As, however, the proofs were thoroughly satisfactory to me, I labored desperately to overcome the difficulty. During my student years I had attained much chemical knowledge, and I knew that most of the resinous products which withstand acid, as well as the fats, wax, tallow, and so forth, can be dissolved and diluted partly in etheric oils and spirits of wine, and partly in alkalies. My problem was to obtain a thin mass which would permit itself to be spread very thinly in cold condition over the copper etching surface, dry quickly, become sufficiently firm after drying without getting tough, and, above all, be something that would not attack the etching surface. A few trials with spirits of wine and various resinous forms gave no satisfaction. The one experiment that I made with oil of turpentine and wax also failed, presumably because I diluted the mixture more than necessary, which caused it to flow too much and dissolve the etching surface, at which time several well-done parts of the engraving were ruined. Besides, this mixture dried only slowly to the degree necessary for working. Fortunately I made no further experiments with this material, because then I should not have invented stone-printing, as I know now how to make a cover varnish that is quite satisfactory. I turned, instead, to an experiment with wax and soap, which succeeded beyond all expectations. A mixture of three parts of wax with one part of common tallow soap, melted over the fire, mixed with some fine lampblack, and then dissolved in rainwater, gave me a sort of black ink with which I could correct faulty spots most easily. Now I needed only practice in order to carry out my project of etching my literary productions in copper. This presented a new difficulty. After I had written on my single little copper plate, etched it, and pulled proofs at the house of a friend who possessed a copper-plate press, I had to spend some hours again laboriously grinding and polishing the plate, a process which also wore away the copper fast. This led me to practice on zinc, which was easier to scrape and polish. An old zinc plate of my mother's was requisitioned at once, but the results were very unsatisfactory, because the zinc probably was mixed with lead, and I had used only aqua fortis instead of more powerful acid.