcandier mentioned a mixture of hemp and wool, known as berlinge, that was widely used in France:
"Hards or Tow, in the modern method of its preparation, being equally mixed with Wool, Cotton, Silk or Hair, is with much credit and advantage wrought into a variety of hose, caps, stuffs, cloths, and many other articles, to the reduction of the usual price of the whole, and the consequent encrease of commerce...
"Now this hards, that was formerly an object of discouragement... by this new operation becomes a matter of very great advantage. By carding them like wool, they produce a fine, marrowy, and white substance, the true use whereof was never discovered till now. It may not only be used alone, as it is, for the making of wadding, which, in many respects, will have the advantage of the ordinary sort; but moreover, it may be spun, and made into very beautiful thread. It may also be mixed either cotton and silk, with wool, and even with hair; and the thread, that results from these different mixtures, affords, by its vast variety, materials for new essays, very interesting to the arts, and of vast utility to several sorts of manufactures...
"The principal advantage that Hemp, intended for these uses, will have over wool, grogram yarn, and cotton, is, that it may be used without spinning or even combing. It will be in no danger from those worms, which commonly eat woolen cloth; and the beauty, as well as the lasting nature and the low price of it, will render it preferable to any other material. The different trials, that have been made of this sort, leave no room to doubt of success in other attempts of the kind."
Japanese Patent #42,193 describes the manufacture of imitation cotton from waste hemp:
"Purified waste hemp fibers are wrapped on a spool to a thickness of about 2 cm and one side is then cut to form a sheet. These sheets are immersed in NaOH solution at 5-15o for 20 minutes, neutralized with dilute sulfuric acid treated with Marseilles soap and non-drying oil as usual, and loosened to a cotton state."
According to Japanese Patent #42,179, threads of hemp, cotton, etc., can be whitened by immersion in a solution of 20 gr BaCl2 or CaCl2, 1 gr of sodium acetate, 1 gr of glycerol or phenol, 20 gr of milk and a small amount of gum Arabic in 180 cc of water. After drying, the textile is passed through cold sulfuric acid, washed with water, then dried in the air by heating.
US Patent #2,450,586 was granted for a process to saccharify hurds (and other agricultural residues), which can be treated to produce C5-6 sugars:
"The cellulosic materials are treated with 1-6% sulfuric acid at 100-120oC to convert pentosans to pentoses and furfural. The treated material is washed free of soluble pentose and other sugars with additional dilute sulfuric acid. The residue is dewatered and dried, then comminuted and mixed with 0.15-0.55 parts of 80-87% sulfuric acid per part of cellulosic material at a temperature below 40o C, whereby a free-flowing powder is formed. The total amount of both dilute and concentrated acid used is not more than 0.35 part/part of the original cellulosic material by weight. The acid-mixed material is then subjected at a temperature of or below 45o C for 1-5 minutes to mechanical mastication so as to develop continuously-changing directional pressures on the solids in excess of 100 lb/sq. in. and just short of the formation of dextrose, whereby a pronounced physical and chemical change is effected to convert powder to a stiff plastic mass. The resulting mixture is then diluted and hydrolyzed to produce dextrose."Hurds also can serve well as a fuel, and can significantly reduce the cost of processing the fiber if used to generate steam power to operate the machinery. During World War II, hemp mills in the USA dried the damp stalks by conveying them through a long, heated drying tunnel before delivering them to the decorticator and scutching machinery. The hurds were burned to provide heat for the drying tunnel.
I. Popescu and I. Afusoae discovered another use for hurds:
"Fermentation can help turn hemp boon [hurds] into a suitable product for soil fertilization. During fermentation the boon reaches almost the same level of assimilable N, K, Mn and Cu as in barnyard manure...
"Hemp boon will absorb up to 792 mg ammonia/gr of boon in ammonium nitrate and sulfate solutions." (10)
When retting is complete, the stalks are dried and sorted by grades, then crushed in a mechanical decorticator. The classic hemp-brake is a manually operated wooden press of intersecting boards that break the stalks so the hurds can be removed. The operation consists of placing some stems with the butt end first across the break, then lowering the upper boards by the handle, breaking the stems as the boards intermesh. This is done repeatedly as the stems are fed in up to the tips. An experienced hemp worker can hand-break about 250 lb of fiber per day.
