Prevention of Stain and Mould in Lumber and Board Products

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2 Prevention of Stain and M o u l d in Lumber and Board

Wood Technology: Chemical Aspects Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 09/10/18. For personal use only.

Products A . J. C S E R J E S I Western Forest Products Laboratory, Vancouver, British Columbia, V 6 T 1X2, Canada

Wood, like a n y o t h e r natural organic material is perishable, and may s e r v e a s a nutrient source for different microorganisms. Some wood-inhabiting m i c r o o r g a n i s m s (decay f u n g i ) can utilize as nutrients t h o s e wood c o m p o n e n t s w h i c h f o r m t h e structural elem e n t s o f wood (cellulose, 1ignin), while others (sapstain fungi and m o u l d s ) consume t h o s e wood c o m p o n e n t s w h i c h r e p r e s e n t t h e cell contents of the tree (starch, other carbohydrates, etc.) and thus no strength loss o f wood o c c u r s . To p r e v e n t g r o w t h o f wood-inhabiting f u n g i it is c u s t o m a r y to treat wood w i t h c h e m i c a l s w h i c h a r e toxic t o fungi. To protect wood in service f r o m deterioration by decay fungi, the chemicals, called wood preservatives, a r e applied in m o s t c a s e s by p r e s s u r e t r e a t m e n t b e c a u s e deep penetration into t h e wood i s essential. Wood p r o d u c t s i n t e n d e d f o r u s e where t h e d e c a y h a z a r d is l o w a r e unlikely to require p r e s s u r e treatment but i n many instances may require superficial fungicidal treatment to prevent fungal discoloration. H i s t o r y of

S a p s t a i n and Mould P r e v e n t i o n

W o o d - i n h a b i t i n g f u n g i n e e d w a t e r , a i r , n u t r i e n t s and a s u i t a b l e temperature for growth. Sapwood i n f r e s h l y c u t l u m b e r has t h e p r o p e r b a l a n c e o f a i r and w a t e r , c o n t a i n s n u t r i e n t s , and t h e r e f o r e i n warm w e a t h e r i t i s r e a d i l y a t t a c k e d b y f u n g i . Abs e n c e o f a n y one o f t h e s e f o u r r e q u i r e m e n t s s t o p s t h e g r o w t h o f t h e s e f u n g i , however t h e f u n g i a r e n o t n e c e s s a r i l y k i l l e d . Among t h e s e f o u r r e q u i r e m e n t s t h e m o i s t u r e c o n t e n t ( M . C . ) o f t h e wood i s t h e e a s i e s t t o c o n t r o l , a n d i f i t i s b e l o w a b o u t 20% f u n g a l growth i s stopped. I n t h e e a r l y d a y s a i r - d r y i n g was t h e m e t h o d u s e d t o r e d u c e t h e M . C . o f wood. H o w e v e r , a i r - d r y i n g i n humid and r a i n y w e a t h e r was r e l a t i v e l y s l o w , o f t e n t a k i n g a s l o n g a s two o r t h r e e m o n t h s t o d r y lumber t o l e s s t h a n 20% M . C . During t h i s period fungi w e r e a b l e t o grow o n ( m o u l d s ) and i n ( s t a i n i n g f u n g i ) t h e l u m b e r . T h i s p e r i o d however, i s too short f o r decay f u n g i to cause an

