Archaeological Ceramic Study Possibilities with a Thermal Gradient

4 Archaeological Ceramic Study Possibilities

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with a Thermal Gradient Furnace F R E D E R I C K R. M A T S O N Department of Anthropology, The Pennsylvania State University, University Park, Pa. 16802

Changes with increasing firing temperatures in some physical properties of clays sampled near archaeological

sites can

bedetermined by firing clay test bars in a thermal gradient furnace and by refiring strips cut from potsherds. A furnace built for this purpose is described, and its possible uses in archaeological

ceramic studies are discussed. An example

is cited of the changes in color, scratch

hardness, and

shrinkage

village

of clays now used by Egyptian

when test bars were fired in the thermal gradient

potters furnace.

The results can be compared with the same physical properties of potsherds. the ancient Egyptian

They aid in a better understanding of potters' problems of working with the

clays and help to establish criteria for classifying

archaeo-

logical ceramic materials.

/

T h e range of colors f o u n d i n a n c i e n t p o t t e r y at o n e site has f r e q u e n t l y i

* ·• l e d archaeologists to classify their m a t e r i a l s i n t o r e d w a r e , y e l l o w ware, gray ware, a n d the like. surfaces

have

also s e r v e d

T h e colors of t h e p a i n t e d o r s l i p p e d

as classificatory aids.

U n f o r t u n a t e l y these

differences i n the fired colors o f the c l a y vessels are t e c h n o l o g i c a l v a r i a b l e s that m a y have been caused b y the time-temperature-atmosphere c o n d i tions u n d e r w h i c h t h e w a r e w a s fired. T h e y d o n o t necessarily h a v e c u l t u r a l significance w h i c h c a n b e u s e d to classify p o t t e r y unless d i s t i n c t l y different r a w m a t e r i a l s w e r e u s e d w h i c h fired to different colors. T h e q u a n t i t y a n d size of C a C 0

3

grains i n clays w e a t h e r e d f r o m limestone

m o u n t a i n s o r h i l l s , s o l u b l e salts present i n t h e clays, a n d t h e rate o f d r y i n g o f t h e p l a s t i c c l a y w a r e c a n a l l influence t h e u l t i m a t e fired c o l o r as m u c h as o r even m o r e t h a n the i r o n content of the c l a y a n d t h e f o r m i n 34

Beck; Archaeological Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

4.

MATSON

Ceramic Study with Thermal Gradient

35

Furnace

w h i c h i t is present. S l i g h t v a r i a t i o n s i n the c l a y sources, m a n u f a c t u r i n g p r o c e d u r e s ( w h i c h c a n i n c l u d e the m i x i n g together of t w o or three clays a n d a p l a s t i c ) , k i l n d e s i g n a n d o p e r a t i o n , a n d fuels u s e d b y the potters i n a s i n g l e p o t t e r y m a k i n g v i l l a g e c a n result i n c o n s i d e r a b l e v a r i a t i o n i n the a p p e a r a n c e of the wares p r o d u c e d .

M a n y archaeologists n o w r e c o g

n i z e that i t is necessary to b e f a m i l i a r w i t h t h e d r y i n g a n d firing p r o p erties of the clays a v a i l a b l e at or near t h e i r sites b e f o r e one attempts to differentiate wares i n terms of t h e i r b o d y c o l o r a n d t h e a p p e a r a n c e

(or

a c t u a l p r e s e n c e ) of a s l i p or of t h e p a i n t e d d e c o r a t i o n , let alone i d e n t i f y some pieces as i m p o r t e d . W i t h t h e a i d of color illustrations H a r r e l l a n d R u s s e l l

(J)

have

s h o w n some of the c o l o r changes that c a n o c c u r i n several c l a y b o d i e s w h e n the f u r n a c e a t m o s p h e r e is v a r i e d f r o m o x i d i z i n g to r e d u c i n g a n d w h e n its m o i s t u r e content is c o n t r o l l e d . B r o w n e l l ( 2 )

has s u m m a r i z e d

t h e roles of s e v e r a l factors i n f l u e n c i n g t h e d e v e l o p m e n t

of

scummed

surfaces o n b r i c k a n d other s t r u c t u r a l c l a y p r o d u c t s . M a t s o n ( 3 ) has d i s cussed the r e l a t i o n s h i p of the firing t e m p e r a t u r e s of a n c i e n t N e a r E a s t e r n p o t t e r y a n d the colors d e v e l o p e d together w i t h other p h y s i c a l p r o p e r t i e s of the wares. Thermal

Gradient

Furnaces

T h e r m a l g r a d i e n t furnaces are u s e d i n g e o c h e m i c a l studies of p h a s e relationships a n d i n c e r a m i c test w o r k , b u t t h e y h a v e not b e e n u s e d i n a r c h a e o l o g i c a l studies. T h e r e are t w o b a s i c types of furnaces.

