US3369921A

US3369921A - Method and composition for treating wood to reduce staining

Info

Publication number: US3369921A
Authority: US
Inventors: Robert E Stutz; Chapman Alfred Dale
Original assignee: Chapman Chemical Co
Current assignee: Chapman Chemical Co
Priority date: 1962-10-04
Filing date: 1962-10-04
Publication date: 1968-02-20
Anticipated expiration: 1985-02-20

Description

3,369,921 METHOD AND COMPOSITION FOR TREATING WOOD TO REDUCE STAINING Robert E. Stutz, Memphis, Tenn, and Alfred Dale Chapman, Palo Alto, Calif., assignors to Chapman Chemical Company, Memphis, Tenn, a corporation of Tennessee No Drawing. Filed Oct. 4, 1962, Ser. No. 228,274

9 Claims. (Cl. 117-47) The instant invention relates to compositions and methods for the treatment of wood, and more particularly, to compositions and methods for the treatment of unseasoned or green wood to reduce the occurrence of staining phenomena.

Although the compositions and method of the instant invention may be used for the treatment of wood for various purposes, such compositions and method are particularly useful in the suppression of the development of the so-called black or sticker stain in certain woods. The instant invention is, however, adapted for the suppression of other types of staining of wood. Specifically, black or sticker stain tends to occur in redwood on air seasoning, and the occurrence of this type of stain is generally assumed to be related to the outward movement of the so-called extractives. Such stain is understood to be formed during seasoning but becomes apparent only in the dressed lumber after it is in place and after exposure to moisture vapor. There are a number of different and even conflicting concepts concerning the cause of such stain, although heretofore the lumber industry in general has accepted the migration of the extractives as the principal cause of the stain. Certain biological explanations have met with less acceptance.

Heretofore the practice of many sawmills resulted in heavy degrade losses in lumber through the action of such staining phenomena. One of the present methods employed for limiting such staining phenomena involves steaming of the lumber for four hours at 212 F. prior to air seasoning. The disadvantages of an operation such as this are apparent, but steaming offers certain advantages other than stain control, in that the subsequent air seasoning period is noticeably shortened, which lowers the yard inventory. Such steaming also causes some changes in the elastic properties of the wood, introducing sagging which can generally be corrected by flat piling prior to blanking and planing.

Another alternative which has been considered heretofore, and is currently being vigorously pursued, involves an attempt to circumvent the normal drying operation by solvent seasoning in a chamber with acetone or other suitable water extractive solvent. This procedure serves to remove water expeditiously, and also a large fraction of the extractives (which extractives may constitute 35% of the dry weight of the wood in some stocks).

Yet another procedure for suppressing such staining involves immediate introduction of freshly sawn stock into a. dry kiln, but the economics of this procedure leave something to be desired.

Although it is not desired to limit the instant invention to any particular theory, the basis for the use of the composition and method of the instant invention finds response in a theory relating to the biological explanation of the staining phenomena. As is appreciated, lumbering procedures call for the conventional harvesting of logs and the temporary collection in storage of logs at the mill site, whereat the sawing procedure is carried out. Indications are that the staining phenomena in question do not occur to any substantial extent in sound logs, but such staining phenomena tend to occur after sawing and conventional air seasoning of such woods as nited States Patent 3,369,921 Patented Feb. 20, 1968 redwood and other non-porous stain susceptible woods, including other coniferous soft Woods, as Western red cedar and white fir, and certain non-porous hard woods such as maple, birch, and persimmon.

In the practice of the instant invention such staining is suppressed, minimized and/or substantially prevented by treatment of such unseasoned green lumber, immediately after it is sawn, with an aqueous system containing a chelating agent for iron. It is believed that, as a result of the sawing operation, the freshly sawn surfaces of the wood are exposed to iron in one form or another and this iron (in the form or condition thus present) is believed to catalyze the staining phenomena. In the practice of the instant. invention the staining phenomena are suppressed by the use of such chelating agent alone or (preferably) in combination with certain other active ingredients which include nitrogen-containing enzyme inhibitors, fungicides (such as polychlorophenols, organic mercurials and tin alkyls), buffers, etc., some or all of which may be combined with the chelating agent in accordance with the practice of the instant invention to form compatible compositions wherein an improved anti-staining synergism is obtained by the combination of such chelating agents and such other active ingredients.

