Materials Degradation Caused by Acid Rain - ACS Publications

20 Elemental Analysis of Simulated Acid Rain Stripping of Indiana Limestone, Marble, and Bronze K. M . Neal, S. H . Newnam, L. M . Pokorney, and J. P. Rybarczyk

Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: September 25, 1986 | doi: 10.1021/bk-1986-0318.ch020

Department of Chemistry, Ball State University, Muncie, IN 47306 Recent work has shown that the Midwest and specifically Indiana is experiencing significant acidic wet deposition. While the natural abundance of limestone in Indiana soil inhibits the major aquatic and forestry damage observed in the Northeastern United States, other less dramatic yet significant effects are being observed in Indiana, primarily degradation of structural materials. Samples of structurally popular Indiana limestone together with marble, micrite limestone, and bronze have been laboratory treated to the equivalent of ten years of leaching at pH=3.0, 4.0, and 5.6 rainfall. The run-off has been elementally analyzed to determine the leaching rates. Also, the effects on limestone and bronze engraving and embossing caused by similar long-term acidic exposure are also provided. These results are then correlated to actual field observations within Indiana. The S t a t e o f Indiana has been c l o s e l y l i n k e d t o t h e a c i d r a i n cont r o v e r s y f o r two primary r e a s o n s . The f i r s t a s s o c i a t i o n arose because o f I n d i a n a ' s r a n k i n g as t h e t h i r d l a r g e s t s u l f u r d i o x i d e e m i t t i n g s t a t e and t h e seventh g r e a t e s t n i t r o g e n o x i d e e m i t t e r , both substances being t h e apparent p r e c u r s o r s o f a c i d r a i n . In a d d i t i o n , I n d i a n a c o n t a i n s seven o f t h e l a r g e s t 50 s u l f u r d i o x i d e e m i t t i n g s t a t i o n a r y power p l a n t s i n t h e U.S. (U2)>These f a c t s have l e d t o I n d i a n a being l a b e l e d as a primary geographic a c i d r a i n s o u r c e . However, b e s i d e s b e i n g merely a source o f a c i d r a i n , I n d i a n a a l s o r e c e i v e s s i g n i f i c a n t a c i d p r e c i p i t a t i o n . The e f f e c t o f t h i s p r e c i p i t a t i o n on one o f Indiana's most commercial p r o d u c t s i s i t s second link t o acid rain. Indiana b u i l d i n g l i m e s t o n e has h i s t o r i c a l l y been a p o p u l a r c o n s t r u c t i o n m a t e r i a l not o n l y a c r o s s t h e n a t i o n , b u t p a r t i c u l a r l y w i t h i n t h e S t a t e o f I n d i a n a . Severe d e g r a d a t i o n o f t h i s p o p u l a r m a t e r i a l has r e c e n t l y been d r a m a t i z e d w i t h t h e i n i t i a t i o n o f a $10.5 M r e s t o r a t i o n p r o j e c t o f t h e most v i s i b l e symbol o f t h e C i t y o f I n d i a n a p o l i s , Monument C i r c l e . T h i s c i r c a - 1 9 0 0 l i m e stone s t r u c t u r e , t h e h e a r t o f t h e m e t r o p o l i t a n a r e a , showed s e v e r e s t r u c t u r a l decay, which was a t t r i b u t e d p r i m a r i l y t o a c i d r a i n ( 3 ) . 0097-6156/ 86/ 0318-0285506.00/ 0 © 1986 American Chemical Society