I n the

first, r e f r a c t o r y c e r a m i c h o l l o w cores are s p i r a l l y w o u n d e x t e r n a l l y w i t h p l a t i n u m or a l l o y w i r e s , the s p a c i n g of the w i r e b e i n g c o n t r o l l e d to produce

the t e m p e r a t u r e g r a d i e n t d e s i r e d .

This

core, p a c k e d

i n an

i n s u l a t e d c o n t a i n e r , is e l e c t r i c a l l y h e a t e d w i t h a v a r i a b l e i n p u t t r a n s f o r m e r . C o m b u s t i o n boats c o n t a i n i n g the samples c a n b e i n t r o d u c e d i n t o the f u r n a c e .

I f a t m o s p h e r i c c o n t r o l is d e s i r e d , a q u a r t z t u b e c a n

be

i n s e r t e d i n t o the core, the test pieces are i n s e r t e d , a n d the gas for t h e c o n t r o l l e d a t m o s p h e r e is i n t r o d u c e d at t h e c o l d e n d . W i t h a r e m o v a b l e t h e r m o c o u p l e p r o b e the g r a d i e n t t h r o u g h the l e n g t h of the f u r n a c e c a n b e easily m e a s u r e d . T h e s e c o n d f u r n a c e is a v a r i a n t of t h e

first.

It is

u s e f u l for l a r g e r test pieces a n d is a w e l l i n s u l a t e d k i l n w i t h a l l of t h e heat a p p l i e d b e h i n d the c l o s e d e n d of a h o r i z o n t a l muffle. S u c h furnaces h a v e b e e n u s e d to s u r v e y p o s s i b l e c l a y sources i n the b r i c k i n d u s t r y i n terms of color d e v e l o p m e n t a n d degree of v i t r i f i c a t i o n . p r o v i d e d a t a u s e f u l i n e s t a b l i s h i n g firing schedules.

T h e y c a n also

B o t h types of f u r

naces c a n be u s e d to s t u d y the m a t u r a t i o n of glazes a n d o v e r g l a z e colors. The

c o r e - w o u n d f u r n a c e is often u s e d i n the glass i n d u s t r y for d e v i t r i f i

c a t i o n studies.


36

ARCHAEOLOGICAL CHEMISTRY

Figure

1.

Test bars of clay from Jarmo in northeastern Iraq tempered phin and with salted (15 wt % NaCl) water

with

F i r i n g tests of clays c o l l e c t e d i n the M e d i t e r r a n e a n a n d N e a r E a s t e r n countries h a v e b e e n d o n e b y the a u t h o r for some years. B r i q u e t t e s m a d e f r o m these clays w e r e

fired

to successively

h i g h e r temperatures

e s t a b l i s h e d s o a k i n g p e r i o d s a n d at times w i t h r e d u c i n g atmospheres These

experiments

have provided

data concerning

with (3).

changes

in

color,

s h r i n k a g e , p o r o s i t y , a n d hardness of the clays w i t h respect

to

firing

t e m p e r a t u r e a n d t i m e . H o w e v e r , t h e y represent t h e r m a l step s a m p l i n g s , a n d several firings are u s u a l l y n e e d e d to c o m p l e t e a s t u d y . W i t h a t h e r m a l g r a d i e n t f u r n a c e , u s i n g l o n g test bars m a d e f r o m the clays or slices c u t f r o m potsherds, v a r i a t i o n s i n k i l n t e m p e r a t u r e c o n d i t i o n s closer to those e n c o u n t e r e d b y the a n c i e n t potters c a n b e r e p l i c a t e d . T h r o u g h the c o o p e r a t i o n

of J o h n W o s i n s k i of the C o r n i n g G l a s s

W o r k s , bars m a d e f r o m t w o N e a r E a s t e r n clays c o l l e c t e d i n n o r t h e r n I r a q w e r e , after a p r e l i m i n a r y firing to 5 5 0 ° C f o r 3 h o u r s to r e m o v e t h e c h e m i c a l l y c o m b i n e d w a t e r i n t h e c l a y a n d the i n i t i a l fired

firing

shrinkage,

i n the C o r n i n g laboratories i n furnaces n o r m a l l y u s e d for d e v i t r i f i -


4.

Ceramic

MATSON

c a t i o n studies.

Study with Thermal Gradient

37

Furnace

T h e clays w e r e p r e p a r e d i n t w o w a y s to c o n f i r m the

color changes a l r e a d y e s t a b l i s h e d i n the step firings of b r i q u e t t e s .

One

s a m p l e of e a c h c l a y was t e m p e r e d w i t h tap w a t e r ; the other w a s t e m p e r e d w i t h w a t e r c o n t a i n i n g 15 w t % s h o w n i n F i g u r e 1.

NaCl.