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a p p r e c i a b l e amount of decay. P l a n i n g a i r - d r i e d lumber l a r g e l y removes surface d i s c o l o r a t i o n caused by moulds, but d i s c o l o r a t i o n i n the wood caused by s t a i n i n g f u n g i cannot be removed. K i l n - d r y i n g does not depend on the weather and, i n a d d i t i o n , due t o the h i g h temperature used, the wood i s e s s e n t i a l l y s t e r i l i z e d . However, i f k i l n d r y i n g i s delayed mould and s a p s t a i n f u n g i may d i s c o l o r the wood. For the p r e v e n t i o n of s a p s t a i n development d u r i n g a i r - d r y i n g , sawmills s t a r t e d to use f u n g i c i d e s as e a r l y as the end of the l a s t century. The f i r s t attempt t o prevent s a p s t a i n i n lumber was i n 1888, according to Bryant ( 4 ) . At the beginning of t h i s century, the use of sodium carbonate and bicarbonate became widespread i n s a w m i l l s , and remained the s a p s t a i n and mould preventi v e u n t i l about 1930 ( 4 ) . The s t u d i e s c a r r i e d out by the Southern Forest Experiment S t a t i o n and the Forest Products Laboratory i n the U.S.A. from 1928 to e a r l y 1930's (31) r e s u l t e d i n a change over from the use of sodium carbonate to the use of the sodium s a l t s of c h l o r i n a t e d phenols and organo-mercurie compounds. Since s a p s t a i n and mould p r e v e n t i v e s based on the t o x i c i t y of i n d i v i d u a l compounds were r e p o r t e d t o f a i l o c c a s i o n a l l y , the use of mixtures of m e r c u r i a l s and the sodium s a l t s of c h l o r i n a t e d phenols was suggested (39,40). Since then the sodium s a l t s of c h l o r i n a t e d phenols, sodium pentachlorophenate and sodium t e t r a c h l o r o p h e n a t e , buffered i n most formulations w i t h borax, w i t h or without organo-mercurials, has dominated the s a p s t a i n and mould p r e v e n t i v e market. In B r i t i s h Columbia the lumber i n d u s t r y v o l u n t a r i l y stopped u s i n g f o r m u l a t i o n s c o n t a i n i n g mercury i n the l a t e I 9 6 0 s . However s a p s t a i n and mould p r e v e n t i v e s c o n t a i n i n g m e r c u r i a l s are s t i l l i n use i n the U.S.A. The o r i g i n a l requirement f o r a s a p s t a i n and mould p r e v e n t i v e treatment was to prevent f u n g a l growth on lumber f o r the p e r i o d of a i r - d r y i n g , or f o r the storage p e r i o d between sawing and k i l n d r y i n g . Thus the chemicals were r e q u i r e d t o be e f f e c t i v e f o r about a two month p e r i o d . T r a d i t i o n a l l y lumber from the P a c i f i c Northwest i s shipped unseasoned, by ocean t r a n s p o r t . Shipping charges are based on volume not on weight, and n e i t h e r a i r nor k i l n - d r y i n g would be j u s t i f i e d economically. A f t e r the Second World War, i n order to reduce handling c o s t s , packaging of lumber was introduced. Although i t was recommended that the importer break up the packages on a r r i v a l and p i l e the lumber f o r d r y i n g , i n p r a c t i c e i t became customary t o break up the packages only when the lumber was a c t u a l l y used. This p e r i o d i s g e n e r a l l y l e s s than one year, but i s sometimes as much as two years (26). During storage of packaged green lumber, c o n d i t i o n s remain favourable f o r fungal growth and s i n c e the p i e c e s are i n c l o s e contact i n the packages, f u n g i , i n c l u d i n g decay f u n g i , may spread from one p i e c e to another i f f u n g i c i d a l treatment was inadequate. 1