It is, therefore, an important object of the instant invention to provide an improved method and composition for the treatment of wood.

'It is another object of the instant invention to provide an improved method and composition for suppressing, minimizing and/or preventing the staining of wood, and particularly the formation of black or sticker stain in wood.

Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed disclosure thereof.

One aspect of the present invention consists in a method of treating freshly sawn unseasoned wood, which comprises bringing the wood into contact with a composition comprising 1 to 60 parts of chelating agent for iron, in 1500 parts of water. I

Although the various active ingredients used in the practice of the instant invention may be compounded as a dry mix for convenience (and even for actual application as such to moist wood) the practical use of the invention ordinarily contemplates the application of such various active ingredients including the chelating agent to the freshly sawn wood in an aqueous system or solution. The bases generally used herein for the aqueous compositions are 700 to 800 parts of Water (which would be approximately gallons, if each part is a pound) or 1500 parts of water (which is approximately 200 gallons on the same basis). As used herein, however, the terms parts and percent mean parts and percent by weight, unless otherwise specified.

As previously indicated, a key to the instant invention resides in the concept of using a chelating agent for iron in the treatment of the freshly sawn wood. In this respect, it will be appreciated that the explanation or theory of the invention is biological in character and it has been found important to employ the treatment of the invention while the wood is freshly sawn. In fact, the treatment of the invention is preferably carried out immediately after the unseasoned wood is sawn, i.e., as soon as is reasonably practical after the wood is sawn in a conventional lumbering operation, and certainly it is preferable to carry out the treatment of the invention within one week after the unseasoned lumber is sawn. In the preferred operation, the sawn lumber is treated in the conventional continuous lumbering operation by a process step immediately after the sawing step, and before the freshly sawn lumber undergoes any storage.

Chelation is an essentially chemical process which involves the formation of a complex usually heterocyclic ringcompound containing a metal cation. A chelating,

agentis a compound capable of forming such complex; and'a chelating agent for'iron is a compound capable of forming such complex'with iron as the metal cation. The molecule, ion or group bound to the metal cation is known as'aligand. In the practice of the instant invention, the chelating agents preferably have two or more such points of attachment, and'such a chelating agent is referred to as a polydentate ligand. Two general types of ligand groups give rise to coordinate bondsbetween ligand and metal; and these are primary acid groups in which the metal ion can replace an acid hydrogen and neutral groups which contain an atom with a free electron pair suitable for bond formation. The chelating agent used in the practice of the instant invention preferably has one or more of such primary acid groups, which may or may not be temporarily neutralized by an easily replaceable ion such as an alkali metal ion, depending on such variable characte'ristics of the aqueous system as the pH. A hexadentate chelating agent found to be particularly useful in the practice of the instant invention is ethylene diaminetetraacetic acid (commercially available as Versene Acid), and its ammonium or alkali metal salts, which include the disodium salt, the trisodium salt (sold as Versene 9), the tetrasodium salt (sold as Versene Powder, Versene 67, Versene 100), and mixtures thereof. Another particularly suitable chelating agent for use in the practice of the instant invention is N,N-di-(2-hydroxyethyl)glycine, and its ammonium and alkali metal salts, such as the sodium salt (sold as Versene Fe-3 Specific"); and mixtures of such sodium salt with the salts of ethylenediaminetetraacetic acid (combining Versene 67 and Versene Fe-3 Specific, sold as Versene Fe-3 Liquid and Versene Fe-3 Powder). Another efiicient chelating agent is N-hydroxyethylethylenediaminetriacetic acid and its salts such as the trisodium salt (sold as Versenol beads and powder). A characteristic radical for each of these chelating agents is as follows: (i.e., containing a preferred carboxylic acid group) As the foregoing formula shows, the N and thetwo Os of the OH groups provide the minimum of three ligands or ions capable of forming coordinate bonds with iron or other polyvalent metal ions in accordance with the phenomenon of chelation.

Other chelating'agentswhich may be used in the practice of the instant invention include'the low molecular Weight polycarboxylic acids, hydroxy carboxylic acids, and beta ketones,- such as citric acid, oxalic acid, salicylic acid, succinic acid, malic acid, maleic acid, acetyl acetone, diglycolic acid, etc. and the ammonium and/or alkali metal salts of the aforementioned acids. In general, such carboxylic acids contain at least one carboxylic acid group plus at least one OH-group (which may be in another carboxylic acid group or in an alcohol group), and such carboxylic acids have the characteristic of being-watersoluble per se and/or water-soluble in the form of their ammonium or alkali metal salts, at least in comparatively dilute concentrations, such carboxylic acids containing 4 to 7 carbon atoms. Gluconic acid may also be used as suchor as an acid phosphate.

