A medium hard cement is made from plaster of Paris, six parts; silex, or fine sand, two parts; dextrine, two parts (by measure). Mix with water until soft; then work with a trowel or knife.
Soft Cement
A good soft cement is made of plaster of Paris, five parts; pulverized asbestos, five parts (by weight). Add water enough to make a soft paste, and use with a trowel or knife. This is a heat-proof compound and is commonly known as asbestos cement.
Very Hard Cement
One of the hardest cements that can be made is composed of hydraulic cement (Portland or Edison), five parts; silex, or white sand, five parts (by measure). Mix with water and use like plaster with a trowel or spatula.
Care must be taken when the parts are combined to see that the cement is free from lumps. These must be broken before the silex, or sand, and water are added. Then the two parts should be mixed together at first and moistened afterwards. The proper way is to place some water at the bottom of a pan; then add the dry mixture by the handfuls, sprinkling it around so that the water can enter into it without forming lumps. Keep adding and mixing until the mass is at the right consistency to work.
All these cements are acid-proof.
Clark’s Compound
For exterior insulation, where the parts are exposed to the weather, a superior compound is made up of mineral pitch, ten parts; silica, six parts; tar, one part (all parts by weight). This is called Clark’s compound, after the man who invented it and used it successfully.
It is heated, thoroughly mixed, and used with a brush or spatula.