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T h i s extended time requirement f o r e f f e c t i v e c o n t r o l of f u n g a l growth by s a p s t a i n and mould p r e v e n t i v e s put more s t r i n g e n t requirements on the f u n g i c i d e s as w e l l as on the a p p l i c a t i o n methods. C h l o r i n a t e d phenols a r e e f f e c t i v e f u n g i c i d e s f o r t h i s purpose, as proven by use (17,26), however they may cause s k i n i r r i t a t i o n ( 4 ) , and an i n c r e a s e of the c o n c e n t r a t i o n i n the t r e a t i n g s o l u t i o n without the proper p r e c a u t i o n s may cause other problems i n h a n d l i n g f r e s h l y - t r e a t e d lumber (17,35). Searching f o r s a f e r and more e f f e c t i v e f u n g i c i d e s t o r e p l a c e c h l o r i n a t e d phenols f o r t h e p r e v e n t i o n o f s a p s t a i n and mould, t h e r e f o r e i s Important f o r both the chemical companies manufacturing s a p s t a i n and mould p r e v e n t i v e s , as w e l l as f o r the lumber i n d u s t r y u s i n g them. An e x t e n s i v e study i n Germany i n the e a r l y I 9 6 0 s (28,29) was c a r r i e d out by t e s t i n g about 300 chemicals f o r t h e i r t o x i c i t y to f u n g i and s t u d y i n g t h e i r p h y s i c a l and chemical p r o p e r t i e s . None of the t e s t e d chemicals were found t o be more e f f e c t i v e than c h l o r i n a t e d phenols. In New Zealand, Butcher found t h a t c a p t a f o l i s a s u i t a b l e replacement f o r c h l o r i n a t e d phenols (5_>6>7) and i t has been w i d e l y accepted f o r use i n t h a t country. In North America, Chapman Chemical Co. i s promoting d i f f e r e n t metal complexes of 8-hydroxyquinoline ( 9 ) , but the acceptance o f these f o r m u l a t i o n s i s slow. The Eastern Forest Products Laboratory, Ottawa, found t h a t ammoniacal z i n c oxide i s an i n h i b i t o r t o f u n g i i n pine lumber (16,33). Sanford Products Corp. i s o f f e r i n g 2^hiocyanomethy3(thio) b e n z o t h i a z o l e f o r p r e v e n t i o n of s a p s t a i n and mould i n Canada and U.S.A., and t r i b u t y l t i n o x i d e emulsion i s marketed i n Japan. Experiments were c a r r i e d out i n Europe t o use benomyl, d i ( g u a n i d i n o - o c t y l ) amine and p-chlorophenyl-3-iodo-propargyl formal t o be used i n s a p s t a i n and mould p r e v e n t i v e f o r m u l a t i o n s . L a b o r a t o r i e s , i n c l u d i n g the Western Forest Products Laboratory, are a l s o t e s t i n g new chemicals. However s a p s t a i n and mould p r e v e n t i v e f o r m u l a t i o n s c o n t a i n i n g c h l o r i n a t e d phenols as t h e main f u n g i c i d a l components, s t i l l dominate the market. 1

Sapstain and Mould P r e v e n t i v e Treatment i n P r a c t i c e An e f f e c t i v e s a p s t a i n and mould p r e v e n t i v e , does not n e c e s s a r i l y make a s u c c e s s f u l treatment. For a s u c c e s s f u l treatment the f o l l o w i n g requirements a r e a l s o necessary: 1. Lumber f r e e from s a p s t a i n , mould and a c t i v e decay; 2. Not more than 24 hours delay of the treatment a f t e r sawing ; 3. The s u r f a c e of lumber should be u n i f o r m l y covered w i t h the t r e a t i n g s o l u t i o n ( a p p l i c a t i o n method); 4. Proper c o n c e n t r a t i o n of the f u n g i c i d e i n the t r e a t i n g solution;

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5.

At l e a s t a temporary p r o t e c t i o n of lumber from the weather a f t e r treatment. P o i n t s 1 and 4 need no f u r t h e r comment. The requirement of 2 i s a l s o obvious, c o n s i d e r i n g that t h i s treatment i s r e s t r i c t e d o n l y t o a t h i n surface l a y e r of the wood and a delay of the t r e a t ment a l l o w s s t a i n i n g f u n g i to penetrate beyond the t r e a t e d l a y e r , where t h e i r growth i s u n i n h i b i t e d . The t h i r d requirement r e f e r s t o the a p p l i c a t i o n method which i s used to apply the s a p s t a i n and mould p r e v e n t i v e s . According to the author's experience, the use of a good a p p l i c a t i o n t e c h nique i s as important as, or perhaps more important than, the e f f e c t i v e n e s s of the f u n g i c i d e i n the f o r m u l a t i o n . The type of a p p l i c a t i o n methods used a r e : 1