T h e color changes of the

( S o m e of the earliest p o t t e r y yet k n o w n w a s b e i n g

m a d e at J a r m o before 6000 B . C . T h e site is w e s t of S u l i m a n i y a h i n n o r t h eastern I r a q . )

T h e c l a y w a s l i g h t b r o w n at the l o w e r

it o x i d i z e d to a l i g h t r e d , a n d t h e n t u r n e d p a l e y e l l o w .

temperatures, A t the h i g h

temperatures w i t h i n c i p i e n t v i t r i f i c a t i o n a green color d e v e l o p e d the f o r m a t i o n of c a l c i u m ferrosilicates i n the c l a y b o d y . expected, friable.

the s a l t - e n r i c h e d c l a y t e n d e d

to

flocculate

with

As would

and was

be

quite

P a l e y e l l o w colors a p p e a r e d at l o w e r t e m p e r a t u r e s t h a n w i t h

the n o r m a l c l a y , a n d v i t r i f i c a t i o n o c c u r r e d at the h i g h e r t e m p e r a t u r e s , m a k i n g i t difficult to r e m o v e the vitreous samples f r o m t h e p l a t i n u m c o m b u s t i o n boats.

Furnace

for

Archaeological

Studies

R e c e n t l y it was p o s s i b l e to construct for a r c h a e o l o g i c a l studies a t h e r m a l g r a d i e n t f u r n a c e s i m i l a r to one u s e d i n the c e r a m i c laboratories of the C o l l e g e of the E a r t h a n d M i n e r a l Sciences of T h e P e n n s y l v a n i a State U n i v e r s i t y . R o b e r t F r a n t z m o d i f i e d the d e s i g n of the e a r l i e r f u r n a c e so that i t w o u l d b e s u i t a b l e for testing a r c h a e o l o g i c a l clays a n d sherds that are a v a i l a b l e o n l y i n s m a l l quantities. T h e t h e r m a l g r a d i e n t f u r n a c e n o w b e i n g u s e d for a r c h a e o l o g i c a l c e r a m i c studies contains a 2 0 - i n c h l o n g S i C muffle that is D - s h a p e d i n cross section a n d is closed at one e n d . T h e flat b a s e of t h e mufflle is 3 % inches w i d e , a n d the m a x i m u m h e i g h t of the a r c h is 2 % inches. F o u r S i C h e a t i n g elements 20 inches i n l e n g t h w i t h a n 8 - i n c h hot z o n e are p l a c e d h o r i z o n t a l l y , one a b o v e the other, i n a C - s h a p e d g r o u p i n g b e h i n d the closed e n d of the muffle. A v a r i a b l e e l e c t r i c p o w e r i n p u t 2 4 0 - V t r a n s f o r m e r w i t h a n o u t p u t of 0 - 2 8 0 V , 56 a m p s a n d 15.7 K V A is u s e d to heat t h e elements. T h e i r t e m p e r a t u r e is m e a s u r e d w i t h a P t - P t / R h t h e r m o c o u p l e p l a c e d b e t w e e n the elements at the closed e n d of the muffle. T h i s t h e r m o c o u p l e is c o n n e c t e d to a p y r o m e t r i c c o n t r o l l e r so that the rate of t e m p e r a t u r e rise, i f d e s i r e d , a n d the h o l d i n g t e m p e r a ture at t h e h o t e n d c a n b e r e a d i l y m a i n t a i n e d . T h e t e m p e r a t u r e g r a d i e n t w i t h i n the muffle is m e a s u r e d b y 12 c h r o m e l - a l u m e l t h e r m o c o u p l e s s p a c e d 1 % inches a p a r t a l o n g the l e n g t h of the side w a l l , % i n c h a b o v e t h e floor level. A m u l t i p l e - p o i n t r e c o r d e r is u s e d to p r i n t out the r e c o r d for these 12 couples. T h e i n s u l a t e d f u r n a c e , 19 inches w i d e , 32 inches l o n g , a n d 23 inches h i g h , is s u p p o r t e d o n a w h e e l e d m e t a l cart. T h e c o n t r o l a n d indicating instruments, i n c l u d i n g an ammeter and a voltmeter, are i n s t a l l e d o n the cart p a n e l b e n e a t h the k i l n . F i g u r e s 2 a n d 3 s h o w the a p p e a r a n c e of the f u r n a c e , a n d F i g u r e 4 shows the i n t e r i o r of the muffle after i t has b e e n l o a d e d w i t h test bars.