Thomas Jefferson wrote: "A hand will break 50 or 70 lb a day, and even to 150 lb if it is divided with an overseer; divide it as prepared." But the work was very tedious, and the slaves complained of it. Jefferson proceeded to invent an improved hemp break, for which he received US Patent #1. He described it in a letter to George Fleming (29 December 1815):
"Flax is so injurious to our lands and of so scanty produce that I have never attempted it. Hemp, on the other hand, is abundantly productive and will grow forever on the same spot, but the breaking and beating is so slow, so laborious and so much complained of by our laborers, that I have given it up... But recently a method of removing the difficulty of preparing hemp occurred to me, so simple and so cheap. I modified a threshing machine to turn a very strong hemp-break, much stronger and heavier than those for the hand. By this the cross arm lifts and lets fall the break twice in every revolution of the wallower. A man feeds the break with hemp stalks... where it is more perfectly beaten than I have ever seen done by hand. I expect that single horse will do the breaking and beating of 10 men...
"Something of this kind has been so long wanted by the cultivators of hemp, that as soon as I can speak of its effect with certainty, I shall probably describe it anonymously in the public paper, in order to forestall the prevention of its use by some interloping patentee." (11)
After breaking hemp, the fiber is "scutched" and "hackled" to remove the hurds, broken fibers, and extraneous material. The fibers are cut into shorter lengths, then combed to remove short and tangled segments. The long fibers are parallelized and smoothed in a hackling machine, then repeatedly drawn through sets of sharp pins or combs to produce a product ready for wet or dry pre-spinning into roving yarn. The low quality scutching and hackling tow discarded from the preseding process is shaken and carded, etc., or it is "cottonized" to remove the sticky pectin and lignin and produce loose bast stock. Then it is wet- or dry-spun into coarse yarn, twine, or other specialty products such as insulation..
The Schlichten Decorticator --- Hundreds of hemp-processing machines, or decorticators, have been patented since Thomas Jefferson made his improvements on the hemp break. Only the design perfected by George W. Schlichten met the needs of the industry. The Schlichten Decorticator promised to revolutionize the hemp industry by eliminating the need for retting. As described in his U.S. Patent #1,308,376, "The fiber produced is at once ready and suitable for carding or combing without any further treatment such as degumming or retting, and leaving the fiber soft, pliant, adhesive, and in its unimpaired natural strength and color..." In 1916, after 18 years of development and $400,000 investment, Schlichten tested the market for the hemp fiber his machine produced. He sold his entire first batch to a spinning plant owned by John D. Rockefeller, and was paid a record premium of $100/ton. Afterwards the mill offered to buy the exclusive rights to the invention, and at a higher price than Schlichten had wanted, but he declined the offer.
Field-dried stalks are introduced to the decorticator on a corrugated feed table or through revolving disks that keep the stalks separated and straight. The stalks pass through denting rollers, then through splitting and spreading rollers. The stalks then pass between a series of primary and secondary breaker rollers. Next, a high speed rotating coarse comber begins to clean the fiber and degums it by separating the non-fibrous products along with the short "tow" fibers. Corrugated softening rolls then massage the fiber and hold it in position for another series of combing and softening rollers. Finally, an endless slatted carrier eliminates any remaining small waste particles and delivers a continuous, folded "sliver" of fiber, ready to be hanked and baled (Fig. 3.1). (12)
The machine came to the attention of the industrialist Henry Timken (inventor of the roller bearing) in 1917, and he was very impressed by its possibilities. He arranged to meet Schlichten and offered him the use of 100 acres of his ranchland in Imperial Valley, California, to grow a crop of hemp to test in the decorticator. The bumper crop was 14-16 feet tall and attracted national attention from coverage by the Pathe, Mutual, and Hearst newsreel companies.
Timken also tried to interest newspaper magnate Edward W. Scripps in the idea of making newsprint from hemp hurds. Timken called Schlichten's decorticator "the greatest invention in the world." Scripps' associates Milton McRae and Edward Chase investigated the feasibility of the proposition. In his enthusiastic report, Chase wrote:
"I have seen a wonderful, yet simple, invention. I believe it will revolutionize many of the processes of feeding, clothing and supplying other wants of mankind.