i ) Dipping* (a) hand d i p p i n g ; (b) d i p p i n g on the s o r t i n g c h a i n ; and (c) b u l k d i p p i n g (of packaged lumber). (2) Spraying. The author's preference f o r s a p s t a i n and mould p r e v e n t i v e treatment i s d i p p i n g . Experience, i n a n a l y z i n g commercially t r e a t e d lumber, i n d i c a t e s that the q u a l i t y of treatment by d i p p i n g i s s u p e r i o r t o spraying (12). Hand d i p p i n g i s used today only f o r experimental purposes. Dipping on s o r t i n g c h a i n i s one of the e a r l i e s t automated a p p l i c a t i o n methods (31). The advantage of t h i s l a t t e r i s , that i t r e q u i r e s a r e l a t i v e l y s m a l l volume of t r e a t i n g s o l u t i o n , which i s easy to handle. Although i t i s a good t r e a t i n g method, because of some problems i n handling wet lumber on the s o r t i n g c h a i n , i t had not been used i n B.C. f o r many years. Recently however i t was reintroduced and w i t h the development of automatic s o r t i n g equipment, t h i s t r e a t i n g method probably w i l l become more popular i n the f u t u r e . Bulk d i p p i n g of packaged lumber was introduced i n B.C. i n the I960's, p r i m a r i l y f o r the treatment of rough sawn lumber. A n a l y z i n g commercially and e x p e r i m e n t a l l y t r e a t e d lumber f o r c h l o r i n a t e d phenols a f t e r bulk d i p p i n g showed that t h i s t r e a t ment r e s u l t e d i n r e t e n t i o n comparable to t h a t of hand-dipped rough-sawn lumber. I t was a l s o observed that w i t h surfaced lumber, although each p iece i n the package r e c e i v e d some treatment, the c o n c e n t r a t i o n s of the f u n g i c i d e were much lower than i n hand dipped lumber, p o s s i b l y due to the r e s t r i c t e d space between p i e c e s (25). Bulk d i p p i n g i s done e i t h e r by d r i v i n g a c a r r i e r w i t h the lumber i n t o the " d i p tank", or d i p p i n g on a p l a t f o r m . The l a t t e r method u s u a l l y u t i l i z i n g automatic equipment, which lowers the lumber placed on the p l a t f o r m i n t o the s o l u t i o n , keeps i t down f o r a short time, and b r i n g s i t up. In some s m a l l s a w m i l l s , the package of lumber i s held under the f o r k s of a f o r k l i f t , which push them under the s o l u t i o n i n an above ground " d i p tank". A major advantage of b u l k d i p p i n g i s that the lumber i s not handled manually a f t e r treatment. A disadvantage i s the l a r g e