Baboian; Materials Degradation Caused by Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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The m a j o r i t y of s t u d i e s examining t h e e f f e c t s o f a c i d r a i n have f o c u s e d on the N o r t h e a s t e r n U.S. and have l a r g e l y i g n o r e d the Midwest. F i s h k i l l s , d y i n g l a k e s , and f o r e s t damage i n the Northe a s t have been f a i r l y d r a m a t i c and w i d e l y p u b l i c i z e d ( 4 - 1 5 ) . Much work has been c o n c e n t r a t e d on ecosystem p e r t u r b a t i o n s , such as the l e a c h i n g of n u t r i e n t s and t r a c e metals from s o i l s (16-19). Concern f o r t h e f o r e s t canopy has r e s u l t e d i n a p l a s t i c polymer c o a t i n g t o be sprayed on i n d i v i d u a l urban t r e e s f o r p r o t e c t i o n ( 2 0 ) . Only r e c e n t l y has work been c o n c e n t r a t e d on t h e e f f e c t s o f a c i d d e p o s i t i o n on human h e a l t h (21,22) and on s t r u c t u r a l m a t e r i a l s (23-25). Ongoing work by t h e F e d e r a l I n t e r a g e n c y Task Force on A c i d P r e c i p i t a t i o n c e n t e r s on a c i d e f f e c t s on m a t e r i a l s (26,27). The study p r i m a r i l y f o c u s e s once a g a i n on e f f e c t s on t h e East Coast, w i t h t h e e f f e c t s on m a t e r i a l s i n the Midwest b e i n g l a r g e l y i g n o r e d . The purpose of t h i s work was t o p r o v i d e d a t a t o f i l l t h a t v o i d . Recent p r e c i p i t a t i o n parameters common t o I n d i a n a have been used t o e s t a b l i s h t h e e x p e r i m e n t a l c r i t e r i a ( 2 8 ) . These i n c l u d e d such data as a c i d c o n t e n t , m e t e o r o l o g i c a l d a t a and c o n d i t i o n s , c h e m i c a l c o m p o s i t i o n of t h e p r e c i p i t a t i o n , e t c . Only one degradat i o n parameter was f o c u s e d upon: the cause and e f f e c t i n t e r a c t i o n of pure H i n l e a c h i n g s t r u c t u r a l m a t e r i a l s common i n I n d i a n a , i n c l u d i n g bronze, marble, and I n d i a n a l i m e s t o n e . Q u a n t i t a t i v e data on t h e c o n t r i b u t i o n of H can then be s e l e c t i v e l y i s o l a t e d from t h e o v e r a l l e f f e c t s of the complex e n v i r o n m e n t a l m a t r i x . With t h i s i n i t i a l s t e p , the e f f e c t s of f u r t h e r m a t r i x components (whether c a t i o n s , a n i o n s , o r compounds), a c t i n g both i n d i v i d u a l l y and/or s y n e r g i s t i c a l l y , can then be pursued. Environmental

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A t h r e e y e a r study m o n i t o r i n g a c i d p r e c i p i t a t i o n a c r o s s C e n t r a l I n d i a n a has j u s t been completed by the a u t h o r s (28) and t h e l o c a l I n t e r n a t i o n a l Rotary Club who donated the manpower f o r sample c o l lection. Over 3000 i n d i v i d u a l samples were c o l l e c t e d d u r i n g approxi m a t e l y 300 i n d i v i d u a l p r e c i p i t a t i o n e v e n t s . Using a c c e p t i b l e a n a l y t i c a l and s t a t i s t i c a l p r o t o c o l , t h e samples were c h a r a c t e r i z e d by m e t e o r o l o g i c a l , p h y s i c a l , and t r a c e c h e m i c a l c o m p o s i t i o n d a t a t o determine p o s s i b l e sources of t h e a c i d i t y . The study y i e l d e d p e r t i n e n t e n v i r o n m e n t a l d a t a t h a t s e t t h e parameters f o r t h i s work. The volume-weighted pH average f o r p r e c i p i t a t i o n of a l l t y p e s ( r a i n , snow, f o g ) a c r o s s C e n t r a l I n d i a n a f o r t h e 1983-85 study was 4.02. The average w i n t e r pH was 4.27, and the average summer pH was 3.73. Summer r a i n s showed t h e most severe a c i d i t y , w i t h numerous i n d i v i d u a l events as low as pH=2.8 and r o u t i n e v a l u e s i n t h e 3.1 t o 3.2 range. Reasonable e x p e r i m e n t a l pH l i m i t s u t i l i z e d f o r t h i s work were t h e r e f o r e s e t a t pH=3.0 and pH=4.0. A t h i r d background c o n t r o l a t pH=5.6 was a l s o u t i l i z e d , t h i s b e i n g the g e n e r a l l y accepted y e t h i g h l y c o n t r o v e r s i a l pH o f pure water caused by the d i s s o l u t i o n of carbon d i o x i d e gas at s t a n d a r d temperature and p r e s sure c o n d i t i o n s w i t h the r e s u l t i n g c a r b o n i c a c i d / b i c a r b o n a t e b u f f e r e q u i l i b r i u m system ( 2 9 ) . The t o t a l water volume o f a l l m o i s t u r e measured at the B a l l S t a t e U n i v e r s i t y Weather S t a t i o n f o r 1984 was 86.79, w i t h 90 cm per y e a r c o n s i d e r e d " t y p i c a l " f o r t h i s r e g i o n of I n d i a n a ( 3 0 ) . The predominant wind d i r e c t i o n f o r t h e above time p e r i o d was South-



