There are three kinds of photographic enlargements used as a basis for crayon portraits, and, with a little experience, the student can determine for himself which kind will prove the most satisfactory.

Free-hand crayons are made on Steinbach and other crayon papers, without any photograph as a basis. Silver enlargements are made on paper coated with a solution of chloride of silver, which the action of the light reduces to salts of silver. This is the oldest form of photography, and has been used since its introduction by Scheele in 1778. Silver enlargements are made by the aid of the sun (and are then called solar enlargements) or they can be made with the electric light.

Platinum enlargements are a recent advance in photographic printing with iron salts, the process which has been worked out and patented by W. Willis, Jr., being a development of such printing. Its principle is that a solution of ferrous oxalate in neutral potassium oxalate is effective as a developer. A paper is coated with a solution of ferric oxalate and platinum salts and then exposed behind a negative. It is then floated in a hot solution of neutral potassium oxalate, when the image is formed.

This process was first introduced by Mr. Willis in 1874, and he has since made improvements. He claims that the platinotype paper does not contain any animal sizing. The early experiments convinced him that the paper upon which the image was to be printed would prove an important factor, as all photographic paper contained animal sizing, which was found to be antagonistic to platinum salts. The action of platinum salts upon a paper containing animal sizing gave it a tint which no amount of acid washing could remove. For the past nine years Mr. Willis has had manufactured for his special use a Steinbach paper, free from the animal sizing, and he also uses a cold developer, thereby causing the paper to retain its original elasticity.

The chief points of difference between bromide enlargements and silver or platinum enlargements are that, in the former, we have the sensitive compound of silver suspended in a vehicle of gelatin, and, in the latter, a thin coating of an aqueous solution of the sensitive salts. In the former process, the image is not shown until the paper has been developed in the bath, while in the latter, the image is shown upon the paper when it is exposed to the light; so that, in the latter, the image or picture has only to be fixed or made permanent, while in the former, it is developed, then fixed. The gelatin bromide paper is coated with a solution of gelatin, bromide of potassium and nitrate of silver, developed with a solution of oxalate of potash, protosulphate of iron, sulphuric acid and bromide of potassium and water, and fixed with hyposulphate of soda. It is manufactured in America by E. and H. T. Anthony & Co. and by the Eastman Dry Plate Company.