Archaeological Chemistry IV - ACS Publications - American Chemical

25 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 2, 2016 | http://pubs.acs.org Publication Date: July 1, 1989 | doi: 10.1021/ba-1988-0220.ch025

Characterization of Historical and Artificially Aged Silk Fabrics S. P. H e r s h , P. A . Tucker, and M. A. Becker

1

College of Textiles, North Carolina State University, Raleigh, NC 27695-8301

Several methods for characterizing the degradation of silk fabrics that have been subjected to natural and accelerated aging by exposure to heat and light are described. Change in color, strength degradation, and increases in amino- and ammonia-nitrogen contents were measured. Among the more notable findings were that the fabrics discolor on the order of 10 times faster when degraded thermally than when degraded by light (at equal levels of degradation as measured by strength loss) and that strength loss is proportional to the increase in ammonia and amino group contents for fabrics degraded by heat or light. The amino group and ammonia contents of 16th-, 18th-, and 19th-century silks range from values typical for artificially aged contemporary silks to 3 times greater for amino nitrogen and 4 times greater for ammonia. All of the tin-weighted historical fabrics (except three dyed with logwood) have higher amino group contents than unweighted fabric. The ammonia contents do not vary much, however.

T H E B A S I C K N O W L E D G E N E E D E D T O S U P P O R T a p p l i e d research o n the conservation a n d p r e s e r v a t i o n of i r r e p l a c e a b l e textiles is inadequate. T o c o n t r i b u t e to the k n o w l e d g e n e e d e d to d e v e l o p better a n d m o r e acceptable 'Current address: Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, M D 21218

0065-2393/89/0220-0429$06.25/0 © 1989 A m e r i c a n C h e m i c a l S o c i e t y

In Archaeological Chemistry IV; Allen, Ralph O.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

430

ARCHAEOLOGICAL CHEMISTRY

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conservation m e t h o d s , w e p e r f o r m e d several studies focusing o n the c o n servation a n d restoration of cotton (1-8). T h e most fragile textile, h o w e v e r , is generally agreed to be silk. References 9 a n d 10 r e v i e w some of the p r o b l e m s associated w i t h silk. T h i s chapter is the c o n t i n u a t i o n of an e a r l i e r study o n the m e c h a n i s m o f silk degradation (II). A major factor that contributes to the fragility of silk is the practice of w e i g h t i n g , w h i c h has b e e n c o n d u c t e d for at least 200 years (12,13). W e i g h t i n g is the a p p l i c a t i o n of 3 0 % - 3 0 0 % of inorganic salts of a l u m i n u m , i r o n , l e a d , t i n , or z i n c to silk fabrics to increase t h e i r b o d y , d r a p e , " s c r o o p , " w e i g h t p e r u n i t area, etc. B y the late 1800s, w e i g h t i n g h a d b e c o m e an accepted m e t h o d o f silk p r e p a r a t i o n . Because sensitivity to e n v i r o n m e n t a l stress is greatly i n f l u e n c e d b y the type a n d amount of w e i g h t i n g agents present i n the silk, an analytical t e c h n i q u e was d e v e l o p e d to d e t e r m i n e the presence of w e i g h t i n g agents. A convenient, nondestructive qualitative p r o c e d u r e for m a k i n g such tests b y X - r a y fluorescence spectroscopy was d e s c r i b e d elsewhere (14). S t r o n g alkalies cause far greater damage i n proteinaceous fibers t h a n do strong acids (15). T h e damage caused b y strong acids, h o w e v e r , is q u i t e severe. U n d e r m i l d e r conditions, the l i g h t stability of u n w e i g h t e d silk is greatest at about p H 10 b u t decreases r a p i d l y as the fabric p H b e c o m e s h i g h e r or l o w e r (16). Because most w e i g h t i n g c o m p o u n d s are h i g h l y a c i d i c , w e i g h t e d silks m i g h t be expected to be e v e n less stable t h a n u n w e i g h t e d silks. T h i s s e n s i t i z i n g effect of w e i g h t i n g has i n d e e d b e e n s h o w n to o c c u r a n d is far m o r e d e t r i m e n t a l to silks exposed to l i g h t t h a n to fabrics stored i n the dark (17, 18). H o w e v e r , damage e v e n d u r i n g dark storage is severe. T w o major routes to silk degradation are oxidation a n d h y d r o l y s i s (16). H y d r o l y s i s of a n a m i d e group splits the p o l y m e r c h a i n to f o r m an a m i n o group a n d a c a r b o x y l group. D e t e r m i n a t i o n of the increase i n a m i n o groups i n the silk, therefore, s h o u l d p r o v i d e a measure of h y d r o l y t i c degradation. O x i d a t i o n , o n the other h a n d , is a c c o m p a n i e d b y the formation of a m m o n i a (16). T h u s , it s h o u l d b e possible to assess the c h e m i c a l b r e a k d o w n b y measu r i n g the a m o u n t of a m i n o a n d a m m o n i a n i t r o g e n present i n the silk. T h e u l t i m a t e objective of this study is to d e v e l o p a t e c h n i q u e for p r e v e n t i n g , or at least r e t a r d i n g , the degradation of silk b y a p p l y i n g stabilizers a n d consolidants to the silk. Before progress can be made, i t is necessary to characterize the state of degradation of historical textiles a n d to establish, i f possible, the mechanisms b y w h i c h these textiles reached t h e i r present state. T h u s , the p r i m a r y purpose of this chapter is to compare the state of d e g radation o f historical silk fabrics w i t h those of c o n t e m p o r a r y silk fabrics that have b e e n artificially aged b y exposure to d r y heat a n d to l i g h t from a x e n o n arc l a m p . P r o p e r t i e s e x a m i n e d for this purpose are tensile strength, color change, a n d the concentration of a m i n o groups a n d a m m o n i a i n the w a t e r extractable components of the fabrics.