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volume of t r e a t i n g s o l u t i o n needed to f i l l the " d i p tank". Spraying i s used mainly f o r the treatment of surfaced lumber and thus the equipment i s u s u a l l y i n s t a l l e d behind a high speed planer. The main problem w i t h t h i s method i s that the n o z z l e s of the spray system o f t e n become t e m p o r a r i l y plugged. Even a very short stoppage of spray, c o n s i d e r i n g the h i g h speed of the lumber (180-350 m/min), may r e s u l t i n l a r g e surface areas of the p i e c e s remaining untreated. These untreated areas are r e s p o n s i ­ b l e f o r f a i l u r e i n the p r o t e c t i o n of lumber t r e a t e d by t h i s method. Retention of Sapstain and Mould P r e v e n t i v e s on Lumber According to V e r r e l l (39) the e f f e c t i v e n e s s of the s a p s t a i n and mould p r e v e n t i v e treatment i s c o r r e l a t e d t o the r e t e n t i o n of f u n g i c i d e s on the lumber. T h i s r e t e n t i o n i s a f f e c t e d by some p h y s i c a l and p o s s i b l y chemical p r o p e r t i e s of wood i n a d d i t i o n to the a p p l i c a t i o n of f u n g i c i d e , as w e l l as by the method of i t s application. In l a b o r a t o r y experiments by the author i t was found that the moisture content of the wood has l i t t l e e f f e c t on the r e t e n ­ t i o n of the t r e a t i n g s o l u t i o n on the lumber f o l l o w i n g a 15 second d i p (14). In another l a b o r a t o r y study (11) i t was found that although a l t e r n a t e w e t t i n g and d r y i n g caused complete l o s s of pentachlorophenol from wood, n e i t h e r l e a c h i n g nor d r y i n g alone caused s i g n i f i c a n t l o s s of pentachlorophenol a f t e r a 1 to 2 day f i x a t i o n p e r i o d i n the wet c o n d i t i o n . This suggests that lumber needs o n l y temporary p r o t e c t i o n from weather, although complete l o s s may occur from the surfaces of lumber i f exposed to weather due to repeated w e t t i n g and d r y i n g (12). Using the r e s u l t s of a l a r g e s c a l e f i e l d t e s t (26), a number of c o n c l u s i o n s about s a p s t a i n and mould p r e v e n t i v e treatments can be made. In t h i s experiment more than 10,000 p i e c e s of 2" χ 4" and 8 f t long lumber were t r e a t e d by hand d i p p i n g i n t o two s a p s t a i n and mould p r e v e n t i v e s , each used at four c o n c e n t r a t i o n s . Both contained sodium t et rac hlorophenat e as the main f u n g i c i d e . A l l p i e c e s were inspected f o r growth of s a p s t a i n and mould a f t e r d i f f e r e n t storage p e r i o d s , and more than 3000 p i e c e s were ana­ l y z e d f o r t e t r a c h l o r o p h e n o l . The c o n c e n t r a t i o n s of t e t r a c h l o r o phenol were ealeulated2on weight per surface area b a s i s (13) and were reported as mg/cm . Considering the l a r g e number of observations the f o l l o w i n g c o n c l u s i o n s were w e l l documented : 1. The same p r o t e c t i o n was observed against s a p s t a i n and mould f u n g i w i t h the same r e t e n t i o n (mg/cm ) r e g a r d l e s s of whether the lumber was rough sawn or surfaced (15). 2. Rough-sawn lumber r e t a i n e d an average of 2.5 to 3 times as much t e t r a c h l o r o p h e n o l as surfaced lumber d i d , when they were t r e a t e d i n the same s o l u t i o n . T h i s means t h a t surfaced lumber should be t r e a t e d i n a s o l u t i o n which i s

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three times more concentrated than t h a t used f o r roughsawn lumber f o r e q u i v a l e n t treatment (15). 3. Large v a r i a t i o n s i n the r e t e n t i o n s (the maximum being 10 χ higher than the minimum) were found i n the lumber t r e a t e d i n the same s o l u t i o n (26). T h i s v a r i a t i o n was about the same on both, surfaced and rough-sawn lumber (hand-dipped I). 4. On the premise that the p r o t e c t i o n depended o n l y on the r e t e n t i o n o f t e t r a c h l o r o p h e n o l , we c a l c u l a t e d t h a t f o r c l o s e 2 t o 100% p r o t e c t i o n , a minimum r e t e n t i o n o f 0.05 mg/cm i s necessary f o r 2 a 2 year p e r i o d and a minimum r e t e n t i o n of 0.04 mg/cm t e t r a c h l o r o p h e n o l f o r a one year p e r i o d (26). There i s no standard f o r s a p s t a i n and mould p r e v e n t i v e treatment, but f o r export lumber most c o n t r a c t s s p e c i f y t h a t t h e lumber should be " e f f e c t i v e l y t r e a t e d " . Savory and Cockroft i n 1961 (30) came t o the c o n c l u s i o n : " i t i s doubtful whether a c e r t i f i c a t e o f treatment employing the words ' e f f e c ­ t i v e l y t r e a t e d ' has any r e a l meaning. I t i s suggested t h a t c e r t i f i c a t e s o f treatment would be more v a l u a b l e i f they s t a t e d the t r e a t i n g chemical and the c o n c e n t r a t i o n a t which i t was a p p l i e d . " T h i s suggestion i s not yet accepted. But f o l l o w i n g up t h e i r suggestion based on the r e s u l t s o f our experiment we c a l c u l a t e d t h a t 13 randomly taken samples would be r e q u i r e d t o estimate w i t h reasonable c e r t a i n t y whether the r e t e n t i o n was w i t h i n the d e s i r e d t a r g e t amount (15). Another f a c t o r which may reduce the e f f e c t i v e n e s s o f a s a p s t a i n and mould p r e v e n t i v e treatment i s the t o l e r a n c e and t h e a d a p t a t i o n o f f u n g i t o f u n g i c i d e s (10,39). However the t o l e r a n c e of f u n g i i s considered when the e f f e c t i v e c o n c e n t r a t i o n o f a f u n g i c i d e i s determined. Experimental