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Southwest, w i t h 71% of t h e t o t a l p r e c i p i t a t i o n o r i g i n a t i n g from t h e Southern (90 -270 ) compass hemisphere. Thus, t h e maximum d i r e c t i o n a l p r e c i p i t a t i o n r e c e i v e d by a s t r u c t u r a l s u r f a c e would be 64 cm from t h e Southern hemisphere o r 26 cm from the N o r t h e r n . This work u t i l i z e d 60 cm as a t y p i c a l y e a r l y d i r e c t i o n a l exposure v a l u e . The average s u l f a t e i o n c o n c e n t r a t i o n f o r t h e s e Indiana r a i n s was i n the 1.0 t o 10 ppm range, w h i l e t h e n i t r a t e i o n l e v e l s were i n 0.1 t o 5 ppm range, w i t h about a 2:1 r a t i o o f s u l f a t e t o n i t r a t e being t y p i c a l . S i n c e the r e l a t i v e c o m p o s i t i o n of t h e a c i d c o n s t i t u e n t s v a r i e d so w i d e l y and s e a s o n a l l y a c r o s s I n d i a n a , i t would be erroneous t o e x p e r i m e n t a l l y u t i l i z e a p a r t i c u l a r l e v e l of s u l f u r i c o r n i t r i c a c i d s . T h e r e f o r e , t o t e s t o n l y t h e e f f e c t s of a c i d l e a c h i n g on s t r u c t u r a l m a t e r i a l s , and not t h e e f f e c t s o f t h e v a r i ous a c i d anion s a l t s , n i t r i c a c i d was t h e o n l y a c i d source i n t h e s i m u l a t e d a c i d r a i n w a t e r . By d e l e t i n g t h e s u l f u r i c a c i d , gypsum f o r m a t i o n (CaSO,*2H 0) on t h e l i m e s t o n e and marble was a v o i d e d . While i t i s p r o b a b l y the major c a t a l y s t of l i m e s t o n e d e g r a d a t i o n , t h e f o r m a t i o n and subsequent p e e l i n g o f t h e s o f t s p o n g e - l i k e gypsum was beyond the scope of t h i s work. The removal of each gypsum l a y e r exposes a v i r g i n l i m e s t o n e s u r f a c e t o t h e a c i d l e a c h i n g p r o c e s s , which would merely m u l t i p l y t h e i n d i v i d u a l a c i d l e a c h i n g p r o c e s s e s of r e l a t i v e l y s t a b l e s u r f a c e s t h a t were s t u d i e d i n t h i s work. 2