As previously indicated, the instant chelating'agents may be used in'the method of the instant invention in a relatively concentrated aqueous system or solution containing an amount ranging from a minimum effective amount of about 1 part to a maximum practical amount of about 60 parts per 1500 parts of water. Also, the instant chelating agent may be used in combination with any of a number of nitrogen-containing enzyme inhibitors and/ or any of a number of fungicides.

The nitrogen-containing inhibitors which may be used in the practice of the instant invention include the known water-soluble azides, such as sodium azide or other alkali metal azides, hydroxylamine (or its salts such as hydroxylamine hydrochloride, etc., which eifectively release the hydroxylamine or ammonium group in the aqueous system), thiourea, aminotriazines such as melamine, alkylene polyamines such as ethylenediamine, and characteristic enzyme inhibitors containing the NHNH- group characteristic of hydrazine. These last mentioned compounds include the hydrazine, its salts such as hydrazine hydrochloride, the carbazides and semi-carbazides (usually available'commercially as the hydrochloride or hydrogen sulfate salts). Essentially the enzyme inhibitors contemplated in the practice of the instant invention are water-soluble nitrogen-containing compounds, which contain in most instances a plurality of nitrogen atoms, usually in the form of --NH and/or NH groups. Such compounds are used in varying concentrations, which may range from an effective minimum amount of about 1 part to an efifective maximum amount of about 60 parts per 1500 parts of water.

Still other compounds which are used in the practice of the instant invention include the conventional fungicides employed in the treatment of wood, which include such materials as the polychlorophenols (and phenates) and the organic mercurials. The organic mercurials which may be used in the practice of the instant invention include C C alkyl mercury phosphates, such as ethyl mercury phosphate, phenyl mercury phosphate, methoxy ethyl mercury acetate, methoxy ethyl mercury phosphate, methoxy ethyl mercury acylate and phenyl mercury acylate, wherein the acyl group in each case is derived from a carboxylic acid whose molecule contains from 2 to 7 carbon atoms and the compound has at least one hydroxy group, e.g., glycolic, lactic, hydroxybutyric, hydroxyvaleric, hydroxycaproic, hydroxyheptanoic, salicylic, or 5-hydroxy salicylic acids. Another heavy metal organic compound which may be used in the practice of the instant invention in substantially the same proportions that the organic mercurials are used is a tri C -C alkyl tin compound such as tributyl tin.

The polychlorophenols which are used in the practice of the instant invention are polychlorophenols having wood preservative function, i.e., effective fungicides for use with Wood, and these phenols are generally considered to be monocyclic compounds having 3 to 5 chloro groups such as symmetrical (2,4,6) trichlorophenol, unsymmetrical (2,4,5) trichlorophenol, 2,4,5,6-tetrachlorophenol, 2,3,4,6-tetrachlorophenol, pentachlorophenol and/ or the corresponding salts thereof such as the alkali metal salts, ammonium salts, C -C alkylol ammonium salts (which include the monoethanol ammonium, diethanol ammonium and triethanol ammonium salts) and the like. The pentachlorophenol (technical grade) is the most commonly used commercially in the treatment of Wood, and pentachlorophenol and/ or its salts are preferred for use in the practice of the instant invention. It will be appreciated that pentachlorophenol per se is not very soluble in most aqueous systems, but in an alkaline aqueous system, wherein the pentachlorophenol is understood to be converted to its corresponding alkaline salt, the resulting pentachlorophenate is relatively soluble. In instances involving the use of the polychlorophenols or phenates, the concentrations used may range from a practical minimum of about 0.1 to a practical maximum of about 20 parts per 1500 parts of water, although the preferred range is about 2 to 20 parts.

In the case of the organic mercurials, however, the preferred range of fungicide is lower, being about 0.1

- to about 2 parts per 1500 parts of water.