Methods

The experiment which made i t p o s s i b l e t o draw s e v e r a l con­ c l u s i o n s on s a p s t a i n and mould p r e v e n t i v e treatment was a l a r g e s c a l e t e s t that needed about three years t o complete. The q u a n t i t a t i v e c o n c l u s i o n s j u s t i f i e d such e x t e n s i v e exper imentat i o n . However, f o r screening new f u n g i c i d e s , such a l a r g e experiment would be too expensive and time consuming. F i e l d t e s t s u s i n g commercially produced lumber w i t h much s h o r t e r storage p e r i o d s (4 months) g i v e a very good i n d i c a t i o n o f the u s e f u l n e s s o f a f u n g i c i d e f o r s a p s t a i n and mould p r e v e n t i o n (7,25,31,40). Ex­ periments were a l s o c a r r i e d out w i t h s m a l l e r samples, up t o 3 f t long and s h o r t e r storage p e r i o d s (2-3 months). The r e s u l t s of these t e s t s a l s o gave a good i n d i c a t i o n o f the e f f e c t i v e n e s s of f u n g i c i d e s (5,31^,40). As an a l t e r n a t i v e t o these t e s t s , where untreated p i e c e s were used a s c o n t r o l s , p i e c e s h a l f dipped i n t o the t r e a t i n g s o l u t i o n were t e s t e d a g a i n s t the untreated other h a l v e s , which served as c o n t r o l s (Chapman Chem. Co.).