"Mr. Schlichten raised 5 tons of hemp stalks to the acre on a 100-acre patch... He will pay the growers $15 per ton for dry hemp stalks delivered to his machine... Thus the farmer gets $75 an acre for this crop which matures in 100 days. The stubble and that part of the leaves and tops which remain on the field (containing in excess of 50% of N), are wonderful fertilizer. Moreover, the hemp kills all weeds. The farmer's land is left in fine condition for immediate planting of other crops.
"From each ton of dry hemp stalks, costing him $15, Mr. Schlichten gets the following:
About 500 lbs. hemp fiber at $0.16/lb ...................................... $80.00
1250 lbs. hurds at $5.50/ton ................................................... $ 3.44
(Worth that figure as stock feed or for paper stock)
250 lbs. leaves, tops, &c. at $5.50/ton ..................................... $ 0.69
From each ton, about .............................................................. $84.13
From each acre, about ........................................................... $420.65
From 100 acre experimental patch, about ......................... $42,065.00One of Schlichten's machines will produce per day (2 shifts of 8 hrs. each):
2 tons of fiber worth about..................................................... $600.00
5 tons of hurds....................................................................... $27.50
1 ton of tops, leaves, &c., worth about $5.50
Total: $633.00"This will be at a total cost of less than $200 --- less than $100 per ton of fiber for growing the hemp, passing it through the machine and baling the output ready for market. One fairly good machinist and three common laborers... are required per machine for each shift. This one new machine... turned out in the two days I was there:
3 tons of fiber worth............................................................. $900.00
About 7 tons of hurds worth................................................ $38.50
About 1 & 2/5th tons of leaves, &c.. $7.70
Total: $946.20"McRae waxed eloquent with praise in a letter to Henry Timken (11 August 1917)
"Mr. Schlichten impressed me as being a man of great intellectuality and ability... he has created and constructed a wonderful machine...
"[That] Schlichten decorticating business... I believe, is one that is worthy of any man devoting his time and money to, because it appears to me that the successful development of that industry means clothing all the people of the world, eventually, at considerably less cost than is now required.
"You know, Lord Bacon had a theory, or philosophy, that in this industrial age the real benefactor of mankind was not one who evolved or preached a beautiful theory, but one who assisted in clothing and feeding the world more economically than in the past... that means shorter working hours, and shorter working hours eventually means the spiritual development of the world..."
The Schlichten decorticator remained dormant until the mid-1990s, when it was redeveloped by Jim Hill, the founder and owner of Hill Agra Sales (Shelburne, Ont.), manufacturer and distributor of bulk vegetable harvest machinery. Jim Hill described the project in an interview with Hemp Magazine (1997):
"We have it working, and it does what we expect of it. The machine is two feet wide and eight feet long. It is on a two wheel trailer with jacks on either end. It is hydraulically driven. The high and low speed rollers have variable speed controls with flow dividers. It can be run by a tractor of 60 horse power using the hydraulic pump on the tractor.
"The [Fibre X Model 1460] machine will do approximately [3-4 tons] per hour. We have worked on other designs for fiber extraction. The Schlichten model is only one of several we have been working on...
"Basically, what I've done is to study the Schlichten papers, patents and drawings. The way that machine is drawn up, there is no possible way that the machine could work, based on that drawing. Back in those days, people changed their drawings from the original machine, so that they couldn't be copied. They never trusted anybody...
"The original machine that he built required four people to operate. That means two people putting the stalks in at one end, one person shoveling the hurds out from underneath, and a fourth person who had to be a good mechanic in order to keep the machine working. There were no bearings. The thing probably sounded like a flock of canaries when it was running. So, between oiling it and keeping the hemp from tangling up in the shafts, he must have been a very busy fellow...
"Our main goal was to maintain all the advantages he had built into his unit; such things as holding the rollers apart from one another, and having them spring loaded. Obviously, we had to put a high grade bearing in and have it protected against wrapping up in the fibers going through. Instead of putting an inside bearing, we put an external bearing that actually has its shaft continue over the mounting plate. This means that there is no chance of getting fibers caught in the bearing...
"Two people can run it. Operating this machine is more just a question of watching it to make sure that the material flows smoothly. It can be hand fed, or machine fed. There are all kinds of developments that can be added to this machine. The hurds that come out underneath the machine can be dropped on a conveyor and blown off into large plastic bags for later use. Our idea for the decorticator is that it should be used out in the field. This puts the dust right back in the ground for fertilizer...