Since woods generally have a tendency to release acid components to an aqueous system in contact therewith, there are particular advantages in the use of alkaline buffers in the case of aqueous systems used in the practice of the instant invention which are adjusted to and maintained at alkaline pHs. For example, such alkaline buffers as the alkali metal borates, perborates, carbonates, and phosphates may be used to practical advantage in the instant invention for purposes of adjusting the pH to and maintaining the pH at an alkaline range of perhaps 9 to 10.5. Such alkaline buffers are preferably used in rather substantial quantities, although the amounts used may range from about 1 part to a practical maximum of about 50 parts per 1500 parts of water.

DEMONSTRATION A The instant invention may be demonstrated using a composition having the following formulation:

20 pounds of disodium ethylenediaminetetraacetic acid (range: 1 to 30 pounds).

20 pounds thiourea (range: 1 to 30 pounds).

0.3 pound of ethyl mercury phosphate (range: 0.015 to 1 pound).

The foregoing is thoroughly dispersed in an aqueous system wherein the pH is adjusted to 4:5-6. The resulting acidic composition is found to be particularly effective in the suppression of stains when applied to freshly sawn unseasoned wood by conventional dipping or spraying processes. The wood used in the particular demonstration is redwood.

The invention may be demonstrated by replacing the thiourea in the aforementioned formulation and procedure with an equivalent amount of hydroxylamine hydrochloride and by replacing the ethyl mercury phosphate with a corresponding amount of phenyl mercury phosphate. The suppression of stains in the manner described is obtained.

Suppression of stains in redwood may also be obtained using the formulation and procedure described in the first paragraph of Demonstration A, except that the thiourea is replaced with a corresponding amount of semi-carbazide hydrochloride (or with a corresponding amount of semi-carbazide sulfate).

DEMONSTRATION B The suppression of staining in redwood may also be demonstrated by the use of conventional dipping or spraying treatment of freshly sawn unseasoned wood samples (which are treated within a matter of a few hours after the sawing operation, which is the treatment time for Demonstration A also), using an aqueous system that is adjusted to an alkaline pH of 9 to 10.5. Such alkaline system contains 20 pounds of tetrasodium ethylenediaminetetraacetic acid (or acetate), again the range is 1 to 30 pounds, and 5 pounds of sodium azide (range: 0.5 to 10 pounds), plus 10 pounds of 10 mol borax (range: 1 to 20 pounds) and 5 pounds of technical sodium pentachlowater.

DEMONSTRATION C A number of compositions embodying the invention plus certain control compositions which do not embody formulations of the invention are tested in a number of test runs (indicated alphabetically in the first column of the following Table I) of concentrations per 100 gallons of water of chemicals used in each of these tests is'set forth in the second column of Table I below, the appearance of the aqueous test solution is described in the third column of Table I below, and the results are described hereinafter. The stock used for the test is freshly cut high extractive heart sinker redwood pieces, which were dipped 6 in the various test solutions within one day after being cut (or sawn).

TABLE I Test Lb./l00 gal. Appearance, Solution A Water Control Blue purple. B 30 lb. Hydroxylamine Clear-pmklsh. O 24 l'lghimreal Purple. D 15 y roxy amin {12 lb. Thiourea... Clear p1 nklSh E 1 lb. NazEDTA 1 Clear pink. F 5 lb. NagEDTA Do. G 10 lb. NaaEDTA 1 Do. H 20 lb. NaaEDTA DO. I {10 lb. NazEDTA D0 1 12 it fr iifi irr 10 a; t

15 lb. Hydroxylamine Clear 10 lb. NatED'lA K 12 lb. Thiourea Purple 10 lb. Bazide i f M 25 lb. Sodium crtrat Clear pinkish. g7 2g g a gip tiliphosphate 11211115111. 2 a urp e. P 20 lb. NajEDTA 1 Purple, some change {10 lb. Bazide 3 zvhgn Bazide added 1 Ethylenediaminetetraacetic acid. 2 Technical sodium pentachlorophenate. 3 1.25 lbs. NaN;;, 5 lbs. NazCOs and 3.75 lbs. NazB4O .Ha0.

A noticeable inhibition of sticker stain is found in the case of each of the tests wherein a chelating agent is used. As the concentration of chelating agent is increased in Test E through H, alphabetically, the sticker stain inhibition is increased, with the best results being obtained using the concentration shown in Test H. In Tests I and J the sticker stain inhibition is greater than that obtained in Test G, although the same concentration of chelating agent was used in each of the tests, and this is believed to be because of synergism between the chelating agent and the nitrogen containing enzyme inhibitor used in Tests I and I (so as to obtain sticker stain inhibition comparable to that obtained using the higher concentration of chelating agent, as in Test H).