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For p r e l i m i n a r y t e s t s of f u n g i c i d e s , d i f f e r e n t l a b o r a t o r y methods were developed. Several methods were described u s i n g a r t i f i c i a l media t o t e s t the t o x i c i t y of chemicals t o m i c r o organisms. One of the f i r s t t e s t s of t h i s k i n d was to use malt agar mixed w i t h the chemicals (19). The r e p r o d u c i b i l i t y of these t e s t s i s much b e t t e r than those which use wood as the medium (21). However, the r e s u l t s obtained w i t h a r t i f i c i a l media r a r e l y c o r r e l a t e w i t h those which are obtained u s i n g wood, and t h e r e f o r e a r t i f i c i a l media are best used f o r s p e c i a l experiments (e.g. a d a p t a t i o n of f u n g i to f u n g i c i d e s (10). Most of the l a b o r a t o r y r e s e a r c h w i t h f u n g i c i d e s intended to be used f o r wood p r o t e c t i o n has been c a r r i e d out w i t h a wood medium. Since no standard e x i s t s to t e s t s a p s t a i n and mould p r e v e n t i v e s , almost as many methods e x i s t as experiments c a r r i e d out (3,12,13,19,20,23,24,30,38). D i s c o l o r a t i o n of Non-Fungal O r i g i n on Lumber D i s c o l o r a t i o n , other than s a p s t a i n and mould (or decay) a l s o occurs on lumber. Some of t h i s d i s c o l o r a t i o n o r i g i n a t e s from the t r e e s and i s r e s t r i c t e d mainly to the heartwood I t may be caused by chemical r e a c t i o n s which do not continue a f t e r the t r e e s are harvested (1,31,34) or i t may be caused by f u n g i i n the l i v i n g t r e e , which may or may not continue to develop a f t e r h a r v e s t i n g (31,34). The o n l y p r o t e c t i o n against d i s c o l o r a t i o n which i s caused by f u n g i t h a t continue a f t e r h a r v e s t i n g , i s k i l n d r y i n g of the lumber ( 1 ) . A non-fungal d i s c o l o r a t i o n developing i n sapwood of lumber of s e v e r a l species i s due t o the o x i d a t i o n of c e r t a i n wood ext r a c t i v e s (2,22_,31) . C o l o u r l e s s carbohydrates or phenolic wood e x t r a c t i v e s , e i t h e r present i n sound wood or produced by m i c r o organisms e.g. b a c t e r i a i n ponded l o g s (18,37), migrate to the s u r f a c e , mainly onto the c r o s s - c u t ends of lumber. On the surface these e x t r a c t i v e s are converted to t a n n i n - l i k e , coloured m a t e r i a l s by o x i d a t i o n , i n most cases c a t a l y z e d by enzymes (2,22). This d i s c o l o r a t i o n i s mostly r e s t r i c t e d to the surfaces of sapwood, and more pronounced on the ends of pines (22,36) western hemlock (2,18) and a few other wood s p e c i e s . S i m i l a r d i s c o l o r a t i o n occurs d u r i n g k i l n - d r y i n g i n pine lumber, and i s caused by a s i m i l a r process. But i n s t e a d of enzymatic o x i d a t i o n , the high temperature a c c e l e r a t e s the formation of the coloured compounds. This d i s c o l o r a t i o n i s u s u a l l y not r e s t r i c t e d to the surfaces of the lumber (39). To c o n t r o l t h i s d i s c o l o r a t i o n i n p i n e s , buffered sodium a z i d e (36,37), and more r e c e n t l y the l e s s hazardous sodium f l u o r i d e have been used ( 8 ) . Recently ammoniacal z i n c oxide (32) and sodium carbonate or bicarbonate (41) have been suggested to prevent t h i s discoloration. The brown s t a i n on western hemlock sapwood i s not c o n t r o l l e d by sodium a z i d e (18). A number of chemicals (reducing agents and a c i d s ) prevent the c o l o u r formation i n l a b o r a t o r y experiments but f

2 . csERjEsi Prevention of Stain and Mould 31 i n p r a c t i c e they o n l y delay i t . The i n e f f e c t i v e n e s s of these chemicals i n p r a c t i c e i s probably due to the l o w e r i n g of the c o n c e n t r a t i o n of the chemicals below t h e i r e f f e c t i v e concen­ t r a t i o n s by r a i n and by the continuous movement of the e x t r a c t i v e s to the surface d u r i n g d r y i n g ( 2 ) . Since t h i s d i s c o l o r a t i o n i s r e s t r i c t e d to the surface, no p r e v e n t i v e measures are taken a g a i n s t i t s formation. This type of d i s c o l o r a t i o n i s l e s s important i n B r i t i s h Columbia than the d i s c o l o r a t i o n caused by f u n g i . Surface d i s ­ c o l o r a t i o n can be r e a d i l y removed by s u r f a c i n g of lumber, and u s u a l l y obj e c t i o n i s r a i s e d o n l y when i t ' s observed on an unfam­ i l i a r wood species as was the case w i t h western hemlock. Literature 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. Sci. 15. 16. 17. 18. 19. 20. 21. 22.

Cited

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32

WOOD TECHNOLOGY: CHEMICAL ASPECTS

23. 24. 25. 26.

27. 28. 29. 30. 31. 32. 33. 34. 35.

36. 37. 38. 39. 40. Bull 41.

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