"The decorticator is portable so you can move it from field to field instead of moving all the material to a central location for processing. That means that several growers can get together and buy the equipment and handle this as any other crop. After that, you can handle the product output many different ways. You can upgrade your product at the farm gate and have that value ready to ship out as goods. This will allow the growers to realize more of the profit available from their crop."
Austrian company Rohemp also manufactures a mobile hemp breaker which separates field-retted hemp into fiber and hurds at a rate of about 1 ton/hour and produces round bales of straw.
The certified classification and value of hemp stalks is determined by the quantity and quality of the long fibers in particular. Fine fibers are most valuable. The content of fine fibers is indicated by a numerical index that is determined by the ratio of the length and diameter of the stalk. Grade I stalks are no more than 0.4 inches (10 mm) thick along at least 60 inches (150 cm, 85%) of the plant's length. Grade II stalks have a maximum thickness of 0.5 inch (12 mm) and a minimum technical length of 40 inches (100 cm). Grade III stalks are a maximum 0.6 inch (14 mm) thick and 25 inches (60 cm) minimum technical length. The maximum moisture content for all three grades is 16%.
No matter what the method of cultivation, all hemp must undergo some primary processing near the farm. Schlichten-type decorticators or "mini-mills" with 50-300 tons daily capacity would be ideal.
Several modern technical developments have made hemp fiber processing more economical and eco-friendly. Traditional methods of retting can pollute the environment and are subject to failure. The new methods produce high yields of standard-quality fiber at competitive prices for industrial purposes. The absorptive properties, temperature stability, etc., can be modified by pre-treatments such as drying, carbonizing, impregnation, and mineralization. A very efficient method of steam explosion (STEX) that produces high-quality hemp fiber has been developed in Europe. STEX enables the production of custom-made fibers that meet special requirements for singular products. The parameters of the process can be steered to optimize the objective qualities of the resultant fibers. It is possible to produce yarns of Nm 10-15 fineness (the running length of thread per gram of yarn weight) without making any special modifications of the process. Even after cottonization, steam-treated hemp has a value of about 60 cN/tex (cotton has a value of about 35 cN/tex).
Cleaned fibers (70 mm) are pre-treated with alkali and introduced to a reaction vessel. High-pressure steam (up to 12 bar, 175 psi) is injected to penetrate between the bast fibers and dissolve the lignin, pectin, etc., for 30 minutes or less. The pressure is released suddenly to explode the fiber bundles without causing disintegration of single cells. The additives and other substrate ingredients are blown out and collected for extraction and recycling. The resultant fibers are washed, rinsed, and dried with hot air. Then the fibers are opened with a saw-tooth opener and any remaining dust or wood is removed with a multi-stage cleaner. Lesser amounts of dyes and other chemicals are required for further processing because the STEX process produces very high purity fibers.
The Xymax Corporation has perfected a patented process using steam explosion to reduce plant material to cellulose, hemicellulose, and lignin. The "Xylanizer" produces "cottonized" or flock hemp that can be refined by existing cotton/wool-processing equipment. It also produces "BondoMass", an inexpensive plastic-like wood which is stronger and more flexible than lumber. Xylanized cellulose also produces twice as much ethanol as fermentation processes, and the residue can be made into paper pulp at a lower cost than from wood. The only byproduct is steam. (13)
Ecco Gleittechnic Gmbh (Germany) has demonstrated a novel ultrasonic breakdown method and apparatus to remove dirt, lignin, and pectin from hemp straw. The resultant fiber is of high purity and consistency, "a completely new type of natural fiber" possessed of "extremely high and fast water absorption, and a high degree of whiteness [70]." Only 1% hydrogen peroxide is necessary to reach 90 degrees of whiteness. The process replaces the retting process and mechanical decortication, and it excludes the degradation of cellulose caused by chemical treatments. The lignin and pectin can be recovered for other uses, such as binders for particle boards.
Alcell Technologies, a division of Repap, has experimented with pulping whole hemp stalks. Separating the longer fibers before processing them is too expensive. Alcell's technology fractionizes the fibers in a Thermo-Mechanical Pulping (TMP) process to produce both newsprint and high-quality wood paper from a single source of fiber.
