It is said that a mixture of eqxial parts of concentrated glycerine, alcohol and water, deeply colored with aniline black, does not freeze in the coldest weather, flows freely from the pen, and does not spread. Our only fear would be that such ink would not dry thoroughly.
Write With Thin Solution Of Starch, and let the correspondent wash with solution of iodine.
Write With Milk, Onion Juice Or Lemon Juice, and let the correspondent expose to heat.
Write With Solution Of Tartar Emetic and wash with any alkaline sulphuret.
Writing done with a solution of 2 parts of potassium chromate, 2 of nitric acid, 2 of sodium chloride in 40 parts of water, turns yellowish-green on gentle warming.
A solution of equal parts of sodium chloride and cobalt chloride in 20 times the amount of water produces lines which turn blue on gentle warming.
Letters may be written on postal cards with these inks, and will remain invisible until washed with the appropriate solution or exposed to heat. To prevent the letters from being seen by close scrutiny the solutions should be very dilute, and to distract the attention of those not in the secret, write some unimportant matter, in lines far apart, and between them write the private matter in secret or sympathetic ink.
Rub together one part of finest lampblack and 2 parts of Prussian blue with a little glycerin, then add 1 part powdered gum arabic, and enough glycerin to form a thin paste.
Dissolve 24 grains of carmine in 3 fl. oz. of water of ammonia, then add 2 fl. drachms of glycerin. Incorporate with this 1/2oz. of powdered gum arabic.
Rub together 6 parts of pure Prussian blue and 1 part oxalic acid with a little water, to a perfectly smooth paste. Let it stand in a rather warm place over night, then rub it with more water, and with 1 part of gum arabic to a thin paste.
Aniline Inks may be made of any desired shade in the same manner. The best way of using these inks is by applying them, by means of a small pad, uniformly to a little cushion, on which the stamps are then inked.
The above formulae have been tested by experience, and are said to give good results. Another set of formulae, also highly recommended, is the following:
Finest lampblack, 10 parts; powdered guru arabic, 4 parts; glycerin, 4 parts; water, 3 parts. Dissolve the gum arabic in the water, add the glycerin, then rub the lampblack with the mixture in a mortar.
Replace the lampblack in the above formula by the appropriate color; chrome-yellow for yellow; red lead or red ochre for red; green, ultramarine, or chrome-green for green; indigo or Prussian blue, or blue ultramarine for blue; umber for brown, etc.