25.

H E R S H ET AL.

431

Historical and Artificially Aged Silk Fabrics


Experimental Design Test Materials, C O N T E M P O R A R Y S I L K . A S r e p o r t e d earlier (11), fabrics w e r e chosen over yarns for treatment because of t h e i r ease of p r e p a r a t i o n and h a n d l i n g . T e n s i l e properties, h o w e v e r , w e r e m e a s u r e d o n yarns extracted f r o m the fabrics to take advantage of the ease of h a n d l i n g fabrics a n d the ease of testing yarns. T h e silk fabric s t u d i e d was an u n w e i g h t e d p l a i n w o v e n C h i n e s e silk habutae (Testfabrics, I n c . , M i d d l e s e x , N J , Style N o . 605) w i t h 126 e n d s / i n . (37.6 denier) a n d 117 p i c k s / i n . (32.1 denier) a n d w e i g h i n g 1.11 o z / y d . T h e fabric h a d b e e n d e g u m m e d (14). F a b r i c was taken from the same bolt for all tests. 2

HISTORICAL FABRICS. F a b r i c s a n d garments analyzed w e r e 11 samples from the study c o l l e c t i o n of the D i v i s i o n of Textiles, N a t i o n a l M u s e u m of A m e r i c a n H i s t o r y of the S m i t h s o n i a n I n s t i t u t i o n , a n d one sample f r o m the M u s e o P o l d i - P e z z o l i . M a n y of the garments w e r e composites consisting of several layers a n d decorative elements. T h e fabrics e x a m i n e d w e r e i d e n t i f i e d as follows: 1. late 19th- o r early 20th-century w e d d i n g dress, b e i g e 2. early 2 0 t h - c e n t u r y green fabric w i t h green satin stripes 3. late 19th- or early 20th-century w h i t e fabric w i t h p i p i n g 4. 18th-century green b r o c a d e fabric b a c k g r o u n d w i t h a and

flower

building pattern

5. late 18th- o r early 19th-century p i n k fabric w i t h e m b r o i d e r e d metallic

flowers

6. late 19th-century t h r e e - l a y e r vest, black top a n d l i n i n g a n d gold i n n e r layer 7. ca. 1901 E d w a r d i a n collar piece, green fabric w i t h b e i g e lace and

black v e l v e t collar b a n d (part of N o . 8)