Experimental Reagents & Equipment. A l l reagent s o l u t i o n s were prepared w i t h doubly d i s t i l l e d and d e - i o n i z e d water t h a t was a l s o f i l t e r e d through a Gelman S c i e n c e s (Ann A r b o r , MI) Water I p u r i f i c a t i o n system t o an ion c o n d u c t i v i t y of 5xt.mho/cm. The s i m u l a t e d a c i d r a i n s o l u t i o n s of pH=3.0, 4.0, and 5.6 were prepared i n t h i s water from c o n c e n t r a t e d n i t r i c a c i d , F i s h e r ACS C e r t i f i e d Reagent Grade. The pH was adj u s t e d u s i n g a Corning Model 5 pH Meter ( M e d f i e l d , MA) t h a t was c o n t i n u o u s l y c a l i b r a t e d w i t h a pH=4.01 M a l l i n k r o d t B u f f a r b u f f e r solution. F i v e s t a n d a r d s each f o r c a l c i u m , magnesium, copper, and z i n c were prepared from 1000 ppm F i s h e r S c i e n t i f i c Atomic Absorpt i o n s t a n d a r d s . A l l s t a n d a r d s , samples, and blank s o l u t i o n s at each pH were a n a l y z e d on a P e r k i n Elmer 306 Atomic A b s o r p t i o n S p e c t r o photometer (Norwalk, CN) a c c o r d i n g t o s t a n d a r d a c c e p t a b l e methods. Samples. Three s t a n d a r d stone samples were o b t a i n e d from t h e c o l l e c t i o n of the B a l l S t a t e U n i v e r s i t y Geology Department. The p r i m a r y sample was a specimen of Salem Limestone ( c a l c i t e , CaCOo), commonly known as I n d i a n a B u i l d i n g Limestone, which was o b t a i n e d i n t h e Bloomington-Bedford a r e a . Because of i t s p o p u l a r i t y as a cons t r u c t i o n m a t e r i a l , t h e Salem Limestone was t h e f o c u s of t h i s s t u d y . I t was s u b d i v i d e d i n t o t h r e e equal s e c t i o n s f o r t h e t e s t i n g of l e a c h i n g r a t e s a t pH=3.0, 4.0, and 5.6. Henceforth, these three samples w i l l be l a b e l e d Salem 3.0, Salem 4.0, and Salem 5.6. The somewhat porous c r y s t a l l i n e f a c e of each s u r f a c e was c u t smooth and square, but not p o l i s h e d . The second stone sample was common w h i t e Georgian marble (metamorphic c a l c i t e , C a C 0 ) . S i n c e marble i s a r e a r r a n g e d and s t r o n g e r c r y s t a l m o d i f i c a t i o n of l i m e s t o n e , i t s h o u l d p r o v i d e a c o n t r a s t i n a c i d l e a c h i n g r a t e s . T h i s sample was square c u t , and i t s f a c e of r e l a t i v e l y l a r g e c r y s t a l l i n e g r a i n s was h i g h l y p o l i s h e d t o a g l a s s 3



