There are so many purposes to which paper is applied that a small volume might be filled with a description of them. The following are those which will probably prove most useful to the amateur:
Paper in sheets, half of which are gummed on both sides, and the other half on one side, and divided into strips and squares of different sizes by perforations, like sheets of postage stamps, are very convenient in many ways - the doubly-gummed answering for fixing drawings in books, labels on glass, etc. It is stated that the mixture by which it is coated is prepared by dissolving six parts of glue, previously soaked for a day in cold water, two parts of sugar, and three parts of gum arabic, in twenty-four parts of water, by the aid of heat.
This is paper impregnated with a so-called sympathetic ink, which alters its color by a change of temperature. The most delicate substance to accomplish this is sulphocyanide of cobalt, originally proposed by Grotthus. This is prepared by adding an alcoholic solution of potassium Btilphocyanide to an aqueous solution of cobaltous sulphate, until no more potassium sulphate separates. The whole is transferred to a filter, and the residue on the filter (potassium sulphate) washed with alcohol. The dilute nitrate may be used as it is, for impregnating paper, or it may be concentrated by very careful evaporation at as low a temperature as possible. The salt may be obtained crystalline by removing the alcoholic menstruum in the vacuum of an air-pump. It forms violet columns, soluble in water with red color. Paper impregnated with the alcoholic solution, or on which tracings have been made with the latter, turns reddish in dry air, but assumes a blue color at the slightest elevation of temperature.
Lay the paper or engraving, face downwards, on a sheet of smooth, unsized white paper; cover it with another sheet of the same, very slightly damped, and iron with a moderately warm flat iron.
Sometimes it is difficult to get a drawing on a sheet of paper of the ordinary sizes when Btretehed upon a board, by reason of the waste edges used to lecure the paper firmly; and again, in stiff papers, such as the "Eggshell," so called, ordinary mucilage does not possess sufficient strength, and glue has to be substituted, to the annoyance of the draughtsman. The following is a very simple way of obviating these difficulties: First moisten the paper thoroughly; then lay it upon the board in proper position, and, with blotting paper, remove most of the moisture for a distance of half an inch or thereabouts from the edges; then take strips of Manila paper (not too stiff) about one and a half inches wide, covered on one side with mucilage, and paste them down on both paper and board, allowing them to lap on the edges of the sheet about half an inch. Keep the middle of the sheet thoroughly wet until the mucilage on the edges has set, when the whole sheet may be allowed to dry gradually. It will be found that this method is quicker and surer than any other, and is of great use where it is necessary to color on mounted paper.