Hurds are useful as papermaking material, but they contain more lignin than the bark. Therefore, hemp bark is more valuable than hurds for papermaking. The lignin must be removed in the papermaking process. Delignification generally is done with chlorine, which is the major source of pollution from pulp mills. Digestion with sodium hydroxide at about 170o C also promotes delignification. Hydrogen peroxide provides a superior alternative, producing high quality pulp with minimal pollution.
novel chemi-mechanical hemp pulping technology developed by the Dutch ATO-DLO allows fiber length to be adjusted, and eliminates the need for a hollander beater. The process uses a twin-screw extrusion pulping machine that combines shear forces and small amounts of alkali and catalyst to remove only about 50% of the lignin. Thus, hemp paper can be manufactured at a lower cost than from wood. (14)
The closed-cycle Alcohol-Ammonia-Sulfite (AAS) pulping process makes possible the production of hemp pulp without pollution. AAS pulping can use the woody core to produce long bast fibers with very low levels of lignin, and it does not require that the bast fiber and woody shives be separated beforehand. The selectivity and "soft" conditions of AAS-pulping enable bast fiber pulping to be separated into low-lignin long fibers and high-lignin short fibers. The AAS-pulping process produces hurd pulp equal in quality and superior in yield to wood kraft pulp, and it is about 15% brighter. Its breaking length also is very high.
The AAS process uses aqueous ethyl or methyl alcohol (water 65:35 alcohol vol %) as a solvent, but polyols (glycerine, diethylene glycol, etc.) or cellosolves (ethylene glycol monoethyl ether, etc.) can be used, though they are more costly. The process cooks the shives with ammonia-sulfite (1:0.2-2.5 % by weight) in aqueous alcohol at 150-185o C for 75-180 minutes. The liquor to shive ratio is from 4-4.5:1 The ammonia consumption is 5-25% of the weight of raw material. The alcohol, water, and by-products (alcohol, acetone, ethyl acetate, etc.), can be recovered for recycling. The yield varies from about 55-68%.
Table 3.3 ~ Colorimetric Identification of Hemp Fiber Reagent Color
Iodine-Sulfuric Acid Blue-green
Zinc cholor-iodide Blue/violet, trace of ellow
Calcium chlor-iodide Rose-red
Aniline sulfate Yellow-green
Ammoniacal Fuchsin Pale red
Ammoniacal Copper Iodide Blue/Blue-green
Ammonia Faint violetA wide range of lignin content has been reported for hemp, from a low of 4-8% to a maximum of 30%. The dry weight of male hemp is 13-16% lignin; females contain 23-25%. As many as 4 chlorinations are necessary to delignify hemp cellulose. If the residual lignin exceeds 0.8%, the fiber is brown and hard to handle. In comparison, straw contains about 15% lignin, sugarcane bagasse about 20% lignin, bamboo is about 23% lignin, jute is 11% lignin, and retted flax is 2% lignin.(15, 16)
When viewed under a polarization microscope, interference colors appear in the orthogonal position, which are different for flax and hemp.
A typical analysis of hemp fiber gives: Ash (0.8%), hygroscopic water (8-9%), aqueous extract (3.5%), fat and wax (0.%), cellulose (78%), lignins and pectins (9.5%). The woody core contains about 7.7% glucan, 6.7% xylan, 1.2% mannan, and 2.1% lignin. The bast fibers contain about 6.7% glucan, 1.5% xylan, 1.9% mannan, and 4% lignin.
Most of the few hemp pulp paper mills each produce about 5 kilotons/year of specialty papers for cigarettes, filters, insulation, security and art. Hemp paper is superior to pulp in most respects, but the paper industry is not equipped to handle hemp pulp, and the cultivation of hemp is not yet extensive enough to supply the market.
The pulp and paper industry also does not use hemp hurds for several technical reasons. Hurds are relatively difficult to delignify. Unbleached hurd pulp produced by conventional processes gives lower yields and has higher Kappa numbers than hardwood pulp. The fiber length of hurd pulp is only half as long as hardwood pulp. Hurd pulp has an extremely low rate of drainage and low mechanical and papermaking properties when prepared by conventional methods. These factors are of little importance when AAS pulping is used.