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like finish. I t was o n l y exposed t o pH=3.0 w a t e r , and h e n c e f o r t h w i l l be l a b e l e d as Marble 3.0. The f i n a l stone sample was a f i n e - g r a i n e d and d e n s e l y - p a c k e d d o l a m i t e - l i k e grey l i m e s t o n e M i c r i t e from t h e Onondaga f o r m a t i o n i n New York. The l a r g e f a c i a l area o f t h i s sample was smooth-cut i n t o which t h e words "ACID RAIN" were m e c h a n i c a l l y engraved. The average depth of t h e s e l e t t e r s was 0.218 cm, which i s comparable t o the average depth o f 0.268 cm f o r t h e l e t t e r s engraved on a 1985 V e t e r a n s ' A d m i n i s t r a t i o n marble g r a v e s t o n e i n Muncie. Micrometers and m a c h i n i s t s ' depth gauges were used t o measure t h e l e t t e r depths p e r i o d i c a l l y i n t h e s t u d y . T h i s sample was l a b e l e d M i c r i t e 3.0. The bronze samples were o b t a i n e d i n 30.5 cm χ 30.5 cm s h e e t s from t h e C.R. H i l l s Co. ( B e r k l e y , MI). The bronze a l l o y was c a s t as A l l o y #220 w i t h 90% copper and 10% z i n c by w e i g h t , w i t h t r a c e l e a d and i r o n i m p u r i t i e s . I t was i n sheet r o l l e d form a t 18 gauge (1.00 mm t h i c k n e s s ) and w i t h a d e n s i t y o f 8.80 g/cm . H i s t o r i c a l l y , bronze was c l a s s i f i e d as c o p p e r / t i n a l l o y w i t h a minimum 10% by weight t i n , w h i l e b r a s s was a c o p p e r / z i n c a l l o y w i t h a t l e a s t 10% z i n c . The contemporary c l a s s i f i c a t i o n o f "bronze" has been expanded to encompass a much w i d e r range o f copper a l l o y s w i t h b r o n z e - l i k e s t r u c t u r e s , p r o p e r t i e s , and c o l o r , but which c o n t a i n no t i n . Thus, a l t h o u g h i t i s c o m m e r c i a l l y c l a s s i f i e d as b r o n z e , t h e sample u t i l i z e d i n t h i s work was h i s t o r i c a l l y b r a s s . T h i s bronze sheet was machine c u t i n t o f o u r s t r i p s , each w i t h dimensions of 15.25 χ 3.75 cm. The l e t t e r s s c r i p t "A" and b l o c k "T" were engraved i n t o each s t r i p , w i t h t h e "T" h a v i n g an average depth of 0.033 cm. Two o f t h e bronze s t r i p s were l e a c h e d w i t h pH=3.0 water as d u p l i c a t e s , a t h i r d sample r e c e i v e d pH=4.0 exposure, w h i l e the f i n a l s t r i p as a c o n t r o l r e c e i v e d pH=5.6 water. These samples w i l l be l a b e l e d Bronze 1-3.0, Bronze 2-3.0, Bronze 3-4.0, and Bronze 4-5.6, r e s p e c t i v e l y . P r o c e d u r e . The M i c r i t e 3.0 was suspended v e r t i c a l l y on a r i n g s t a n d , w i t h a 15 cm p o l y e t h y l e n e f u n n e l and 100 ml b o t t l e assembly d i r e c t l y beneath t o c o l l e c t t h e water r u n - o f f . To s i m u l a t e r a i n , t h e pH=3.0 s o l u t i o n was sprayed on t h e engraved stone u s i n g a c o n v e n t i o n a l 400 ml handpump a e r o s o l s p r a y e r , which was s e t t o d e l i v e r a medium d r o p l e t s i z e m i s t a t a d i s t a n c e o f 20 cm from t h e s t o n e , t o ensure complete coverage o f t h e f a c e by t h e c i r c u l a r s p r a y p a t t e r n . Only d i r e c t r u n - o f f from t h e stone was c o l l e c t e d , w i t h t h e remainder o f the s p r a y p a t t e r n d i s p o s e d o f as waste. The stone was sprayed o v e r a t e n minute "exposure" p e r i o d o r u n t i l 65 ml were c o l l e c t e d i n p r e c a l i b r a t e d b o t t l e s ( t h e amount b e i n g e q u i v a l e n t t o a 1.05 cm r a i n on t h e s u r f a c e area o f t h e M i c r i t e ) . The c a l c i u m and magnesium l e v e l s were measured on t h e Atomic A b s o r p t i o n i n s t r u m e n t . The stone was a l l o w e d t o a i r d r y between c o l l e c t i o n s , w i t h two hours b e i n g t y p i c a l due t o t h e p o r o s i t y o f t h e s t o n e . T h i s procedure was used f o r 300 s i m u l a t e d r a i n s . The engraved "ACID RAIN" l e t t e r s and a l l o t h e r dimensions were measured a f t e r e v e r y 30 e v e n t s . Sample weight changes on t h i s and o t h e r stone samples were not measured due t o i n c o n s i s t e n t water r e t e n t i o n . The Marble 3.0 and a l l t h r e e Salem Limestone samples were leached by a m o d i f i e d p r o c e d u r e . The s p r a y i n g was r e p l a c e d by s e t t i n g each stone i n t o a 14 cm p e t r i d i s h t o g e t h e r w i t h 100 ml o f t h e r e s p e c t i v e pH water f o r t h e s t a n d a r d 10 minute exposure t i m e . The



