Methods & Aims in Archaeology

by W. M. Flinders Petrie

Where the stucco is thicker, about 1/16 inch, but wholly shattered into minute chips, none over ¼ inch across, a different treatment was necessary, as at Medum. The mass of plaster and stucco was laid face down, the mud cut away behind it till about a square inch of shattered plaster was bared at the back; this was covered with a thin coat of fresh plaster (mixed in the palm of the hand); then another square inch was bared and coated, and so on, until the whole of the mud was removed and the old stucco all lay smeared with a thin coat of fresh plaster on the back. A large slate was then cut to size; a pudding of liquid plaster was poured on to the stucco and pressed out as thin as could be with the slate. When it was set, the old painted stucco was thus securely cemented on to the slate; light, tough, and portable, it travelled to America in perfect state.

The third method is where the surfaces are curved. By cutting away the back as thin as is safe, and setting in a firm backing of cement, even this difficult subject may be dealt with, and removed safely.

Gold.

Metals do not require much treatment in the field; but it is needful to understand the condition of them in order to know how they can be safely treated. Gold should be cleaned as little as possible, as the old red surface is the best appearance of it; a little brushing with camel-hair brush and plain water to remove the dust is generally enough. Where there is much silver in it, as in electrum, the surface is dark with chloride of silver; this may be removed with strong ammonia or cyanide of potassium. Gold-foil often requires straightening out into its former shape, but it must not be burnished in so doing, as that expands the form.

Silver.

Silver is one of the most troublesome metals, as it is so very readily attacked by chlorine and sulphur; and, moreover, it undergoes a colloidal rearrangement by which it breaks readily into irregular curved grains, and it is in this state as rotten as rotten brass. If deeply corroded nothing can well be done to it; the lumpy crust shows more of the original form than the metal would show if bared. When the corrosion is but slight it may be removed, either by solution in strong ammonia or cyanide of potassium, or by reduction. To bring the chloride into the state of porous metal, it is only needful to place it with zinc or iron in a solution of salt or weak vinegar or lemon juice, and in a few hours the whole of the chlorine has gone over to the fresh metal. The powdery silver left can be mainly brushed away in water, and a little picking with a bone point will loosen it entirely. Of course, the whole of the silver removed has come out of the body of the metal, which is left porous and tender, although the face may be unbroken. It will not bear, therefore, the same cleaning as new and strong metal. In the case of silver coins in fine condition, each coin should be reduced separately, and the whole of the old silver weighed with it before cleaning it away, so as to recover the original weight. Silver must never be put bare in a tin box, as the chlorine forms chloride of tin, which deliquesces, and then attacks the iron and stains the silver with brown rust. Often there is both chloride and lime on the surface, and alternations of ammonia and weak acid are required for cleaning.

Copper.

Copper objects are distinguished from bronze by retaining usually their pliability. This renders them much easier to clean, as they are seldom deeply corroded, and the red oxide upon them will generally flake off clean by blows, and leave the original face in perfect condition. A very light hammer should be used, and sharp scaling blows be given, so as to flake off even half-an-inch breadth of scale at once, without ever touching the old face. In hollows which cannot so easily be struck, an iron nail may be used as a punch, and struck so as to crush the red oxide little by little. A copper object which scales freely is a treat to clean, as the old face can be entirely bared, and appears of a beautiful red-brown colour with all the detail quite perfect. Very thin copper may, however, have entirely passed into green carbonate, if buried in a damp soil; and in this case nothing can be done except washing off the earth and dirt.

Bronze.

Bronze and brass need much more care than copper, as they contain a mixture of alloys of very different oxidability; hence much of the material all through the mass will have moved up to the surface and been corroded there, while the form and size of the original may at present contain only half the metal in a very porous and brittle condition. In some cases bronzes may be scaled by blows like copper, and they then appear in their best condition. But more often they are too brittle, or the corrosion adheres too tightly, for it to be thus removed. For cleaning off small quantities of green carbonate, vinegar left to stand for some days does well. But the proper solvent of both carbonate and oxide is dilute hydrochloric acid, about 1 to 10 or 20 of water, as this will not attack the metal, but only the corroded parts. The objection to this solvent is that it leaves a thick mud of white oxy-chloride of copper, which is difficult to brush off, and which stains the skin green in handling. The treatment is to brush off as much as can be easily removed, and then pickle in hyposulphite of soda, which dissolves the white coat; if used hot and strong this will clean the metal to a bright metallic condition. After all these solutions, a long washing in many waters for two or three days is needed to remove all trace of salts which might afterwards make further corrosion. Minute traces of chlorides are specially dangerous, as they decompose with carbonic acid in the air, forming carbonate, and liberating the chlorine to attack more metal; thus a trace of chloride will eat through any amount of copper. The extent to which bronzes should be cleaned, should be ruled by the fullest display of original workmanship: so long as more detail can be shown more crust should be removed. But, if possible, some of the coat of red oxide should be left on plain parts as a guarantee of the age of the work. To bare bronzes entirely, and then oil and smoke them, is barbarous treatment, to be seen in some museums. If something is desired over the bare metal, the bronze may be left in a shallow pan of water, soaking for some weeks, by which it will gain a tinge of red oxide over it which is suitable and pleasing. Another mode of scaling is to heat the bronze over a fire or in melted lead, and then plunge in cold water, which loosens the scale from it. It often happens that a bronze has the original face broken up by corrosion, and then no cleaning is of any use, the mass of green carbonate shows more than any other surface would do. This last and worst state is indicated by cracks in the outer coat, due to further expansion of the inner body. A cracked bronze is best left alone.