The production of paper from hemp hurds is a relatively simple process. The hurds must be sorted or screened according to size so as to ensure uniform quality of the finished fibers. Smaller pieces are reduced sooner than large hurds by the caustic cooking process, and the over-treated hurds result in a lower yield of cellulose fiber, and a mixture of over- and under-treated fibers. The hurds must be sieved to remove dirt before cooking.
Such preparations are unnecessary if the Schlichten decorticator, AAS pulping, or steam explosion is employed. Edward Chase pointed out the advantages of Schlichten’s mechanically decorticated hurds in his report to Howard Scripps:
"On page 22 of [USDA] Bulletin #404, paragraph 2, you will note:
"The weight of hurds which are capable of being charged into a rotary (digester) is a decidedly unfavorable factor.
"(This in comparison with the weight of a cubic foot of wood as now charged into the digesters at the paper mills.) This would not be the case with the hurds from the Schlichten machine... Dry hurds in hydraulically pressed bales would weigh about as much per cubic foot as wood.
"You will also note that the bales of hurds from the retted hemp must be covered, which would not be the case with the Schlichten hurds. Also, hurds from retted hemp must be screened or sorted, and the various sizes treated separately and differently. None of this work is necessary with the Schlichten hurds..."
The following excerpts from a "Digest of Conversation of Mr. G.W. Schlichten with Mr. M.A. McRae..." (3 August 1917), illustrate the enormous potential of hemp hurds for purposes of paper production:
"In speaking about the rise in price of paper, etc., Mr. Schlichten said ...
"I came in contact with Mr. Merrill [sic] of the Paper Plant Investigation Bureau of the Agriculture Department; he is the head of it... I gave to Merrill some hurds... The hemp hurds is a practical success and will make paper of a higher grade than ordinary news stock. The Government has made on a large and practical scale paper --- a beautiful sheet --- and I can show you governmental reports printed on paper made from hemp hurds... This paper has been made from hurds produced from the fermented [retted] hemp... but I produce from the unfermented stock, and therefore the inner part is more valuable for paper stock because it has a certain amount of natural glue contained in it, which acts in the cooking as a natural binder for the fiber...
"In the cooking and beating of these hurds, less caustic soda, resin, and ... clay will be needed than when ground wood is used;
"Sulphite must be mixed with ground wood pulp, but not with [hurd] pulp...
The USDA report by Merrill states:
"This comparison, satisfactory in many respects, develops two factors which are decidedly unfavorable to hemp hurds, namely, raw-material storage and digester capacity, and they must be taken into full account in considering the paper-making value of this material... Material progress was being made at the conclusion of this preliminary work...
"Calculations on the raw product and acreage for a permanent supply for a pulp mill producing 25 tons of fiber a day for 300 days per annum, or 7,500 tons per annum, give the comparison between hurds and wood..."
[Table 3.4]
Comparison of Wood & Hemp Hurds
The most important point derived from this calculation is in regard to areas required for a sustained supply, which are in the ratio of 4 to 1. Every tract of 10,000 acres which is devoted to hemp raising year by year is equivalent to a sustained pulp-producing capacity of 40,500 acres of average pulp-wood lands. In other words, in order to secure additional raw material for the production of 25 tons of fiber per day there exists the possibility of utilizing the agricultural waste already produced on 10,000 acres of hemp lands instead of securing, holding, reforesting, and protecting 40,500 acres of pulp-wood land." (17)
Paper is manufactured by "cooking", the process by which fibrous raw matrials are reduced to cellulose pulp by chemical treatment with alkali. The most satisfactory results are obtained with sufficient caustic solution (29.5% sodium hydroxide at a concentration of 107 gr/liter having 84% causticity) to supply 25-30% actual NaOH (calculated from the dry weight of hurds in the charge). Merrill used a larger amount of caustic than was necessary in his experiments because his steam supply was problematic. The batches were contained in a rotary charger at 1/2 rpm. After 5 minutes, steam (120 psi) was admitted at such a rate that the charge was heated to 170o C in 1 hour. The heating was continued 5 hours, then the steam pressure was released, and the stock was emptied into a tank to be drained and washed.
The cooked stock, brown in color, was washed for 1 hour in a cylinder covered with 60-mesh wire cloth so as to remove dirt and chemical residues. The water was drained off, the stock was steam-heated to 40o C, and a solution of 11.3% chlorine bleaching solution and 1/2 pint of sulfuric acid was added, equivalent to 10% of the weight of the fiber. The mixture was bleached overnight, then drained and washed. If the color was not sufficiently white, more bleach was added, and the process was repeated.