8. ca. 1901 E d w a r d i a n afternoon r e c e p t i o n dress, bodice. 9. late 19th-century skirt, beige e m b r o i d e r e d face fabric a n d b r o w n l i n i n g fabric. 10. 1880-1900 dark b l u e a n d black j a c q u a r d p a t t e r n fabric. 11.

late 19th-century q u i l t e d b e d s p r e a d , o l i v e w i t h p i n k stripes.

12. 16th-century F l e m i s h tapestry (being restored), d e t e r i o r a t e d silk y a r n sample. T h e y a r n was fragmented into segments 1 to 2 m m long.


432


Artificial Aging by Heat. T h e p r o c e d u r e d e v e l o p e d earlier ( I I ) was followed. F a b r i c pieces (15 X 15 cm) w e r e p l a c e d o n racks c o v e r e d w i t h a F i b e r g l a s screen (7- X 3-cm mesh) i n a forced convection o v e n p r e h e a t e d to 150 °C. T h e screen was u s e d to p r e v e n t any e n h a n c e d degradation that m i g h t b e caused b y d i r e c t contact w i t h the m e t a l rack. T h e c o n t r o l silk fabric


was h e a t e d for u p to 4 days i n 6-h i n c r e m e n t s , a n d t h e n i m m e d i a t e l y p l a c e d i n a desiccator that contained silica g e l to k e e p the silk d r y w h i l e cooling.

Artificial Aging by Light. Pieces o f silk fabric (20 X 7 cm) w e r e m o u n t e d i n standard s p e c i m e n holders as specified i n the A m e r i c a n A s s o ciation o f T e x t i l e C h e m i s t s a n d C o l o r i s t s ( A A T C C ) Test M e t h o d 1 6 E - 1 9 8 2 , "Colorfastness to L i g h t : W a t e r - C o o l e d X e n o n A r c L a m p , C o n t i n u o u s L i g h t " (19). Samples o f the c o n t e m p o r a r y fabric w e r e exposed to l i g h t for 2-day intervals for u p to 20 days i n a water-cooled x e n o n arc fading apparatus W e a t h e r - o m e t e r , m o d e l E S 2 5 (Atlas E l e c t r i c D e v i c e s , C h i c a g o , I L ) . T h e W e a t h e r - o m e t e r was operated at 5 0 °C w i t h a n arc intensity o f 1500 W . T h e relative h u m i d i t y was m a i n t a i n e d at 3 0 % ± 5%. T h e sample was exposed to 108 k j / m at 2-day intervals. U p o n r e m o v a l from the W e a t h e r - o m e t e r , the samples w e r e p l a c e d i n acid-free tissue paper, t h e n e q u i l i b r a t e d u n d e r standard test conditions for future testing. 2

Parameters

to Measure

Degradation,

B R E A K I N G

S T R E N G T H .

W a r p yarns w e r e extracted from the fabrics, a n d t h e i r b r e a k i n g loads w e r e d e t e r m i n e d at a gauge l e n g t h of 5.0 c m a n d a rate o f extension o f 50 m m / m i n o n a tensile testing m a c h i n e (Instron 1123) as set forth i n the A m e r i c a n Society for T e s t i n g a n d M a t e r i a l s ( A S T M ) Test M e t h o d D 2 2 5 6 - 8 0 , " B r e a k i n g L o a d (Strength) a n d E l o n g a t i o n of Y a r n b y the S i n g l e - S t r a n d M e t h o d " (20). N o r m a l l y , 21 measurements w e r e made from each fabric sample. T h e b r e a k ing strength of the yarns extracted from the control fabric was 100.0 gf (2.6 gf/denier) w i t h a standard d e v i a t i o n of approximately 8 gf. C O L O R D I F F E R E N C E . Silk discolors w h e n exposed to heat a n d light. T h e color change is taken as one measure of the extent of degradation. T h e color difference o f each treated sample was evaluated o n a D i a n o M a t c h Scan spectrophotometer against a standard u n t r e a t e d silk sample. T h e color differences A E * are r e p o r t e d i n C I E L A B color difference units ( C D U ) for I l l u m i n a n t D ^ , a n d w e r e calculated b y (21) a b