38


M a n y e x p e r i m e n t a l firing schedules c a n be d e s i g n e d for t h e f u r n a c e . I d e a l l y , the f u r n a c e s h o u l d b e m a i n t a i n e d at the d e s i r e d t e m p e r a t u r e s , b u t this presents p r o b l e m s of t h e r m a l shock w h e n c l a y test specimens are i n t r o d u c e d a n d r e m o v e d . A s a m p l e firing s c h e d u l e is s h o w n i n F i g u r e 5. T h e m a x i m u m desired temperature ( 1 0 8 0 ° - 1 1 0 0 ° C ) was attained i n 3 h o u r s at the h o t e n d of the muffle ( t h e r m o c o u p l e 12, w h i c h is ΐ4 i n c h f r o m the e n d w a l l ) . C o u p l e 1 w h i c h is 4 % inches w i t h i n the o p e n e n d , a p l u g f o r m e d f r o m i n s u l a t i n g b r i c k o c c u p y i n g the first 2 % inches, h a d not yet r e a c h e d 4 0 0 ° C at the e n d of the first 3 hours. A f t e r a n a d d i t i o n a l 3 h o u r s , the t e m p e r a t u r e s at t h e r m o c o u p l e s 8 - 1 2 h a d s t a b i l i z e d . T h e s e five couples c o v e r a d i s t a n c e of a b o u t 5 ΐ 4 inches. T h e temperatures at couples 1-7 c o n t i n u e d to rise, b u t at s l o w e r rates after the first 4 h o u r s . T h e g r a d i e n t across the 12 couples at the e n d of t h e 6-hour

Figure 2.

firing

Thermal gradient furnace with control panel


was

MATSON


Figure

Figure 4.

3.

Thermal gradient furnace couple distribution

Furnace

showing

Muffle of thermal gradient furnace bars

thermo-

loaded with test


40


TIME - HOURS

Figure 5. A time-temperature firing schedule of the thermal gradient furnace at the 12 thermocouple points of measurement 4 7 5 ° C over a d i s t a n c e of 15 inches. T h i s p r o v i d e s a n a m p l e t e m p e r a t u r e r a n g e to evaluate a c l a y . I f o n e wishes to h a v e t h e c l a y b a r at its h o t e n d a p p r o a c h c o m p l e t e v i t r i f i c a t i o n , a n d there is a d e q u a t e p r o t e c t i o n f o r t h e muffle, t h e h o l d i n g t e m p e r a t u r e at c o u p l e 12 c a n safely b e i n c r e a s e d to 1150°C.

T h i s firing s c h e d u l e has b e e n c i t e d to i n d i c a t e the shortest t i m e

i n w h i c h satisfactory results c a n b e a c h i e v e d w h e n t h e k i l n is at r o o m temperature

at t h e start of t h e

firing.

A

slower

heating

schedule

is often m o r e d e s i r a b l e a n d of course extends t h e l i f e of t h e h e a t i n g elements. T h e rate of c o o l i n g c a n , i f necessary, b e c o n t r o l l e d as w e l l . I f the p o w e r is t u r n e d off at t h e e n d of t h e 6-hour

firing,

the temperature

i n t h e muffle d r o p s r a p i d l y , a n d t h e samples c a n b e r e m o v e d after 3 - 4 hours of c o o l i n g .


4.

Ceramic

M A T S O N


41

Furnace

T h e test pieces f o r the firing c a n b e p r e p a r e d i n m a n y f o r m s , d e p e n d i n g u p o n t h e i n f o r m a t i o n sought.

W h e n s t u d y i n g clays i t has b e e n c o n

v e n i e n t to f o r m bars w h o s e p l a s t i c l e n g t h is 7 inches a n d w h o s e cross section is no m o r e t h a n % i n c h . T w o s u c h b a r s , p l a c e d one b e h i n d the other, p r o v i d e a s u i t a b l e test s t r i p of c l a y .

T h e y c a n be p l a c e d o n a

r e f r a c t o r y slab or i n l o n g c o m b u s t i o n boats.

B e c a u s e of the s m a l l w i d t h

of the b a r s , f o u r clays c a n b e test fired at one t i m e w i t h o u t s t a c k i n g . S t a c k i n g c o u l d cause t r o u b l e i n the hot z o n e i f some of the clays b e g i n to v i t r i f y . O n e d i f f i c u l t y w i t h l o n g n a r r o w bars is that s o m e t e n d to w a r p badly during firing

firing

i f t h e y are f o r m e d f r o m clays that d e v e l o p

high

s h r i n k a g e . F o r s u c h clays a series of m u c h shorter bars p l a c e d i n a

l i n e one b e h i n d the other, or l a r g e r bars w i t h a s q u a r e cross a b o u t 1 i n c h w i d e c o u l d be u s e d .

section

S t r i p s c u t f r o m one l a r g e p o t s h e r d

w i t h a d i a m o n d i m p r e g n a t e d s a w a n d p l a c e d i n a g r a d i e n t l i n e c a n also be tested i n the f u r n a c e . Archaeological

Test Problems

W h e n t r y i n g to e s t a b l i s h t h e m a x i m u m a n d m i n i m u m

firing

tem

peratures r e a c h e d b y a n c i e n t potters a n d to estimate the d u r a t i o n of t h e firings,

d a t a o b t a i n e d f r o m test bars m a d e of the l o c a l clays a n d f r o m

strips c u t f r o m potsherds, a l l fired i n the t h e r m a l g r a d i e n t f u r n a c e , are essential.

