(2557) Blackboard against a wall.

Manufacturer and builder 1, 1880

— The very best thing is to cover the wall with plaster of Paris. As plaster of Paris alone sets too quickly to enable you to make a good job of it, it is better to mix some fine slacked lime with it. Mix the lime first with water, then mix the plaster with water also, and work the two rapidly together, and put it on the wall after first wetting the latter. But perhaps it is better to employ a regular plasterer for this, who can make you a surface as level and smooth as Bristol board. After the wall is well set and dry, paint it with boiled linseed oil and lampblack, and after this give it a coat of whet may be called artificial slate, made by mixing shellac varnish with lampblack and a very little of the finest emery or pumice stone, as to render the surface a little gritty, that it may bo written on with chalk. Without the addition of some gritty substance it would be too smooth to write on; but if you put in too much, the surface will be at rough that it is difficult to obliterate the chalk mark. When the Cooper Union was organized in 1859, we had several thousand square feet of hard finished wall treated in this way, so as to change it into blackboards for the several classes in mathematics and mechanics taught in some eight or ten different rooms, and for the classes in perspective and architectural drawing, and there blackboards gave great satisfaction. After a few seasons the cost of artificial slate had to be renewed, as in time it wears too smooth and loses its deep black color.