AE*

a b

= [(AL*)

2

+ (Δα*)

2

+ (Afe*) f 2

(1)

w h e r e A L * is the change i n lightness, from l i g h t e r ( + ) to d a r k e r (-), Δ α * is the change i n shade from r e d ( + ) to green (-), a n d Δ&* is the change i n shade f r o m y e l l o w ( 4- ) to b l u e (-) w i t h respect to a standard (untreated silk


25.

HEESH ET AL.

433


fabric). T h e variables L * , α*, a n d L*

are d e f i n e d as

= 116(Y/Y f -

16

0

a* = 500 [(X/X f 0

(2)

- (Y/Yof]

(3)

(Z/Z f]

(4)

fc* = 2 0 0 [ ( Y / Y f 0

0

w h e r e X , Y , a n d Ζ are the t r i s t i m u l u s values for the sample, a n d X , Y , Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 2, 2016 | http://pubs.acs.org Publication Date: July 1, 1989 | doi: 10.1021/ba-1988-0220.ch025

0

and Z

0

0

are the values for the reference w h i t e .

AMINO GROUP CONTENT. E a c h fabric was g r o u n d i n a W i l e y m i l l fitted w i t h a N o . 40 m e s h screen. T h e concentration of a m i n o groups was t h e n d e t e r m i n e d c o l o r i m e t r i c a l l y b y the reaction of 20 m g of the g r o u n d silk fabric w i t h n i n h y d r i n b y u s i n g the p r o c e d u r e d e s c r i b e d e a r l i e r (11, 22). T h e r e action of n i n h y d r i n w i t h c o m p o u n d s c o n t a i n i n g α-amino groups a n d w i t h a m m o n i u m salts forms a c o m p o u n d k n o w n as R u h m a n n ' s p u r p l e that has a m a x i m u m absorption at 570 n m . Stock solutions of dZ-leucine a n d a m m o n i u m c h l o r i d e w e r e p r e p a r e d for calibration. S u i t a b l y d i l u t e d aliquots of the stock solutions w e r e t h e n a n a l y z e d b y u s i n g the n i n h y d r i n p r o c e d u r e . T h e c a l i bration curves o b t a i n e d (shown i n F i g u r e 1) w e r e v i r t u a l l y i d e n t i c a l a n d fit the f o l l o w i n g l i n e a r least-squares regression equations: l e u c i n e [ - N H ] ( n m o l / m L ) = 59.45 A + 0.0585 2

NH C1 [NH ](nmol/mL) 4

3

= 60.28 A + 0.450

nmoK-NH^or

(r (r

2

2

= 0.9980)

= 0.9995)

(5a) (5b)

NH^/mL

Figure 1. Calibration curve for amino group (dl-kucine) and ammonium ion (ammonium chloride) analysis by ninhydrin method.


434


w h e r e [ - N H ] a n d [ N H ] are the concentrations of a m i n o groups a n d a m m o n i a , r e s p e c t i v e l y ; A is the absorbance of the test solution at 570 n m ; a n d r is the c o r r e l a t i o n coefficient. Because the analysis of the silk fabrics b y the n i n h y d r i n m e t h o d gives the s u m of [ - N H ] a n d [ N H ] , the a m i n o group concentration was calculated b y subtracting the i n d e p e n d e n t l y m e a s u r e d a m m o n i a concentration. 2

