Dictionarium polygraphicum. Secret Writing.

Dictionarium Polygraphicum:
Or, The Whole Body of Arts Regularly Digested.
Vol II.
London: Printed for C. Hitch and C. Davis in Pater-noster Row, and S. Austen in St. Paul's Church Yard. MDCCXXXV.
1735
Secret Writing.
Put powder of alum into water, and what you write the letters will not appear; but put the paper into water, and then you may read it, or juice of spurge will do.

To Write letters that cannot be discovered.
Take a sheet of white paper, double in the middle of it, then cut holes through both the half-sheets, cut the holes like the panes of a glass window, or any other fashion you please; then with a pin prick two little holes at each end, and cut your paper in two halves, and give one half to your friend, to whom you design to write, and keep the other half your self.
When you write, lay your cut paper on half a sheet of wri ting paper, and stick two pins through the two holes, that it stir not, then write your mind to your friend thro' these holes; then takeoff the paper with the holes, and write any thing, what you please, to fill up the vacancy. And when your friend receives the letter, let him lay his cut paper on it, putting the pins into the holes, and then what you wrote not to the purpose is covered, and he discovers your mind.

Another way.
Write the letter with common ink on one side, then turn the paper, and write on the other side with milk what you would have secret, (with a clean pen) and let it dry; then when it is to be read, let the written side be held next the fire, and the milky letters will appear bluish on the other side, and may be perfectly read.

An exquisite method of invisible Writing.
The first ink. Take a pennyworth of litharge of gold or silver unprepared, pound it in a mortar, then infuse it in a vial half full of strong vinegar, shake them well together, and let them stand to settle, and being clear, write upon your paper with a new pen, and it will not appear at all.
The second Ink. Burn cork 'till it has done smoking, extinguish it in aqua-vita, or spirit of wine, dry it, and mix it with Water, and a little gum-Arabick, to the consistence of thin paste; when you would write with it, make it thinner with common water, and write upon what is written with the forementioned ink.
The third Ink. Take yellow orpiment and quick-lime, of each an ounce, pound them in a mortar, and put them in four ounces of common water, and stir them well; this water will take away the second ink, and make what was written with the first appear.

Writing not to be read but in water.
Write with the juice of spurge or alum-water, dry it, and it will not be legible without wetting.

To make black writing vanish, and appear again.
Dissolve burnt tartar in common water, and filtrate it; strike it over the writing, and it will disappear.

To make the writing appear again.
Dissolve an ounce of white vitriol in a pint of water, filtrate it, strike the paper over with it, and the writing will presently appear as before.

To write with ink, which will vanish in five days.
Infuse an ounce of sal-armoniac four or five days in strong water: make of it ink with a piece of touchstone beaten fine, and what you write with it will be gone in five days.

To write with an ink that shall vanish in twenty-four hours.
Boil galls in strong water, put to it some vitriol, a little sal-armoniac, and a little gum-Arabick, and it is done.