The best results in the process of furnishing in a beating engine was obtained with a charge of 16.5% sulphite, 22.3% soda poplar, and 61.2% hurd stock loaded with 22% clay and 1.38% resin size. The furnish was given a hard brush for 1 hour and given a blue tint before it was run on a papermaking machine with no problems whatsoever. Merrill commented that, "Experienced paper-makers commented very favorably on the running of this furnish and the quality of the paper produced," which was classed as a No. 1 machine-finished printing paper. The USDA Bulletin No. 404 was printed on the hempen paper they produced. The bulletin then continues with a proviso:
"The weight of a cubic foot of hurds is about 5.4 lb compared to 8.9 lb/cu ft for poplar-chips charge. This represents a cooking charge of 60% of the weight of a poplar-chips charge, yielding about 38% as much fiber as a wood charge. The smaller weight of a hurd charge constitutes one of the main objections to the use of hemp hurds in paper manufacturing, but the weight of the charge can be increased by steaming or tamping. The relatively high cost of hemp fiber pulp is due to the inefficient processes currently used to produce pulp. In addition, hemp is harvested once a year --- usually in August --- and it needs to be stored until used at the mill, resulting in higher costs."
Cooking of hemp fibers with water gives a product which can be used for Kraft paper. Digestion with sodium hydroxide gives a pulp containing as much as 20% pentosans. The Ritter-Kellner method is more suitable: the fiber is cooked at 140-150o for 12-13 hours with 4% sulphite liquor to produce pulp containing about 64% crude cellulose of which 10% is pentosans(18)
As explained above, hemp fiber is not entirely practical for paper-making when it is subjected to soda and sodium anthraquinone pulpings. Furthermore, the bast and woody fibers of hemp are so different from each other that it is impractical to pulp them together. The qualities of hemp fiber are different from other pulp materials; it requires special beating and refining processing and equipment. The process is much less efficient when conventional papermaking machinery is modified for hemp pulp. For example, it is not worthwhile to use a disk refiner, whereas low-speed hollanders are suitable. It is also characteristic of hemp pulp that the "freeness" increases during the papermaking process. It must be watched closely to prevent over-beating.
Hollander beaters impair the drainage properties of hemp pulp to such extent that the paper-making machine must be run at a slower speed in order to dewater properly. Thermo-Mechanical Pulping (TMP) eliminates the need for hollander beater. TMP employs mechanical shear forces combined with small amounts of alkali and catalysts.Wade Chute, Senior Research Engineer of Agrifibres at the Alberta Research Council, made a comparative study of the properties of mechanical pulps made from hemp bast fiber and whole-stalk mechanical pulps. Significant problems were encountered with fiber tangling and plugging of the refiner inlet. The primary processing procedure was therefore made particularly aggressive to prevent recurrence of the problem, but this manner of processing caused excessive damage to the fibers, resulting in significantly lower quality results than were expected. Chute found that whole-stalk mechanical pulps are weaker than bast fiber pulps, resulting in lower tear strength because short hurd fibers are present. More refining energy is required to process whole stalk hemp pulps, compared to bast fiber pulps.
The French BiVis process, developed in 1975, uses a twin-screw extruder to cut the long bast fibers to a length that conventional papermakers can handle without adversely affecting the drainage. The BiVis rocess has been adapted to handle hemp fiber efficiently with an overall yield of 75-80% w/w (compared to about 50% efficiency for chemical pulping). The process uses hydrogen peroxide (40 kg/ton) and alkali (50 kg/ton) to produce chlorine-free paper with 82 point brightness. Water consumption is 12 m3/ton, and most of the waste is biologically degradable.
US Patent #982,170 describes an electrified preparation of hemp for the manufacture of paper:
"Hemp is cut into small pieces, then boiled in 0.5-8% sulfuric acid. Immerse the hemp in a solution of Na2CO3 and NaCl, and subject it to the action of an electric current."
US Patent #2,099,400 describes "An improved method of producing bleached pulp from hemp tow (hurds)":
"[Subject] the tow to a cooking operation in a digester with a solution of water of approximately 2-1/2 to 4 times the dry weight of the fiber, and 14% to 20% sodium hydroxide and 1% to 4% sulfur, the quantitites of the chemicals employed in the solution being based on the dry weight of tow; then wash the cooked fiber to remove shive fiber and water soluble impurities, and bleach the fiber."
