Fig. 44. and Fig. 45.
Dagger sheath before and after treatment by Finkener’s method.

A short digression may be here made in order to discuss the question whether the composition of the bronzes undergoes any alteration. Three analyses[150] of Egyptian bronzes before and after reduction by Finkener’s method show that the change in composition is so slight as to be immaterial. It is of course obvious that greater differences will be seen in the results of the analyses before and after reduction of bronzes which are in an advanced state of oxidation, for in this case chlorine, oxygen, water, and carbonic acid constitute an appreciable proportion of the total weight. But even in these cases the analysis made after the reduction shows very slight variation from that of the original metal.

OsirisOsirisIbis
BeforeAfterBeforeAfterBeforeAfter
Reduction
Tin2·162·274·304·218·668·46
Copper77·8377·4579·6679·7488·5388·75
Lead19·2319·8615·5115·581·691·95
Iron0·120·140·280·240·210·20
Nickel &
Cobalt		0·290·240·200·170·300·29
Arsenic0·170·230·17present0·32present
Antimonytracetracetrace—0·20present

The two latter bronzes were tested qualitatively only for arsenic and antimony, and when the three objects were washed the hydrated tin-oxide described on p. [130] was only found in the case of the Ibis. In this connection it should not be forgotten that slight differences in the quantities may be due to errors in the analysis as well as to a want of homogeneity in the alloy.

Krefting’s Method. This method is similar to that used for the reduction of iron (see page [108]). The layer of oxidized material is removed in several places by filing, hammering, or rubbing with emery cloth until the metal is exposed. The object is then wrapped round with strips of zinc, and placed in a 5% solution of caustic soda. The hydrochloric acid produced in the process of reduction acts upon the soda to form sodium chloride. Here too the greatest care must be taken that the steeping is sufficient.

Personally I prefer Finkener’s method, for potassium cyanide is more easily washed out than soda, and also, although poisonous, is less caustic.

Krefting’s method however has proved of considerable success in some cases, notably in the treatment of some 40-50,000 Roman copper coins at the Berlin Museum. These were, with few exceptions, covered with a crystalline layer resembling green malachite or blue azurite and were quite illegible. Various unsatisfactory attempts were made to clean them with ammonia, with warm and cold acids of different kinds, with acid and iron nails, and by electric current both in an acid solution and in a solution of potassium cyanide. The following method finally proved satisfactory[151]:

Krefting’s Method Applied to Oxidized Copper Coins.

“A thin plate of zinc with a bright metallic surface is perforated with a brad-awl, having a diameter of from 2 to 5 mm., until there are about 50 or 60 holes in each square metre. This is placed with the sharp edges of the holes uppermost on a row of glass rings (or crystallizing dishes will serve the purpose) 20 mm. in height resting upon the bottom of a large glass vessel. The coins, which in this case were 20 mm. in diameter, were then placed on the zinc plate, so that 7 or 8 of them occupy a space of 1 square decimetre. Another similarly perforated plate is laid upon them, and upon this more coins are arranged in the same way, and so on until there are six or eight double layers. A perforated zinc plate is then placed on the top with the sharp edges of the holes turned downwards, and over this a few zinc plates which have been previously used. The whole pile is surmounted with weights or stones resting upon glass rings or inverted glass dishes in order to press the sharp edges of the holes into the closest possible contact with the coins. A 5% solution of caustic soda is then poured over the whole, the immediate result of which is an evolution of gas. The reduction of the coins is usually complete in fifteen to eighteen hours, after which they should be well washed. After several rinsings in cold water they are placed, about 1000 at a time, in a large vessel fitted with a perforated false bottom containing hot water, which should be renewed three or four times every day. After four days the coins are wiped with a cloth and thoroughly dried on a warm oven plate or in a drying chamber at a temperature of about 212°F. [100°C.]. They are then brushed with a bristle brush before a dust extractor, a procedure rendered necessary by the fine metallic dust from the coins, which then assume a light or dark brown colour such as is seen on copper coins which are in actual circulation. The practice of placing the coins whilst still wet into melted paraffin wax at 260°F. [120°-130°C.], which gives a dark appearance even to the brightest, has the disadvantage that the wax prevents the use of sealing-wax for taking impressions, and is therefore not recommended.