As it is important that the plaster should not be spread over the surface by passing and repassing the trowel for too long a time, the fastest workman will always be the best one to employ. When sulphate of iron is used, the slabs are of the colour of iron rust; but if linseed oil boiled with litharge be passed over the surface, they assume a beautiful mahogany colour, and offer a certain superficial elasticity to the tread. If a coat of hard copal varnish be added, the colour becomes very beautiful.
On spreading a 2 or 3-in. layer of limed plaster in a room, and treating it in the way above described, is produced a floor which is smooth, and which, in most cases, fulfils the office of an oak floor, but which has the advantage over the latter of costing one fourth. - (Scient. Amer.)
(10) Reducing And Enlarging Plaster Casts
Ordinary casts taken in plaster vary somewhat, owing to the shrinkage of the plaster; but it has hitherto not been possible to regulate this so as to produce any desired change, and yet preserve the proportions. Hdger has, however, recently devised an ingenious method for making copies in any material, either reduced or enlarged, without distortion.
The original is first surrounded with a case or frame of sheet metal or other suitable material, and a negative cast is taken with some elastic material, if there are undercuts; the inventor uses agar-agar. The usual negative or mould having been obtained as usual, he prepares a gelatine mass, resembling the hektograph mass, by soaking the gelatine first, tfyen melting it and adding enough of any inorganic powdered substance to give it some stability. This is poured into the mould, which is previously moistened with glycerine to prevent adhesion. When cold, the gelatine cast is taken from the mould and is, of course, the same size as the original. If the copy is to be reduced, this gelatine cast is put in strong alcohol and left entirely covered with it. It then begins to shrink and contract with the greatest uniformity. When the desired reduction has taken place the cast is removed from its bath. From this reduced copy a cast is taken as usual. As there is a limit to the shrinkage of the gelatine cast, when a considerable reduction is desired, the operation is repeated by making a plaster mould from the reduced copy, and from this a second gelatine cast is taken and likewise immersed in alcohol and shrunk.
It is claimed that even when repeated there is no sacrifice of the sharpness of the original.
When the copy is to be enlarged instead of reduced, the gelatine cast is put in a cold water bath, instead of alcohol. After it has swollen as much as it will, the plaster mould is made as before. For enlarging, the mould could also be made of some slightly soluble mass, and then by filling it with water the cavity would grow larger, but it would not give so sharp a copy.
Stereotyping (iv. 217-28).
While the previous article on this subject conveyed an account of the processes of stereotyping as generally carried on, so much original research and applied science is contained in Prof. Bolas's series of Cantor Lectures on the subject, at the Society of Arts in 1890, as to warrant a supplementary notice embodying his remarks.
An early method, which is worth reproduction on account of its simplicity, was known as " polytype." According to this, the page type, or the original to be copied, is slightly oiled, and fixed face downwards on a block of wood, supported, at some little distance, over a paper or cardboard tray, into which melted type-metal has been poured. Just as the type-metal begins to show distinct signs of solidification, the block carrying the page of type is allowed to fall on the soft metal; and on separating the two, a reverse or mould is obtained. This reverse or mould, being now fixed upon the lower face of the drop-block, is allowed in its turn to fall on the surface of type-metal contained in a paper tray, this metal being at the point of solidification as before. The paper tray is of course crushed in each case, and to regulate the thickness of the " strike," metal gauges are fixed alongside the paper tray, and in such position that the frame or chase containing the original, when down, shall rest upon them.
It is easy to copy this method by the aid of an ordinary stamping press provided with a quick screw. Fig. 296 conveys a good idea of a simple form of apparatus originally used, and which was constructed of hard wood. The arrangement of parts is sufficiently obvious without further explanation, excepting that it may be mentioned that the rod carrying the catch which releases the block should stretch from one standard to the other, the catch being taken out of the staple in the drop-block by putting the wire in torsion. This process is still in occasional use for the rapid reproduction of small typographic ornaments or blocks, but in such cases the matrix is generally a thick electrotype cast made from the original block. For initial letters or ornaments about 1/2 in. square, there is probably no quicker or better method of reproduction than that of striking the matrix into semi-fluid type-metal.
At another period Firmin Didot adopted the plan of forcing the forme of type by dead pressure against a sheet of soft lead, and the matrix or reverse thus obtained served for obtaining printing plates by the method of striking into semi-fluid type-metal just described. It is easy to obtain a reverse in soft sheet-lead by dead pressure; ana if the lead matrix is placed in the ordinary stereotype casting-box, casts may be obtained from it in ordinary type or stereotype metal, • these alloys melting at a lower temperature than the lead. In a similar way a lead matrix may be made by driving an ordinary type punch - ways into a piece of lead, and the lead matrix being adjusted to an ordinary hand-mould of the right body size, it is easy to cast a number of types in it, which are about as good as the original; this being often a very convenient process to follow when a few extra types are wanted in a hurry.