3


2

3

AMMONIA CONTENT. T h e concentration of a m m o n i a n i t r o g e n i n the water-extractable c o m p o n e n t s of the fabric was d e t e r m i n e d c o l o r i m e t r i c a l l y b y N e s s l e r s reaction (23). U n d e r alkaline conditions, N e s s l e r s reagent (mer c u r i c i o d i d e - p o t a s s i u m i o d i d e solution) reacts w i t h a m m o n i a that has b e e n released from a m m o n i u m salts b y the alkali present to p r o d u c e a y e l l o w c o m p o u n d b y the f o l l o w i n g reaction: 2K HgI 2

4

+ N H

3

+3KOH-* Hg OINH 2

2

+ 7KI + 2 H 0 2

A 5 0 - m g sample of each g r o u n d fabric was i n t r o d u c e d into a 5 0 - m L E r l e n m e y e r flask, a n d 2 0 - 3 0 m L of d e i o n i z e d - d i s t i l l e d water was a d d e d . A f t e r a p p r o x i m a t e l y 15 m i n , each solution was filtered t h r o u g h ashless filter p a p e r into a 1 0 0 - m L v o l u m e t r i c flask. A 2 - m L aliquot of Nessler's reagent (ΑΡΗA, F i s h e r Scientific C o . ) was a d d e d to the 1 0 0 - m L solution. A f t e r at least 10 m i n , b u t not longer than 20 m i n , the absorbance of the solution at 425 n m was m e a s u r e d o n a B a u s c h a n d L o m b Spectronic 20 spectrophotometer. A solution s u b m i t t e d to the same treatment b u t to w h i c h no silk was a d d e d was u s e d as the blank. C a r e m u s t be taken to p r e v e n t c o n t a m i n a t i o n of the water a n d reagents, because a n u m b e r of organic materials interfere w i t h the m e a s u r e m e n t . T h e c o l o r e d solution f o r m e d after a d d i n g the N e s s l e r s reagent d e v e l o p e d some t u r b i d i t y on the samples degraded 10 days or m o r e b y light. T h u s far, the reason for this c o m p l i c a t i o n has not b e e n established. T w o solutions w e r e p r e p a r e d for each historical silk sample. N e s s l e r s reagent was a d d e d to one, a n d d e i o n i z e d - d i s t i l l e d water was a d d e d to the other, so that a n i n d e p e n d e n t m e a s u r e m e n t c o u l d be made of the a m o u n t of discoloration c o n t r i b u t e d to the solution b y the silk a n d its degradation products and contaminants, i f any. F o r these measurements, d e i o n i z e d - d i s t i l l e d water was u s e d as the blank, a n d a separate absorbance meas u r e m e n t was made o n the solution of water a n d N e s s l e r s reagent. T h e total absorbance A of the solution c o n t a i n i n g the a m m o n i a - N e s s l e r s c o m p l e x is the s u m of three absorbances: A = A

f l

+ A„ + A

(6)

s

w h e r e A is the absorbance of the a m m o n i a - N e s s l e r ' s complex, A is the absorbance of the N e s s l e r s reagent, a n d A is the absorbance of the soluble silk constituents a n d degradation products. A is o b t a i n e d b y subtracting A a n d A f r o m A . T h e absorption of silk degradation products, A , is n e g l i g i b l e a

n

s

n

s

s


25.

HERSH ET AL.

435


for the c o n t e m p o r a r y silks, whereas a correction for A m u s t be made i n the analysis of the h i s t o r i c a l samples to correct for any l e a c h i n g of d y e s soil, a n d other contaminants. A stock solution of N H C 1 dissolved i n d e i o n i z e d - d i s t i l l e d water was p r e p a r e d for calibration. A l i q u o t s of this solution w e r e f u r t h e r d i l u t e d to obtain the solutions for the calibration c u r v e . E a c h solution was t h e n s u b j e c t e d to the treatment just d e s c r i b e d . T h e calibration c u r v e ( F i g u r e 2) was fit b y the l i n e a r least-squares regression equation s

?