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s t o n e s i d e s , i n a d d i t i o n t o t h e f a c e , were t h u s a l s o p a r t i a l l y exposed t o t h e water, and t h e s e s i d e a r e a s were i n c l u d e d i n t h e f i n a l exposure a r e a c a l c u l a t i o n s . S i n c e each s t o n e had d i f f e r e n t exposed s u r f a c e a r e a s b u t i d e n t i c a l 100 ml r a i n v o l u m e s , e a c h s t o n e r e c e i v e d d i f f e r i n g s i m u l a t e d r a i n d e p t h s a s f o l l o w s : M a r b l e 3.0 = 1.59 cm, S a l e m 3.0 = 1.73 cm, and S a l e m 4.0 & 5.6 = 1.96 cm. A l l l e a c h i n g r e s u l t s w e r e n o r m a l i z e d t o 1.00 cm t o c o m p e n s a t e f o r t h e v a r i a t i o n s . The f o u r b r o n z e s a m p l e s w e r e e a c h s p r a y e d w i t h s i m u l a t e d r a i n a c c o r d i n g t o t h e p r o c e d u r e f o r t h e M i c r i t e 3.0 a b o v e . E a c h b r o n z e was s p r a y e d u n t i l 30 ml was c o l l e c t e d i n t h e c o l l e c t i o n b o t t l e s , b e i n g e q u i v a l e n t t o 1.0 cm o f s i m u l a t e d r a i n . T h e s e w a t e r s a m p l e s w e r e t h e n a n a l y z e d f o r c o p p e r a n d z i n c on t h e AA i n s t r u m e n t . An a i r d r y t i m e o f 30 m i n u t e s was t y p i c a l . E a c h B r o n z e 3.0 was s u b j e c t e d t o 300 s i m u l a t e d r a i n s , w h i l e t h e B r o n z e 4.0 & 5.6 r e c e i v e d o n l y 150. E n g r a v e d l e t t e r d e p t h s a n d o v e r a l l d i m e n s i o n s o f e a c h w e r e m e a s u r e d a f t e r e v e r y 30 e v e n t s . R e s u l t s and D i s c u s s i o n P h y s i c a l O b s e r v a t i o n s . M i c r i t e : T h e M i c r i t e 3.0 e x h i b i t e d a n o t i c a b l e c o l o r c h a n g e a f t e r 16 cm o f r a i n , f r o m a d a r k g r e y t o a g r e y w h i t e w h i c h w h i t e n e d g r a d u a l l y t h r o u g h o u t a l l 300 r a i n s . No g r o s s d i m e n s i o n a l changes were d e t e c t a b l e , w i t h t h e e x c e p t i o n o f smoother and more r o u n d e d c o r n e r s . A f t e r 50 r a i n s , t h e s u r f a c e o f t h e e n g r a v e d "ACID RAIN" l e t t e r s was b r o a d e n i n g , w i t h t h e e d g e s o f e a c h l e t t e r becoming l e s s s h a r p and d e f i n e d . T h i s l e t t e r b r o a d e n i n g p r o g r e s s i v e l y i n c r e a s e d t h r o u g h o u t t h e experiment. T a b l e I summarizes the engraved l e t t e r depth changes as a f u n c t i o n o f s i m u l a t e d r a i n d e p t h ( 6 0 cm b e i n g e q u i v a l e n t t o o n e y e a r o f d i r e c t i o n a l r a i n ) : Table I.

D e p t h s o f E n g r a v e d "ACID RAIN" L e t t e r s i n M i c r i t e 3.0

Total Rain Depth,cm

Average L e t t e r Depth,cm

S t d . Dev.

12 26 34 42 60 84 114 138 173 213 243 273

0.0194 0.0575 0.0746 0.0879 0.1016 0.1026 0.1051 0.1064 0.1099 0.1137 0.1162 0.1187

0.0143 0.0332 0.0379 0.0334 0.0333 0.0330 0.0328 0.0323 0.0336 0.0343 0.0346 0.0349

The l e t t e r d e p t h m e a s u r e m e n t s w e r e made r e p e a t e d l y a t t h e same r e p r e s e n t a t i v e l o c a t i o n on e a c h l e t t e r . R e l a t i v e l y l a r g e b u t c o n s i s t e n t standard d e v i a t i o n s probably r e s u l t e d from c r y s t a l s t r u c t u r e inhomogeniety, v a r i a t i o n s i n l e t t e r engraving, e t c . Figure 1 i l l u s t r a t e s two d i s t i n c t r a t e s o f l e a c h i n g . The g r e a t e s t r a t e o c c u r e d i n




