Traditional Powder Process
Early etching methods for zinc and copper were cumbersome and imprecise due to the fact that the chemical removal of metal from non-image areas was performed in all directions. As a result, etching of the plate surface was often performed not only in the desired direction but also resulting in a reduction in the width of the lines and dots of the printed image, as well as undercutting the halftone dots, thus creating a below-surface size smaller than the printed surface. The solution to such problems is to deposit a etchant resistant material about the sidewalls of the etched lines and dots. In Europe, the method known as "gillotage”, which involves rolling waxy ink on the sidewalls of lines and dots, is commonly used. In the United States, the most widely used process is "powdering "process in which a resin powder is brushed onto the sides of partially etched lines and dots and melted by heat to provide a coating that resists etching. The operation is always repeated for several times - etching, application of protective material, and etching again - before sufficient depth is achieved. The result of this process depends on the skill of the operator and on ambient conditions such as temperature and relative humidity, as these conditions affect the performance of the powder.
Innovative solution- powderless process
An important breakthrough in solving this problem was the introduction of a powderless etching process for magnesium plates. Experimenters found that by adding an oily material and a surfactant (wetting agent) to the nitric acid bath and controlling the etching conditions of the plate, they could produce relief characters with sufficient etching depth and with no loss of printed area during the etching process. Later, this process was adapted to zinc etching, which was soon adopted by engravers around the world.
With the major obstacles to zinc and magnesium etching overcome, attention turned to copper, and in 1954 it was discovered that a powderless etching process for copper could be achieved by adding an organic compound (thiourea) to the iron chloride etchant. Further improvements to the process were subsequently made and new compounds were introduced to be added to the etching bath.
