Well, I made a mistake. I took Dunninger at his word. In his Monument to Magic, in a small blurb, Dunninger states you can paint a solution of Niter onto paper and when it invisibly dries, cause the writing to turn brown by putting a hot matchhead to the words.
Unfortunately, after I wrote a routine into my television show using this, I tried it. This doesn't work. Not only does it not work, it spectacularly does not work. I've certainly learned a lesson here (don't script until trying) but I need something that does work now.
Anyone have suggestions (besides lemon juice) on how to make invisible writing appear on paper when flame/heat is applied?