290

t h e f i r s t Vk y e a r , w i t h a 50% o v e r a l l i n c r e a s e i n depth. T h i s i s p r o b a b l y t h e r e s u l t o f t h e p u l v e r i z a t i o n and weakening o f t h e i n t e r i o r w a l l s and f l o o r o f t h e l e t t e r s caused by t h e e n g r a v i n g p r o c e s s . T h i s weakened m a t e r i a l c o u l d then be l e a c h e d a t a s i g n i f i c a n t l y more r a p i d r a t e than t h e s u r f a c e M i c r i t e . F o l l o w i n g t h i s stage, the depth r a t e i n c r e a s e slowed a p p r e c i a b l y f o r t h e next 3% y e a r p e r i o d t o a c o n s i s t e n t but v e r y s l i g h t i n c r e a s e . At t h i s p o i n t , t h e M i c r i t e l e t t e r f l o o r was p r o b a b l y v i r t u a l l y i d e n t i c a l t o t h e s u r f a c e M i c r i t e w i t h v e r y s i m i l a r s t r i p p i n g r a t e s . The engraved l e t t e r s were s t i l l s t r e s s p o i n t s , as shown by t h e c o n t i n u e d l e t t e r broadeni n g , r e s u l t i n g i n a s l i g h t r i s e i n t h e s t r i p p i n g p l a t e a u . In a d d i t i o n , t h e l e t t e r s u r f a c e s were r e c e i v i n g t w o - d i m e n s i o n a l exposure on s i d e and t o p v e r s u s o n l y one-dimension f o r t h e normal s u r f a c e . T h i s c o u l d a l s o c o n t r i b u t e t o t h e s l i g h t depth i n c r e a s e l o n g - t e r m . From t h e above, i t i s p r o j e c t e d t h a t t h e l e t t e r depth change would e v e n t u a l l y reach z e r o and become n e g a t i v e as t h e l e t t e r s broadened and t h e s u r f a c e l e a c h e d s l i g h t l y f a s t e r than t h e l e t t e r i n t e r i o r s , r e s u l t i n g i n the l e t t e r s "fading' . This process i s r e a d i l y observed on U.S. Government M i l i t a r y V e t e r a n s ' g r a v e s t o n e s and o t h e r n a t u r a l l y "weathered" monuments. Depths o f V.A. marble s t o n e s i n Muncie's Beech Grove Cemetery were measured f o r t h e p e r i o d 1930-1985. The r e s u l t s are summarized i n Table I I : 1

Table I I . Depths o f Engraved Marble L e t t e r s i n V.A.

Date

#

2-1985 6-1983 1978 1-1975 12-1969 1-1967 4-1961 5-1951 8-1947 11-1940 1-1935 11-1931

Letters

13 14 14 11 11 12 14 14 12 12 9 11

Average

Depth,cm

0.2688 0.2778 0.3097 0.3380 0.2803 0.2646 0.2329 0.2054 0.1947 0.1770 0.0923 0.1259

Std.

Stones

Dev.

0.0489 0.0408 0.0231 0.0384 0.0131 0.0576 0.0357 0.0414 0.0460 0.0515 0.0172 0.0327

Random s t o n e s a l l w i t h a w e s t e r l y exposure were chosen a t a p p r o x i mately f i v e y e a r date i n t e r v a l s . Standard d e v i a t i o n s were p r o b a b l y l a r g e due t o t h e wide v a r i a t i o n o f response t o e n v i r o n m e n t a l f a c t o r s and t o t h e marble's own s t r u c t u r a l v a r i a t i o n s . At t h e same l o c a t i o n , a g r a n i t e f a m i l y g r a v e s t o n e a l s o o f f e r e d a unique s t r i p p i n g r e c o r d . V e r t i c a l l y arranged on t h e western f a c e o f t h i s stone were embossed name and date l i n e s : 1913, 1939, 1957, 1941, r e s p e c t i v e l y t o p t o bottom. Table I I I summarizes t h e average embossed l e t t e r h e i g h t s above t h e stone f a c e f o r t h e l a s t n i n e l e t t e r s o f each l i n e :


20.

NEAL ET AL.


Table I I I . H e i g h t s o f Embossed G r a n i t e L e t t e r s , Muncie,

Date


1913 1939 1941 1957

#

Letters

9 9 9 9

Average

Height,cm

0.0751 0.1270 0.1778 0.2362

Std.

IN.

Dev.

