In my book Make Your Mark: Explore Your Creativity and Discover Your Inner Artist (Chronicle Books, 2004) (http://www.amazon.com/Make-Your-Mark-Creativity-Discover/dp/B0007ZNV1I/ref=sr_1_5?s=books&ie=UTF8&qid=1416922505&sr=1-5&keywords=make+your+mark) there is a chapter on Invisible Ink. You can paint with lemon juice, milk, and other substances, even Champagne, allow the paper to dry, then bake it in an oven. The sugars in the juice or milk brown at a lower temperature than most papers, and your secret mark will be revealed. You may have done this as a child, writing with a q-tip on paper, and heating it over a lightbulb–sending messages back and forth to your friends this way.
To bake, heat your oven to 350 degrees, and slide a cookie sheet in the oven to preheat as well. When your lemon juice artwork is dry, you can slip it into the oven with tongs. The time it will take for your mark to “develop” in the oven will depend on the kind of paper you used, as well as how much lemon juice you used. Sometimes it will take forever for the marks to look a little bit golden, and then all of the sudden, it will get very dark, very quickly.
In addition to secret writing, you can make art-marks this way, painting with lemon juice with a brush, or spraying it across a stencil. Here, I put lemon juice in a Preval sprayer and sprayed juice across weeds:
You can put lemon juice in a blank stamp pad (you can buy a tray with a lid, that has a soft pad empty of ink at most art supply stores that sell to rubber stampers and scrapbookers, or you can pour a little lemon juice on a paper towel in a shallow dish and use that) and stamp with it, baking it:
You can even make an “inkblot” with lemon juice. Fold and unfold a piece of paper. Drip lemon juice on the folded piece of paper, fold and unfold. Allow to dry, and bake as above:
This is a fun project for kids, too, and useful for school projects where a page has to look aged. My son asked if there was any way to age his colonial recruitment handbill for social studies class recently. We didn’t have lemon juice, but had milk, and between tearing the edges and crumpling the paper a little, with milk sprayed through a plant atomiser, he made a pretty convincing document! Hope I am not raising a forger…