The acid applied with the brush provokes a reaction on the plate which is in function of the concentration of the mordant and the amount of time it is left to bite. It is quite easy to experiment by putting a few drops of mordant, each at a different concentration, onto a plate and letting them bite. One may also notice the effect of time by much the same means. It is only by printing that the results obtained will be obvious. On the other hand a practiced etcher can judge the results without having recourse to printing and he can use the acid as if it were an ink.
Powdered sulphur is mixed with oil until the consistency of a paste is reached.
Spread this paste in successive layers using a brush. In any case, all of these techniques are used to bring about various tints rather than blacks even though the latter can be achieved with high concentrations or a series of superimpositions. Be careful, however, not to destroy the characteristics of brush etching by a brutal etching technique which would be in contradiction with the velvety texture of the wash tints achieved by brush etching.