Careful

evaluation must be

shrinkage, a n d porosity

measurements,

made

of

together

the with

color, a

hardness,

pétrographie

analysis of m i n e r a l o g i c a l changes t h a t h a v e o c c u r r e d i f a t h o r o u g h s t u d y is to b e c o n d u c t e d .

I d e a l l y , a l a r g e series of sherds s h o u l d b e s t u d i e d

before the test pieces are selected for r e f i r i n g , a n d one s h o u l d h a v e some k n o w l e d g e of the v a r i a b i l i t y i n the l o c a l c l a y . S i n c e potters w o r k i n g i n M e d i t e r r a n e a n a n d N e a r E a s t e r n v i l l a g e s t o d a y d o not fire t h e i r w a r e s u n d e r c o m p l e t e l y u n i f o r m c o n d i t i o n s , there is n o reason to expect that a n c i e n t practices w e r e a n y different.

T h e r e f o r e i t is essential that one

h a v e some k n o w l e d g e of the r a n g e i n p h y s i c a l v a r i a b i l i t y of the potsherds f r o m e a c h t i m e l e v e l i n a w e l l - e x c a v a t e d site. S o m e of the factors that influence the q u a l i t y a n d a p p e a r a n c e of the v i l l a g e wares p r o d u c e d t o d a y are the state of r e p a i r of the k i l n , the size a n d p o s i t i o n i n g of t h e pieces i n the k i l n l o a d , the k i n d a n d a m o u n t of f u e l u s e d ( seasonal a v a i l a b i l i t y a n d c o s t ) , the w e a t h e r c o n d i t i o n s at different times of the y e a r ( w i n d s , r a i n , frost, or h e a t ) , the k n o w l e d g e a n d s k i l l of the m a n firing the k i l n ( u s u a l l y the p o t t e r ) , a n d the q u a l i t y d e m a n d s of the m a r k e t for w h i c h the w a r e is p r o d u c e d .

T h e m a x i m u m firing t e m p e r a t u r e i n v i l l a g e k i l n s

s u c h as those just d i s c u s s e d varies f r o m a b o u t 800° to 1000 ° C .

Occa

s i o n a l l y 1050 ° C is r e a c h e d b u t is not m a i n t a i n e d . S l a g g i n g a n d s l u m p i n g result f o r most e a r t h e n w a r e p r o d u c t s i f yet h i g h e r temperatures attained.


are

42


W h e n one is c o n c e r n e d w i t h p o t t e r y fired o n a h e a r t h i n a p i l e of b r u s h or p a c k e d i n other o r g a n i c m a t e r i a l s u c h as d u n g cakes, the m a x i m u m t e m p e r a t u r e s a t t a i n e d for a n y significant t i m e w i l l b e m u c h l o w e r — u s u a l l y 6 0 0 ° - 9 0 0 ° C , a n d often less.

T h e a n c i e n t p o t t e r y m a d e b y the

I n d i a n s of the A m e r i c a s a n d b y peoples i n several other parts of t h e w o r l d was

fired

i n this m a n n e r .

Some

I n d i a n s of the s o u t h w e s t e r n U n i t e d

States, for e x a m p l e , c o n t i n u e this p r a c t i c e t o d a y .

Vessels are at times

i n t e n t i o n a l l y b l a c k e n e d at the e n d of the firing w h i l e they are s t i l l hot b y s m o t h e r i n g t h e m i n o r g a n i c m a t e r i a l s . Sherds f r o m vessels fired i n t h e w a y s just m e n t i o n e d c a n b e u s e f u l l y refired at the cooler e n d of the t h e r m a l g r a d i e n t f u r n a c e after the o r i g i n a l color changes f r o m the

surface

t h r o u g h the core h a v e b e e n n o t e d for a n a d e q u a t e series of sherds. T h e r m o c o u p l e p r o b e measurements b y S h e p a r d ( 6 )

of I n d i a n p o t

tery b e i n g m a d e i n the s o u t h w e s t e r n U n i t e d States s h o w e d

maximum

temperatures i n the range 6 2 5 ° - 9 4 0 ° C , the latter b e i n g most e x c e p t i o n a l . P o t t e r y m a d e i n P a p u a a n d N e w G u i n e a , a c c o r d i n g to L a u e r ( 7 ) , fired

f r o m 650° to 920 ° C .