In Test K, the sticker stain inhibition is at least as good as that of Test I and Test J; and the same is true of Test L.

In Test M the chelating agent used is less effective than the chelating agent of Tests G and H, even though the concentration of chelating agent is greater in Test M. The sticker stain inhibition obtained in Test M is, however, superior to that obtained in Test N.

The results obtained in Test 0 are comparable to those obtained in Test H, and the results obtained in Test P are better than those obtained in Test 0.

DEMONSTRATION D In another series of tests board specimens 1" x 6" X 4' of high extractive generally heavy or high moisture content A and B grade redwood were selected, and the specimens were tested by dipping one half their length in the test solutions A through H specified in Table II below:

TABLE II A.-5 lb./ gal. (NH OH) 2H SO Tech. B.10lb./100 gal. (NH OH) 2H SO Tech. C.--l5 lb./l00 gal. (NH OH) 2H SO Tech. D.3 lb./100 gal. thiourea technical E.--6 lb./100 gal. thiourea technical F.12 lb./100 gal. thiourea technical G.25 lb./100 gal. Na EDTA H.H O Control The test results obtained after storage for at least nine months, in a conventional air seasoning procedure, established clearly that the best results in sticker stain inhibition are obtained using the procedure of Test G, which involves the use of a Versene chelating agent for iron.

It will be understood thatmodifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.

7 DEMONSTRATION E The suppression of staining in redwood may also be demonstrated by the use of conventional dipping or spray ing treatment of freshly sawn unseasoned wood samples which are treated within a few hours after the sawing operation, using the various aqueous systems A through F, alphabetically, referred to in Table III below:

TABLE III [Amounts in lbs. per 100 gal. H1O] Test Range A B C D E F (1) NazEDTA 1-30 10 5 (2) NarEDIA-.- 1-30 10 5 10 5 (3) Sodium Azide 0. 5-10 5 5 5 5 (4) Sodium Pentachlorophenol 1-10 1 1 2 1 (5) Wax emulsion -100 40 40 30 (6) Glycol 10-100 40 2O 30 (7) Urea 10-100 4O 2O 1 A substantially stable Wax-in-water emulsion formed of 50% microcrystalline wax (range to 75%), 5% triethanolamine (range 0.5 to 10% alkylolaminc) and remainder substantially water.

2 Propylene glycol, but all or part may be replaced by HO-(RO) u-H wherein n is an integer from 1 to 3 and R0 is a C2C3 alkoxy group, as in ethylene, diethylene, triethylene, and dipropylene glycols.

ferred results, but they may be replaced by substantially equal amounts of any of the other chelating agents hereinbefore described. In addition, the sodium azide of Item 3 may be replaced by any of the other nitrogen-containing enzyme inhibitors hereinbefore described and/ or used in other demonstrations herein. Also, it will be appreciated that the wax emulsion (Item 5) may be a commercially available wax emulsion sold under the trade name Millbrite, which may include emulsion stabilizers, nonionics, etc. Also, the pentachlorophenol component may be incorporated initially as such in the wax emulsion. The wax emulsion, the glycols and urea are seasoning aids used in the art and the essential novelty herein resides in the use of these materials in combination with the specified chelating agents and preferably also with a nitrogen-containing enzyme inhibitor such as sodium azide.

It will also be appreciated that the polychlorophenol component may be replaced in Test E above, and optionally 5 pounds of thiourea, hydroxylamine hydrochloride and/or semi-carbazide hydrochloride may be included in the practice of the invention.

Another aspect of the instant invention involves the incorporation of effective amounts of alkaline buffers of conventional types which include the known borates (borax, etc.), the known carbonates (usually sodium carbonate) and the known phosphates (which include trisodium phosphate, sodium polyphosphate, sodium pyrophosphate, the sugar phosphates such as sodium salts of gluconic acid phosphate, etc.). In the foregoing Tests A through F, alphabetically, such buffers may be used individually or in combinations in amounts ranging from about 1 to 50 pounds, and specifically an improved buffing effect is obtained in the foregoing Tests B and C by adding to the compositions 5 pounds of borax, sodium pyrophosphate, sodium polyphosphate, or the like.