4

[ N H ] ( n m o l / m L ) = 321.5 A 3

a

0.439

-

(r

2

= 0.9979)

w h e r e [ N H ] is the concentration of a m m o n i a i n nanomoles of N H milliliter. 3

(7)

3

per

Results and Discussion Heat Degradation, STRENGTH. T h e b r e a k i n g strengths of yarns extracted f r o m the fabric after h e a t i n g at 150 °C are s h o w n i n F i g u r e 3. C o n s i d e r i n g " p r o p e r t y k i n e t i c s " as d e s c r i b e d b y A r n e y and C h a p e l a i n e (24), the data suggest a t y p i c a l zero-order reaction scheme i n w h i c h the strength S decreases at a constant rate w i t h t i m e t expressed as follows: dS/dt =

-k

0


(8)


436

A R C H A E O L O G I C A L CHEMISTRY

HEATING TIME (hours) Figure 3, Strength loss and total color change of silk fabric as a function heating time at 150 °C.

of

w h e r e k is the z e r o - o r d e r rate constant. W h e n the data are p l o t t e d as the l o g a r i t h m of strength as a f u n c t i o n of t i m e , h o w e v e r , a straight l i n e is also o b t a i n e d . S u c h a c u r v e indicates a first-order reaction i n w h i c h the rate of strength loss (or degradation) is d i r e c t l y p r o p o r t i o n a l to the a m o u n t of m a terial present at any g i v e n t i m e . 0

dS/dt =

(9)

-kS

w h e r e k is the first-order rate constant. T h e first-order rate constant k can be estimated from the slope of the In S vs. t c u r v e , whose equation is g i v e n by In S =

-kt

+ a

(10)

w h e r e k is the slope of the l i n e a n d a is the i n t e r c e p t o n the S-axis. T h e slope k i n equation 10 is the same as the rate constant i n equation 9 as can be s h o w n b y differentiating e q u a t i o n 10 w i t h respect to i : (1/S) (dS/dt) =

-k


(11)

25.


HERSH ET AL.

437

and dS/dt =

(12)

-kS

A l i n e a r regression of the strengths l i s t e d i n Table I a n d s h o w n i n F i g u r e 3 gives t h e least-squares e q u a t i o n for the loss i n strength as a function of heating t i m e


S(gf) = - 0 . 6 8 4 i ( h )

+ 99.29

( r = 0.9941)

(13)

2

w h e r e t (h) is t h e h e a t i n g t i m e i n hours. T h u s , the strength decreases at t h e rate of 0.684 g f / h w h e n h e a t e d at 150 ° C ; 0.684 g f / h is also t h e z e r o - o r d e r rate constant. T h e strength of the y a r n was r e d u c e d to about 3 0 % o f its original value after 4 days of heating. T h e regression equation that uses t h e natural l o g a r i t h m of the strength a n d generates the first-order rate constant is S(gf) = - 0 . 0 1 1 0 In t(h) + 4.67

(r

2

= 0.9807)

(14)

Table I. Properties of Silk Fabric After Heating at 150 °C Heating Time (h)

Breaking Strength (gf)

Total Color Change, ΔΕ (CIELAB units)

0

100.0

0.00

6

98.3

6.71

12

88.2

11.79

18

84.9

16.24

24

80.4

18.20

30

79.8

20.42

36

79.3

22.32

42

71.1

25.44

48

65.8

26.99

54

58.9

28.78

60

58.0

30.66

66

53.3

31.39

72

51.4

34.15

78

48.0

34.86

84

42.4

36.79

90

39.4

37.79

96

30.9

39.39

Rate of Change Zero-order Range First-order Range

Ammonia Concentration

0

Amino group Concentration 1

11.1

53.9

16.8

66.5

22.1

69.3

32.6

68.3

44.4

72.4

37.1

74.9

37.8

95.5

43.0

105.3

55.5

107.2

c

16.4 0
6

4.35 0 < t < 8

1.48 0 < t < 8

-

-

_

c

c c c c

1

d

rf d d d

d

Rate of Changi Zero-order Range First-order Range

-

14.5% 0 < t < 20

-

"Units are micromoles of NH per gram of silk. Units are micromoles of NH per gram of silk. "Solution was turbid. Sum of ammonia and amino group concentrations. ''Same units as for the corresponding column per day. 3

fe

2

rf


-


25.