20 m i n u t e s after the firing

firing

T h e highest temperatures are r e a c h e d

was about

begins, b u t t h e y are n o t m a i n t a i n e d . T o t a l

t i m e varies f r o m a b o u t 16 to 114 m i n u t e s . S o m e p o t t e r y w i t h w e l l - o x i d i z e d surfaces that has b e e n either k i l n

or h e a r t h fired m a y h a v e d a r k gray to b l a c k cores. W h i l e this c a n result f r o m l o w t e m p e r a t u r e short firing treatment, it m a y also b e c a u s e d the too-rapid

firing

of

fine-textured

b l a c k c o r i n g of some b u i l d i n g b r i c k ) .

W h i l e the c o r e color c a n result

e n t i r e l y f r o m the presence of c a r b o n , b l a c k F e 0 3

4

m a y also be responsible.

A f t e r s u c h w a r e is refired, t h e r e w i l l often b e a r e s i d u a l color

difference

b e t w e e n the core z o n e a n d the surfaces t h a t m u s t b e c o n s i d e r e d estimating original

firing

by

w a r e ( a s i t u a t i o n analogous to t h e

temperatures.

Iron i n a reduced

when

state, p a r

t i c u l a r l y F e O , c a n exert a fluxing a c t i o n o n the c l a y b o d y at r e l a t i v e l y l o w temperatures.

A s a result, sherds after a n o x i d i z i n g r e f i r i n g , even

to 9 0 0 ° - 1 0 0 0 ° C , m a y never d e v e l o p

t h e o p t i m u m color i n t h e i r cores

t h a t is f o u n d i n the test bars of c l a y . T h e surface

flashing

of t h e w a r e

a r i s i n g f r o m flame i m p i n g e m e n t a n d other factors i n the o r i g i n a l

firing

c a n also affect the surface color i n w a y s w h i c h are difficult to r e p r o d u c e c o m p l e t e l y i n a test f u r n a c e . T h e surfaces of a n c i e n t p o t t e r y w e r e often d e c o r a t e d w i t h a f e r r u ginous s l i p t h a t c o u l d b e i n t e n t i o n a l l y fired to a r e d or a b l a c k color.

The

c o l o r c h a n g e w a s the result of not o n l y k i l n a t m o s p h e r e a n d t e m p e r a t u r e , b u t at times also of the degree of v i t r i f i c a t i o n of the s l i p . C o l o r v a r i a t i o n s i n b o t h the p a i n t e d d e c o r a t i o n a n d i n the c l a y b o d y of sherds f r o m one a r c h a e o l o g i c a l site a n d t i m e are s h o w n i n F i g u r e 6. T h i s series of sherds w a s e x c a v a t e d at T e l l H a l a f i n n o r t h e a s t e r n S y r i a a n d comes f r o m p o t t e r y


4.

M A T S O N

Ceramic


43

Furnace

m a d e a b o u t 5000 B . C . A c o m p l e t e t e c h n o l o g i c a l analysis of this w a r e s h o u l d start w i t h t h e s t u d y of t h e v a r i a t i o n s i n c o l o r of t h e b o d y a n d p a i n t , t e x t u r a l a n d m i n e r a l o g i c a l c o n s i d e r a t i o n s , r e f i r i n g tests, a n d i f pos sible, c o m p o s i t i o n a l analyses of t h e p a i n t a n d b o d y w i t h t h e a i d of a s c a n n i n g e l e c t r o n m i c r o s c o p e . O n c e t h e sherds h a v e b e e n s t u d i e d v i s u a l l y , r e f i r i n g tests i n t h e t h e r m a l g r a d i e n t f u r n a c e c a n g r e a t l y accelerate t h e analysis a n d a i d i n i n t e r p r e t i n g t h e a p p e a r a n c e of this a t t r a c t i v e p o t t e r y .

Figure

6.

Potsherds from Tell Halaf in northeastern Syria with varying grees of slip-painted and body oxidation

de-

M o r e t h a n 3000 years after H a l a f times t h e G r e e k potters p e r f e c t e d the t e c h n i q u e s u s e d b y t h e H a l a f i a n s a n d a c c u r a t e l y c o n t r o l l e d t h e p r o d u c t i o n of b l a c k a n d r e d s l i p d e c o r a t e d w a r e s .

S o m e of t h e p o s s i b l e

t e c h n i q u e s f o r p r e p a r i n g slips u s e d i n p a i n t e d d e c o r a t i o n s u c h as those f o u n d o n G r e e k vases h a v e b e e n d i s c u s s e d b y W i n t e r (4).

T h e inclusion

of p o t a s h f r o m l e a c h e d w o o d a s h i n the finely f r a c t i o n a t e d c l a y a n d t h e m a n n e r of firing s t r o n g l y i n f l u e n c e d t h e a p p e a r a n c e of t h e fired w a r e . T h e rate o f c o o l i n g of test pieces after firing m u s t b e c o n t r o l l e d f o r o p t i m u m color to b e d e v e l o p e d .

