Taking a piece of glass, smooth board, marble or oil cloth, brush the surface with a little salad or lubricating oil, or linseed oil. There should be no free oil on the surface or streaks of oil as such will cause an uneven surface. When using glass it is possible to insert a diagram or pattern of the shape underneath on a piece of paper as a guide to the bars or retaining walls of the mold. These retaining bars or walls may be of various materials. Strips of wood or plaster are excellent and strips of linoleum, metal and even glass are used. If four pieces of wood about ten or twelve inches long and one and one-half inches wide are used they can always be adjusted to fit any dimension from twelve inches down, by being placed as shown in the diagram.
TO OIL THE RETAINING BARS, brush the surface to come in contact with the plaster and then hold the wood in position over the diagram below, using modeling wax or clay to keep it in position. The clay of course should always be in position on the outside of the wood and should never be in the space into which the plaster is to be poured.
PLASTER HARDENS SLOWLY IN COLD WEATHER and hardens rapidly in a warm temperature. Salt added to plaster will cause it to set more rapidly and to harden more firmly. No exact proportion can be given—just a little sprinkled in a pan of plaster will cause it to set more rapidly.
DIFFERENT PLASTERS SET AT DIFFERENT PERIODS. As has been described before, casting plasters can be secured in quick-setting, medium-or slow-setting mixtures. Medium-setting or slow-setting will be found to be good average mixtures for use.
TO RELEASE THE PLASTER TILE after it has hardened (generally a half hour will insure the hardening action as being completed), the bars can be released and the tile gently lifted at one corner will cause it to come apart from the oiled surface. If glass is used the glass can be placed upright and the separating of the tile from the glass can be watched as indicated by the moisture suction disappearing as the tile is gradually separated. If oilcloth is used the tile can be turned over and the oilcloth peeled off easily. This of course is necessary only where any sticking occurs, for most times the tile will separate easily. If sticking does occur it is generally some fault of the oiling for it needs only one or two little spots overlooked to cause considerable trouble as the plaster will stick to any part that has been skipped in the oiling.
Plaster dries rapidly and will dry in the sun more rapidly. When plaster is damp it can be scraped or carved easily. Temperature and the age of the plaster affect its drying periods.
LARGE TILE MOLDS should be strengthened by having burlap strips or wire imbedded into the back while the plaster is soft. This creates a stronger layer than where plaster alone is used. Wood strips should not be used for backing plaster unless thoroughly dried as otherwise it causes cracking by its shrinkage, and it is best not to use it for reinforcing.
TO CORRECT FAULTS IN PLASTER CASTS use a little of the plaster scraped from the back to fill in holes or defects. If a corner or portion is broken off, gouge a hole or cavity so that a little plaster mixed and placed on that section will become firmly connected. As it hardens, a little of it can then be scraped with a knife to connect correctly with the surrounding parts or surface.
A SECOND PRACTICE PROBLEM is to take the plaster tile and incise a line pattern in the surface. To make the incising easier, dip the tile in water and then trace the design previously prepared onto the plaster. Pressure alone on the paper with a pencil will make an indentation on the plaster that can be easily followed in the incising.
THE DESIGNS FOR INCISED PATTERNS are best where the lines enclose a shape. The parts are more comprehensive and confusion of the lines will not result if simple outlines are used. In the chapter on Design, line patterns are shown that are adaptable to incised work.