Methods & Aims in Archaeology

by W. M. Flinders Petrie

Casting.

Fig. 33. Paper squeeze. XII Dyn. Goddess Nekheb.

Casting.

Fig. 34. Plaster cast from paper. Philistine. XX Dyn.

Casting.

Beside the direct material for publishing in plates, it is often desirable to take casts and impressions, both for future reference and also as a step toward a photograph ([Fig. 34]). The making of paper impressions or squeezes has already been noted. Casting with plaster of Paris is the principal mode of reproduction, and is such a detailed business in itself that only a few notes can be given here, such as might possibly be wanted in field work. The fine work for museum purposes is outside of our aim here. The main point in handling wet plaster is rapidity; and for that everything must be ready, and the exact plan of work and amount of plaster settled beforehand. A basin should be used with water equal to about two-thirds of the volume of plaster required. Into this shake or sift dry plaster rapidly, until the water is just filled up with it, and no free water left on the top; it is then well proportioned, and should be violently stirred with a large flat spoon or slip of wood and poured out in an even stream, beginning with the middle if a flat mould, and flattening it out to the edges. It is best to have rather too little than too much; as a fresh lot can be mixed, with the hardened pieces of the first lot, to serve for a backing; the first lot being, of course, spread over the whole face to begin with. Strings, or strips of butter-muslin, should be put through the mass, if it is large, so as to prevent it falling to pieces if broken later on. Excellent casts are made with a thin skin of plaster on a backing of muslin put on a frame; but this requires more skill than plain work. About 10 minutes after casting the back should be scraped down level, or planed with a wide-mouthed hand plane, which is a very useful tool in finishing casts. No cast of any large size should be left without even support for some hours after casting, as it will settle out of shape if strained. Small quantities of plaster are best mixed with a pocket knife in the palm of the hand.

Moulds for casting are usually of clay for a large scale, but that is not likely to be used in the field-work. The division of the clay is best done by bedding threads along the face of the object at the lines required, and then pulling them up to cut the clay. The face of the object requires French chalk (steatite powder) on it to prevent cohesion; oiling or greasing spoils the face of the original. For field-work paper moulds are best, and the preparation of these as wet squeezes has been already described. To fit the squeeze for use as a mould, it should be heated and brushed with melted beeswax on the face, without necessarily soaking it through. Any places that are shiny when cold should be warmed and rubbed with cotton wool, so that the face is the true paper cast. Then slightly oil between each plaster casting, or else the warmth of the setting plaster will make the wax stick to it. Several casts can be taken from one paper, if it is carefully handled in peeling it from the plaster each time. Paper impressions of cylinders are best made with blotting-paper, unrolled wet, and left to dry. To remove the cockling of drying, spread a thin coat of stiff paste on card, and press the paper squeeze lightly on it.

Guttapercha moulds are best if many copies are required. To get a sharp impression in this tough material a preliminary mould should be made, of the right shape, but not sharp on the face. This should be thoroughly cooled in water for an hour or more, and then a small quantity of guttapercha from boiling water should be laid in the hard mould and the object pressed in very rapidly and with maximum pressure. Thus the hot material is forced firmly against every part and takes a brilliant impression. Such moulds are used for electrotyping as well as for plaster work. To produce a smooth face to a lump of hot guttapercha, it should be pulled outwards from the middle to all sides by thumbs and fingers, so as to produce a fresh torn face over the whole upper surface.

Sealing-wax is one of the handiest materials, and is used professionally for all the coin reproductions that are published. Only the best wax is of any use for impressions. It should never be allowed to burn or blaze, nor even to boil, but should be gently heated until a large mass will fall quite readily. The object should be wetted moderately just before impressing. So soon as the wax is tough the object should be lifted slightly to make certain that it has not stuck, and then pressed down again till cold. If it has stuck it must be pulled away at once, and the wax picked off while tough. Sealing-wax casts must be oiled before plaster is put to them; and oil does not soften or deteriorate sealing-wax if left on for years. Beeswax, or, better, the mixture called “dentist’s wax,” makes good impressions, and may be used for moulds.