We claim:

1. A composition for the treatment of wood comprising 1 to parts by weight of ethylenediaminetetraacetic acid, 1 to 30 parts by weight of thiourea enzyme inhibitor, and 0.01 to 1 part by weight of organic mecurial fungicide, in 700 to 800' parts by weight of water adjusted to an acid pH.

2. A composition for the treatment of wood comprising 1 to 30 parts by weight of ethylenediaminetetraacetic acid, 1 to 30 parts by weight of hydroxylamine enzyme inhibitor, and 0.01 to 1 part by weight of organic mercurial fungicide, in 700 to 800 parts by weight of water adjusted to an acid pH.

3. A composition for the treatment of woodcomprising 1 to 30 parts by weight of ethylenediaminetetraacetic acid, 1 to 30 parts by weight of semi-carbazide enzyme inhibitor, and 0.01 to 1 part by weight of organic mercurial fungicide, in 700 to 800 parts by weight of water adjusted to an acid pH.

4. A composition for the treatment of wood comprising 1 to 30 parts by weight of ethylenedi-aminetetraacetic acid, 0.5 to 10 parts by weight of azideenzyme inhibitor, and 1 to 10 parts by weight of polychlorophenol fungicide, in 700 to 800 parts by weight of water ad justed to an alkaline pH.

5. A composition for the treatment of wood comprising 1 to 30 parts by weight of ethylenediaminetetraacetic' acid, and 0.5 to 10 parts by weight of sodium azide.

6. A composition for the treatment of wood comprising 1 to 30 parts by weight of ethylenediaminetetraacetic acid, and 1 to 10 parts by weight of sodium pentachlorophenate.

7. A method of treating unseasoned lumber, which comprises sawing the lumber into pieces of desired size and, immediately after sawing the lumber, bringing the sawn pieces into contact with a composition consisting essentially of 1 to 30 parts by weight of ethylenediamine tetraacetic acid, 1 to 30 parts by weight of thiourea enzyme inhibitor, and 0.01 to 1 part by weight of organic mercurial fungicide, in 700' to 800 parts by weight of water adjusted to an acid pH.

8. A method of treating unseasoned lumber, which comprises sawing the lumber into pieces of desired size and, immediately after sawing the lumber, bringing the sawn pieces into contact with a composition consisting essentially of 1 to 30 parts by weight of ethylenediaminetetraacetic acid, 0.5 to 10 parts by weight of azide' enzyme inhibitor, and 1 to 10 parts by weight of polychlorophenol fungicide, in 700 to 800 parts by weight of water adjusted to an alkaline pH.

9. A method of treating unseasoned lumber, which comprises sawing the lumber into pieces of desired size and, immediately after sawing the lumber, bringing the sawn pieces into contact with a composition consisting essentially of 1 to 30 parts by weight of ethylenediaminetetraacetic acid, 0.5 to 10 parts by weight of azide enzyme inhibitor, and 1 to 10 parts by weightof polychlorophenol fungicide, in 700 to 800 parts by weight of water adjusted to an alkaline pH of 9 to 10.5'by an alkaline buffer selected from the group consisting of borates, carbonates, phosphates and mixtures thereof.

References Cited UNITED STATES PATENTS 181,651 8/1876 Dixon 117-59 764,913 7/1904 Chapman 167-387 2,331,268 10/1943 F'lenner et al. 117-.-147 2,549,358 4/1951 Bacon et al. 16738.7 2,735,742 2/1956 Godehn 8--137.5 2,922,728 1/1960 White 8'137.5 3,070,494 12/1962 Goldstein et al '117147 MURRAY KATZ, Primary Examiner.

WILLIAM D. MARTIN, Examiner.

Claims

7. A METHOD OF TREATING UNSEASONED LUMBER, WHICH COMPRISES SAWING THE LUMBER INTO PIECES OF DESIRED SIZE AND, IMMEDIATELY AFTER SAWING THE LUMBER, BRINGING THE SAWN PIECES INTO CONTACT WITH A COMPOSITION CONSISTING ESSENTIALLY OF 1 TO 30 PARTS BY WEIGHT OF ETHYLENEDIAMINETETRAACETIC ACID, 1 TO 30 PARTS BY WEIGHT OF THIOUREA ENZYME INHIBITOR, AND 0.01 TO 1 PART BY WEIGHT OF ORGANIC MERCURIAL FUNGICIDE, IN 700 TO 800 PARTS BY WEIGHT OF WATER ADJUSTED TO AN ACID PH.