A Preliminary Report on the Relative Effectiveness of Ultrasonic Cleaning versus Soaking inthe Conservation of Clay TabletsAuthor(s): Brian LewisSource: Iraq, Vol. 43, No. 1 (Spring, 1981), pp. 76-78Published by: British Institute for the Study of IraqStable URL: http://www.jstor.org/stable/4200134 .

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76

A PRELIMINARY REPORT ON THE RELATIVE

EFFECTIVENESS OF ULTRASONIC CLEANING VERSUS

SOAKING IN THE CONSERVATION OF CLAY TABLETS

By BRIAN LEWIS

This paper reports on an attempt to apply the technology of ultrasonic cleaning to the conservation of clay tablets and to measure the effectiveness of ultrasound

against the desalting technique now in use. It is hoped that the results of these

preliminary experiments will inspire a thorough investigation into the possible advantages of employing an ultrasonic cleaning system. The author would not recommend switching from the standard soaking method to ultrasound before a

good deal more study.

The conservation of cuneiform tablets involves the removal of water-soluble and

less soluble salts present in the clay. This is usually accomplished by baking the

tablets to approximately 15000 F and later washing them in cool, circulating water.

Salt deposits that are not removed through conservation tend to crystallize, damaging the surface of the tablet with its inscription and causing internal disintegration as

well.

According to the practice of the British Museum, four weeks are recommended

for the extraction of salts from cuneiform tablets of average thickness.1 At Yale, the

rule has been to allow 10-14 days for the leaching of water-soluble salts. In both

cases the conservation process is slow and requires three to five weeks alone for

baking and soaking. Additional time must be set aside for brushing residues of less

soluble salts from the surface and for mending broken tablets. Primarily because

of the time factor, long-term programmes involving the conservation of entire

collections have never been practical. Furthermore, unless a way is found to

accelerate significantly the preservation process, many potentially important docu-

ments will suffer irreparable saline damage before they can be made to surrender

their valuable data. The use of an ultrasonic cleaning system for the more efficient

removal of salt deposits in clay tablets might prove an attractive alternative to the

standard practice of soaking.2 Ultrasonics refers to the study and application of high frequency sound waves

1 See R. M. Organ, Design for Scientific Conservation of Antiquities (London, 1968), 201 ; and C. A. Bateman,

" The Treatment of Cuneiform Tablets in the British Museum ", Preservation and Reproduction of Clay Tablets and the Conservation of Wall Paintings, Colt Archaeological Monograph Series, III (London, 1966), esp. 17.

2 The idea of using an ultrasonic cleaning unit to remove surface impurities from clay tablets was suggested to me by Dr. Bernard Jerome, D.D.S., in the course of a conversation precipitated by his keen interest in Biblical and Near Eastern Studies. In

addition, Dr. Jerome graciously volunteered his private ultrasonic unit for, so far as I know, the first test of the effects of high frequency sound on a cuneiform tablet. I owe a special debt of gratitude to Dr. Elizabeth R. Jewell, Assistant Curator of the Yale Babylonian Collection, for her technical advice and assistance. I also wish to thank Prof. Stephen J. Lieberman of the University Museum, Philadelphia and Miss Judy Giordan of the Chemistry department of the University of Maryland for their many helpful suggestions.

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ULTRASONIC CLEANING IN THE CONSERVATION OF CLAY TABLETS 77

beyond the range of human audibility.3 Ultrasonic cleaning units have been in use

for a long time and have been employed in such diverse fields as dentistry, the

jewellery trade, and heavy industry. Ultrasound is an extremely effective way of

cleaning delicate objects that might be damaged by other means. In addition, it

has the advantage of working in a fraction of the time required by more conventional

methods. Ultrasound cleans through the phenomenon of cavitation. The object to

be treated is placed in a liquid medium, the solvent, inside an ultrasonic tank.

Sound waves generated through the medium cause the molecules of the solvent to

expand and contract forming microscopic cavities or bubbles. At a certain point the bubbles collapse, interacting with the surface of the object to rid it of impurities.

In an attempt to verify the effectiveness of the standard soaking procedure, to

ascertain the length of time demanded for the extraction of most salt deposits, and

to develop and evaluate a new technique based on the principles of ultrasonics, the

author embarked on a series of tests, the preliminary results of which he now reports.4 For the purpose of determining the relative efficiency of the standard soaking

procedure versus the ultrasonic technique, an unaccessioned tablet was selected

that was already broken into two pieces.5 One fragment was tested using the current

Results of Halogen Determination6

Standard Soaking ( Weight of Sample : ig'6216 g)

Time

(hours) 24 48 96

144 192 240 286

334 382 430 478 526 574 622 670 718 756 800

Cl present (mg)

??3 2'2

4*1 60

7'9 9*4

II ?? ?2?2

13*2 ?4?? 15*2 ?6??

