They come from only one place in the world, a quarry in Bavaria where the deposits are hard and even. Almost two hundred years ago, a struggling artist-writer looking for a cheap way to print discovered by chance how grease and water behave on limestone. A few drops of oil adhered to some words written with greasy ink but not to areas wet with water. Apparently he had a polished slab around his studio for grinding colors. He reasoned that he could print grease marks from polished limestone if he etched them with acid.
Artists still draw with grease pencils on old-fashioned limestone to make lithographs.
After a lithograph drawing session with Mr. Leo, we need to etch his image pretty much the way lithographers have done for two centuries. Etching chemically locks the drawing into the lithographic stone. When we etch, we use acid mixed with gum-arabic. After we spread it around on the image for a while we rub down the gum to protect the areas that are not supposed to print. Sometimes people ask if we change the artist's drawing when we etch it.