H E R S H ET AL.


441

IRRADIATION TIME (days) Figure 6. Strength and total color change of silk fabric as a function radiation time (xenon arc).

of ir-

IRRADIATION TIME (days) Figure 7. Logarithm of strength of silk fabric as a function of irradiation time (xenon arc). In Archaeological Chemistry IV; Allen, Ralph O.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

442


straight l i n e . T h e s e data suggest that m o r e t h a n one m e c h a n i s m m i g h t be responsible for the degradation. T h e In s t r e n g t h - t i m e relationship, h o w e v e r , provides an excellent fit to first-order kinetics as follows:


In S(gf) =

-0.145f(days)

+ 4.821

(r

(19)

= 0.9947)

2

T h u s , w h e n exposed to artificial light, the strength decreases exponentially as a f u n c t i o n of t i m e at a rate o f 14% p e r day. I n contrast, the first-order heat degradation rate at 150 °C (equation 14) was (0.011 X 24 h/day)/0.145) or 1.82 times faster. COLOR D I F F E R E N C E . U n l i k e those samples aged b y heat, the color of those exposed to l i g h t increased m u c h less. T h e measurements are l i s t e d i n T a b l e I I a n d are s h o w n i n F i g u r e 6. T h e regression equation d e s c r i b i n g the total color difference at exposure times of 6 days or longer as a f u n c t i o n of t i m e is g i v e n by: Δ £ «

= 0.221f(days) + 2.716

(r

(20)

= 0.9740)

2

I n this case, the color changed about 0.22 C D U p e r day c o m p a r e d w i t h a rate of 7.03 C D U p e r day for silk heated at 150 °C. T h u s , d u r i n g the l i n e a r p o r t i o n of the aging c u r v e , the color changes about 32 times faster w h e n h e a t e d than w h e n irradiated. AMINO GROUP CONTENT. A l t h o u g h the data are scattered, F i g u r e 4 shows that the a m i n o content increases w i t h irradiation, b u t at a m u c h l o w e r rate t h a n that at w h i c h a m i n o groups are f o r m e d b y h e a t i n g . T h e h e a t e d samples ( F i g u r e 5) contain m o r e amino groups, at constant strength, t h a n do the light-aged samples. T h e relationship b e t w e e n strength and the a m i n o group content of silk d e g r a d e d b y l i g h t is d e s c r i b e d b y SfeOee =

- 3 . 5 5 [ - N H ] ^ m o l / g o f silk) + 283.4

(r

2

2

= 0.8089)

(21)

T h u s , the strength decreases at a rate of 3.55 gf p e r mole of - N H p e r g r a m of silk w h e n exposed to light. T h i s rate is about 3 times faster than that o f silk exposed to heat. 2

AMMONIA CONTENT. T h e concentrations of a m m o n i a present i n the samples are l i s t e d i n Table I I a n d s h o w n i n F i g u r e 4. T h e concentration appears l i n e a r u p to a p p r o x i m a t e l y 40 μπιοί of N H / g of silk (8 days) a n d is described by 3

[ N H U ^ m o l / g o f silk) = 4.345i(days) 3

8

+ 9.172

(r

2

= 0.9906)

(22)

A s i n d i c a t e d i n T a b l e I I , the a m m o n i a concentrations of the samples i r r a d i a t e d 10 days or l o n g e r c o u l d not b e m e a s u r e d because the solutions b e c a m e In Archaeological Chemistry IV; Allen, Ralph O.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

25.

Historical and Artificially

H E R S H ET AL.

Aged Silk Fabrics

443

t u r b i d . T h e reason for the t u r b i d i t y has not yet b e e n d e t e r m i n e d . T h e strength vs. [ N H ] r e l a t i o n is d e s c r i b e d b y the e q u a t i o n 3

S(gf) H ]

Archaeological Chemistry IV - ACS Publications - American Chemical

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