I n c o o l i n g the test bars f r o m r e d - b u r n i n g

N i l e m u d c o l l e c t e d at Q e n a , E g y p t that are seen i n F i g u r e 7, t h e colors v i s u a l l y c h a n g e d d u r i n g t h e last 200 ° C of c o o l i n g as r o o m t e m p e r a t u r e


44


w a s a p p r o a c h e d . H a d the c o o l i n g b e e n too r a p i d , the i r o n present w o u l d not have fully oxidized. T h e t h e r m a l g r a d i e n t f u r n a c e c a n also b e u s e d to c h e c k t h e a r c h a e o l o g i c a l significance of the colors of m o d e r n b u i l d i n g b r i c k , h o l l o w b u i l d i n g b l o c k s , a n d roofing tiles m a d e t o d a y i n regions w h e r e a n c i e n t p o t t e r y is b e i n g e x c a v a t e d a n d s t u d i e d . It is easy to o b t a i n samples of t h e clays n o w i n use, the k i l n s c a n be v i s i t e d , a n d the c o l o r r a n g e i n the

Figure

7.

Test bars of Egyptian clays fired in the thermal furnace

fired

gradient

p r o d u c t s c a n q u i c k l y b e e s t a b l i s h e d . F i g u r e 8 shows thé c o l o r v a r i a t i o n s i n a stack of b r i c k fired i n a scove k i l n n e a r B a g h d a d . O n e c a n o b s e r v e the v a r i a t i o n s f r o m t a n to r e d , the y e l l o w - t o - w h i t e surface s c u m o n some b r i c k , a n d the c o n c h o i d a l l y s p a l l e d y e l l o w - g r e e n o v e r t i r e d corners of b r i c k t h a t w e r e too n e a r the flames. T h e t h e r m a l g r a d i e n t testing of clays u s e d


4.

Ceramic

M A T S O N


45

Furnace

Figure 8. Color variations in contemporary brick formed of Tigris River deposited clay and fired in a scove kiln near Baghdad

for c o n t e m p o r a r y c e r a m i c p r o d u c t s a n d the s t u d y of the i n d u s t r i a l w a r e s themselves is d e s i r a b l e , e s p e c i a l l y w h e n the a n c i e n t p o t t e r y is l i m i t e d i n q u a n t i t y or c a n n o t b e e x p o r t e d f r o m t h e c o u n t r y i n w h i c h i t w a s excavated. Egyptian A

Clay Studies

specific

archaeological

ceramic

problem

that has b e e n

partly

s t u d i e d u s i n g the t h e r m a l g r a d i e n t f u r n a c e is that of the v a r i a t i o n i n the p h y s i c a l properties of the clays u s e d b y the v i l l a g e potters of

Egypt

t o d a y — t h e N i l e m u d together w i t h clays f r o m the eastern o r w e s t e r n deserts.

T h e a n c i e n t potters h a d the same r a w m a t e r i a l s , a n d t h e y p r o

d u c e d m a n y c e r a m i c wares.

B y s t u d y i n g the clays i t has b e e n possible

to d e t e r m i n e w h y t h e a d d i t i o n of N i l e m u d to the desert clays i m p r o v e s t h e i r w o r k i n g p r o p e r t i e s a n d w h y vessels m a d e of N i l e m u d alone t e n d to b e coarser.

T h e results h a v e b e e n r e p o r t e d i n d e t a i l elsewhere

T h e test bars of the N i l e m u d w e r e r e d d i s h b r o w n ( 5 Y R 5 / 4 ) M u n s e l l c o l o r n o t a t i o n at the 6 0 0 ° C z o n e of the g r a d i e n t (2.5YR 5/6)

(5). i n the

firing,

red

at 8 5 0 ° C , a n d d a r k e n e d w i t h i n c i p i e n t v i t r i f i c a t i o n to w e a k

red (2.5YR 4/3)

at 1 1 0 0 ° C after a h o l d i n g p e r i o d of 30 m i n u t e s .

The

s c r a t c h hardness, u s i n g M o h s ' scale, i n c r e a s e d f r o m 3.0 to 6.5 for the " m a l e " ( b e t t e r w o r k i n g c l a y i n the p o t t e r s t e r m i n o l o g y ) , a n d f r o m 2.3 to 5.0 for the " f e m a l e " m u d . T h e N i l e m u d s h r a n k far m o r e w h e n


fired

46


t h a n d i d t h e other clays—cf. t h e lengths o f t h e test bars i n F i g u r e 7 — for t h e y w e r e a l l t h e same l e n g t h ( 7 i n c h e s ) w h e n f o r m e d .