?7?? ?8??

?9?? ?9?6 20* ?

20-3

Ultrasonic Cleaning ( Weight of Sample : 12-5137 g)

Per cent

weight 0-006 o-oii 0-020

0-030 0-040 0-047 0-056 0-062

0-067 0-071 0-077 o-o8i

0-087 0-091 0-097 0-099 O-102

O-IO3

Time

(hours) 24 48 96

144 192 240 286

334 382 430

Cl present i?g) 31 5*5 6-4 9-2

II -2

13-0 14*5 15*3 i6-o 16-6

Per cent

weight 0-024 0-043 0051

073 ?089 ?103 ?"5 ?122 ?127

0132

3 For a discussion of ultrasound and its practical applications, see Jack Blitz, Fundamentals of Ultra- sonics, 2nd ed. (London, 1967); and Dale Ens- minger, Ultrasonics, The Low- and High-Intensity Applications (New York, 1973). 4 I am very grateful to Prof. William W. Hallo, Curator of the Yale Babylonian Collection, for generously underwriting the cost of testing and for permitting the use of previously unaccessioned tablets from the Collection in these experiments. 6 The break appeared to be of recent origin, and

one assumed that the percentage of salt by weight would be nearly the same in both fragments. An Ur III tablet was chosen simply because of its availability. It measures 3-6 cm ? 3-4 cm X i-i cm; its provenance is Drehern. The tablet has now been accessioned into the Yale Babylonian Collection (YBC 16556). ? The tests were conducted at the Baron Consulting Co., Milford, Connecticut under the supervision of Dr. Harry Agahigian, Ph.D., Chief Consultant.

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78 BRIAN LEWIS

method of soaking. The piece was washed in distilled water at a temperature of

70o F. A magnetic stirrer simulated the effect of circulating water. Samples of

liquid were removed at intervals of 48 hours and checked for halides. The test was concluded when no additional significant amounts of salt were obtained. The other fragment of the tablet was placed in an ultrasonic cleaning unit with distilled water as the solvent. The frequency of the sound waves was 40 kHz and the tempera- ture was kept at approximately 70o F by means of a cooler. Samples of liquid were collected and analysed.

An extract of liquid from the standard procedure was tested for other halides such

as fluorides, iodides, and bromides. None was detected.

Substances other than halides were found to be present in the extract including carbonates, sulfates, phosphates, and nitrates.

Summary and Conclusions

1. Using the soaking technique, over 33 days elapsed before the increase in the

amount of halide extracted became insignificant. The piece yielded a total of

20-3 mg Cl representing 0*103% of the sample weight. A follow-up test was later

conducted to determine whether any appreciable amount of salt remained that had

not been removed by soaking. The fragment was subjected to ultrasonic cleaning for

96 hours7 and the liquid tested. An additional 6 -79 mg of chloride was found,

nearly a third of the amount obtained after 33 days of soaking ! This brought the

total salt removed to 27-09 mg or 0 ? 138% by weight.

2. The fragment washed in an ultrasonic cleaning unit yielded 0-103% of its

weight in chloride (13-0 mg CI) in 10 days as compared to 33 days of soaking. A total of 16-6 mg of chloride was extracted before the amount of salt removed

became insignificant. This figure represents o-132% of the sample weight.

3. No visible surface damage appeared in the fragment originally subjected to

ultrasonic cleaning. At the end of the test period, the surface was found to be

cleaner and the inscription and seal impression more legible than the piece that was

soaked, although some brushing was still necessary.

4. Ultrasonic cleaning of cuneiform tablets appears to be a faster8 and more

efficient way of eliminating water-soluble salts than the method of soaking currently

practised. Although the ultrasonic procedure has yet to be tested fully and refined, the preliminary results are encouraging and offer the hope of a major breakthrough in conservation technology.9

7 At a temperature of 128o F and a frequency of 80 kHz.

8 By one-half to two-thirds of the time spent in soaking. Recent findings indicate that increasing the temperature in the ultrasonic unit to 130o F will accelerate the rate of reaction and reduce the period of salt extraction to one-fourth of the time required by soaking.

9 Some concern has been voiced that ultrasonic treatment might damage or destroy poorly preserved tablets that soaking would not. In an attempt to determine the effects of ultrasonic cleaning on fragile

and friable tablets, a number of fragments from broken tablets were treated by both methods for purposes of comparison. The fragments were all in poor condition, and if one piece was judged to be more brittle than its counterpart (from the same tablet), it was placed in the ultrasonic group. There was not one instance where a soaked fragment showed less deterioration after cleaning than the fragment from the same tablet that was treated ultrasonically. The author intends to publish the complete results of all his tests at a later date.

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