Sadly, the mass-production of hemp paper probably will not become established as a major industry until the insatiable demand for wood has utterly decimated the forests of the world (circa 2020 AD) and there is no other alternative.
A "Market Analysis for Hemp Fiber as a Feed Stock for Papermaking" was suppressed by the Justice Department in 1997 after an anonymous chemical engineer, employed by a public institution in Wisconsin, released the paper. A bootleg copy was obtained and published "in the name of academic freedom" in Hemp Magazine (May 1998), excerpted here:
"The value of bast fibers as a component in paper pulp is widely acknowledged. An analysis of the bast fibers shows that they are composed of 70% cellulose and 8% lignin. Given that this material is chemically quite different than the hurds, it likely would have to be processed separately, but would likely have a 70% yield to fiber. If one does a weighted average of 50% yield for the hurds and 70% for the bast fibers, one obtains a value of 55% fiber yield from retted hemp stalks: (0.25)(70%) + (0.75)(50%) = 55%...
"Making assumptions about hemp yield per acre (3.9 tons/acre/year) and the pulp yield per ton of retted hemp (55%), one can estimate the number of acres of hemp required per year to meet the current Wisconsin demand:
(3,178 tons pulp/day (360 days/yr) = 533,000 acres (3.9 tons hemp/acre/yr)(0.55 tons pulp/tons/hemp).
"The price of bleached pulp varies widely, $300 to $1000/ton, due to fluctuations in supply and demand. Give this wide variation estimating the value of hemp fiber is rather difficult. It is likely that fiber formed from the hurds will be viewed as similar to hardwood fiber. The current price of bleach pulp is near $425/ton. The production costs will be similar to the production cost of fiber from wood. In fact, an implicit assumption of the following analysis is that only minor modifications to a pulp mill would be required to switch from wood to hemp. A recent analysis of the pulp making process suggests that the raw material, chemical and energy costs for pulping and bleaching wood chips is $233/ton of ECF bleached pulp. Of this cost, $155 was the cost of the wood chips, assuming wood chips cost $55/ton. If one includes a 50% increase in the cost/ton to account for labor, overhead and capital, one finds that the break even point is likely near $350/ton. If one assumes that the average yield from the hemp fibers would be 55%, then a direct replacement for wood chips would suggest a value of $75/ton for the retted hemp stalks. This price is based on numbers that were generated in 1993. If one uses the chemical price index to adjust this to 1997 one gets a value of $85/ton... A more realistic future value is likely $100-125.
"It is likely that the bast fibers would be reviewed as a higher value material on the pulp market. If, for example, one were able to produce fibers similar to cotton linters or cotton rags, then the market would likely offer $1000/ton of fiber. To translate this value to a price of the raw materials one must make several assumptions. If one assumes that the processing costs are the same as that for wood, $195/ton, that the yield to fiber is 70%, and that the required profit margin is $100/ton processed, the paper company could pay $500/ton for the bast fibers: ($1000-$295)/1.43 tons hemp/ton fiber = $493/ton hemp.
"If one uses a value of $100/ton for hurds and $500/ton for bast fibers, the estimated market value of retted hemp stalks is $200/ton. A study of hemp cultivation in Iowa suggested an average yield of 3.9 tons/acre. Combining the market price and the yield per acre one obtains a crop value of $780/acre. Since the production, storage and transportation costs will be similar to those of corn, $300/acre, a farmer could make a profit of $480/acre growing hemp. If the farmer were to only market the fiber, however, the profit drops to $190/acre...
"The profitability, for the farmer, hinges on the separation of the bast fibers from the hurds and the selling of the bast fibers at a higher price. For the purpose of this analysis, it was assumed that the paper industry would use the bast fibers, but it is also likely that other markets, e.g., textiles and building materials, could be found for them. Furthermore, one must develop markets for both the hurds and the bast fibers, if this enterprise is to be viable."
Table 3.1 ~ Traditional Hemp Processing
Table 3.2 ~ Typical Breakdown of Green & Dry Hemp
Figure 3.1 ~ The Schlichten Decorticator (USP # 1.308,376 )
3.6 ~ References