T h e clay

f r o m B a l l a s , n o r t h o f L u x o r , w a s f r o m t h e w e s t e r n desert, w h i l e t h a t f r o m Q e n a , also n o r t h o f L u x o r , a n d f r o m H e l w a n , just s o u t h o f C a i r o , w e r e o b t a i n e d east o f t h e N i l e . A l l three areas p r o v i d e d calcarous clays Ithat fired f r o m l i g h t r e d d i s h b r o w n at t h e l o w e r t e m p e r a t u r e s to v e r y p a l e b r o w n at 1 1 0 0 ° C . T h e y h a d v a r i a b l e degrees o f s h r i n k a g e , b u t a l l w e r e f a r less t h a n that o f t h e N i l e m u d . T h e B a l l a s c l a y , a n a n c i e n t sea b e d deposit, d e v e l o p e d a hardness o f 7 at 1 1 0 0 ° C , b u t the other b r o w n Clays h a d a hardness o f o n l y 3. T h u s o n e c a n r e a d i l y u n d e r s t a n d w h y potters c o m b i n e clays w h e n p r e p a r i n g t h e p l a s t i c b o d y . T h e clays u s e d a n d t h e i r p r o p o r t i o n s , together w i t h t h e a d d i t i o n o f aplastics, v a r i e d i n t h e several p o t t e r y m a k i n g v i l l a g e s . A n u n d e r s t a n d i n g o f these p h y s i c a l p r o p e r t i e s h e l p e d i n i n t e r p r e t i n g t h e f a b r i c a t i n g a n d firing c o n d i t i o n s of e x c a v a t e d E g y p t i a n p o t t e r y f r o m t h e same r e g i o n s ; f u r t h e r , t h e y m a y m a k e i t possible t o i d e n t i f y wares t r a n s p o r t e d some distance i n a n c i e n t times f r o m t h e i r places o f m a n u f a c t u r e .

S u c h studies l e a d to a better

u n d e r s t a n d i n g o f past t e c h n o l o g i c a l p r a c t i c e s a n d h e l p to e s t a b l i s h effec tive criteria for sorting a n d classifying the voluminous ceramic y i e l d from E g y p t i a n a r c h a e o l o g i c a l excavations.

Conclusion T h e selective uses o f t h e t h e r m a l g r a d i e n t f u r n a c e i n a r c h a e o l o g i c a l c e r a m i c studies m u s t b e d e t e r m i n e d t h r o u g h t h e close c o l l a b o r a t i o n o f the archaeologist r e s p o n s i b l e f o r t h e c e r a m i c s a n d t h e t e c h n o l o g i s t d o i n g the l a b o r a t o r y w o r k .

U n l e s s e a c h p r o b l e m is c l e a r l y defined a n d t h e

a r c h a e o l o g i c a l c l a y samples a n d potsherds h a v e b e e n a c c e p t a b l y c h o s e n , firing

studies c a n at best b e p a r t o f p r e l i m i n a r y surveys. T h e y s h o u l d b e

m u c h more.

T h e y s h o u l d h e l p us to u n d e r s t a n d t h e significance o f t h e

v a r i a t i o n s f o u n d i n p o t t e r y e x c a v a t e d a n d t h e d e g r e e of t e c h n o l o g i c a l k n o w l e d g e a n d c o n t r o l d e m o n s t r a t e d b y t h e potters w h o m a d e t h e w a r e s as p a r t i c i p a t i n g m e m b e r s of a n a n c i e n t c o m m u n i t y . Literature

Cited

1. Harrell, George O., Ralston Russell, Jr., "Influence of Ambient Atmosphere in Maturation of Structural Clay Products," Ohio State Univ. Eng. Exp. Sta. Bull. 204 (1968). 2. Brownell, W. E., "Scum and Its Development on Structural Clay Products," Structural Clay Products Institute, Chicago, Ill., Research Report No. 4 (1955). 3. Matson, Frederick R., "A Study of Temperatures Used in Firing Ancient Mesopotamian Pottery," in "Science and Archaeology," Robert H. Brill, Ed., pp. 65-79, M.I.T. Press, Cambridge, Mass., 1971.


4.

M A T S O N


Furnace

47

4. Winter, Α., "Die Technik des griechischen Töpfers in ihren Grundlagen," in "Technische Beiträge zur Archäologie," Vol. I, pp. 1-45, Römish-Germanisches Zentralmuseum zu Mainz, 1959. 5. Matson, Frederick R., "Technological Studies of Egyptian Pottery—Modern and Ancient," Cairo Solid State Conf., 2nd, April 1973, in press. 6. Shepard, Anna O., "Ceramics for the Archaeologist," Carnegie Inst. Wash., Publ. (1965) 609,

77-93.

7. Lauer, Peter K., "Preliminary Report on Ethnoarchaeological Research in the Northwestern Massim, T.P.N.G.," Asian Perspectives (1973) 14, 69-75.

RECEIVED July 20,

1973.


Archaeological Ceramic Study Possibilities with a Thermal Gradient

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