0.0021 0.0142 0.0210 0.0365

F i g u r e 2 i l l u s t r a t e s t h e r a t e s of l e t t e r changes f o r both t h e V.A. marble engraved l e t t e r s and t h e g r a n i t e embossed l e t t e r s . The c o r r e l a t i o n between t h e two r a t e s was s u r p r i s i n g i n view o f t h e g r e a t e r bond s t r e n g t h o f t h e g r a n i t e c r y s t a l . The s i m i l a r i t y c o u l d r e s u l t p a r t i a l l y from t h e g r e a t e r s t r e s s on t h e p r o t r u d i n g embossed l e t ters. Of note are t h e a c c e l e r a t e d changes f o r both t h e marble and g r a n i t e i n t h e 1935-40 p e r i o d , p o s s i b l y due t o a l o c a l e n v i r o n m e n t a l p e r t u r b a t i o n . A c l o s e comparison o f t h e e x p e r i m e n t a l F i g u r e 1 w i t h t h e 1985-75 V.A. marble p e r i o d o f F i g u r e 2 shows t h a t t h e marble l e t t e r s a l s o i n c r e a s e d i n depth o v e r t h e i n i t i a l 10 y e a r e x p o s u r e , which i s v i r t u a l l y t h e same as t h e e x p e r i m e n t a l M i c r i t e 3.0. Howe v e r , as d i s c u s s e d above, t h e depth i n c r e a s e was then r e v e r s e d a f t e r 10 y e a r s , and t h e l e t t e r s broadened and faded due t o s u r f a c e decay. Salem Limestone: A f t e r 9 cm o f r a i n , both t h e Salem 3.0 & 4.0 l i m e s t o n e changed c o l o r s l i g h t l y from l i g h t tan-brown towards a white-brown, becoming more pronounced w i t h c o n t i n u e d exposure. Pitt i n g o f t h e s u r f a c e was o b v i o u s a f t e r 43 cm of r a i n and became p r o g r e s s i v e l y deeper and more numerous, w i t h t h e Salem 3.0 showing t h e greatest effect. S u r f a c e area dimensions o f t h e Salem 3.0 remained r e l a t i v e l y c o n s t a n t , a g a i n w i t h t h e r o u n d i n g o f c o r n e r s . Sample depth d e c r e a s e d s l i g h t l y by an average o f 0.1 cm. The Salem 5.6 showed v i r t u a l l y no o b s e r v a b l e changes. M a r b l e : Marble 3.0 showed v e r y l i g h t p i t t i n g s u r r o u n d i n g t h e i n d i v i d u a l marble c r y s t a l s , which was minimal compared t o both t h e Salem 3.0 & 4.0. The Marble 3.0 showed no c o l o r change, and t h e s u r f a c e remained r e l a t i v e l y m i r r o r - l i k e . No changes i n t h e p h y s i c a l dimensions of a r e a o r depth were d e t e c t a b l e . Bronze: The d u p l i c a t e Bronze 3.0 samples both e x h i b i t e d numerous p i t t i n g and d i s c o l o r a t i o n a r e a s . The random p i t t i n g may be t h e r e s u l t of the z i n c d i s s o l v i n g t o provide c a t h o d i c p r o t e c t i o n f o r the copper, due t o z i n c ' s l a r g e 0.76 V o x i d a t i o n p o t e n t i a l . The p i t t i n g o c c u r e d a c r o s s t h e e n t i r e s u r f a c e and i n s i d e t h e l e t t e r s . Since the s t r i p s were l e a c h e d v e r t i c a l l y , t h e s o l u t i o n accumulated a t t h e b o t tom w h i l e d r y i n g , c a u s i n g a g r e y - g r e e n area of s e v e r e d i s c o l o r a t i o n . T h i s m a t e r i a l was p r o b a b l y a c o m b i n a t i o n o f green C u ( 0 H ) C 0 and b l u e - g r e y Zn^iOH^COo, both of which are n a t u r a l o x i d a t i o n p r o d u c t s . The o v e r a l l c o l o r of both samples changed from a dark brown-gold t o a l i g h t w h i t e y e l l o w - g o l d . No d e t e c t a b l e changes i n dimensions o f t h e s t r i p s o r engraved l e t t e r s were d e t e c t e d , e x c e p t a t t h e bottom of t h e b l o c k "T" where a s l i g h t depth i n c r e a s e o f 0.002 cm was obs e r v e d i n both samples. T h i s p r o b a b l y r e s u l t e d from t h e v e r t i c a l p o s i t i o n i n g which a l l o w e d t h e base o f t h e "T" t o r e t a i n t h e w a t e r . 2

2


3

291

2

9


2


TOTAL AVG DEPTH CHANGE, CM

iHMmiiimnmlmm.i,lu,,,,,,,!,,,,,,

0

20

40

60

ill ι ill m m ι n i l il

80

130

120

140

160

Imiiiiiiimmmlimiimlniimnlimmnlnminil

180

200

220

240

268

290

SIMULATED RAIN, CM

F i g u r e 1. L e t t e r depth changes f o r engraved l e t t e r s f o r t h e M i c r i t e sample a t pH=3.0. ENGRAVED MARBLE & EMBOSSED GRANITE AYG LETTER DEPTH,

Materials Degradation Caused by Acid Rain - ACS Publications

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