What is a printmaking technique which uses acid to inscribe a design on a metal plate?
The art form you are referring to is called "etching." In the process, acid or "mordant" is used.Mordant is a dye with a "polyvalent metal ion" that allowed it to adhere to cloth when it was dyed, and it could be acidic or alkaline. Using acid or mordant allowed the artist to "engrave" a design in metal. This process was used on "unprotected metal" to achieve a certain design. This design created a relief—in other words, the acid would eat away at the desired surface: what was left was a raised pattern in the metal. The pattern that emerged was untouched by the acid, and would have two dimensions, rather than a single dimension, such as a painting. This process made the image appear raised, but because of the technique used, offered more stability to the image that the use of stone would. It was.
...the most important technique for old master prints...
This process, along with engraving, is still a popular process today.
Etching is the basic process in printmaking where acids are used to "etch" a design into the metal surface in preparation for producing the final prints and is a part of the broader category of intaglio printmaking (which refers to the process of cutting into a surface). There are many techniques associated with etching. The process has been around for a very long time and was often employed by many famous artists, including Callot, Rembrandt, and William Blake. The process is still widely used by modern artists. Even though some of the substances used have been slightly changed to reflect modern technologies, the techniques are generally the same.
Aqua tinting is also another form of etching that comes under the branch of printmaking, where it utilizes rosin on a metal plate to resist the acid being poured on the metal plate. This helps to create the effect where different tones are being created, thus the name, "aqua tinting".