In etching, what substance is typically used to protect the metal plate before applying acid?

In the process of etching, a protective layer is applied to the metal plate to prevent acid from corroding areas that should remain untouched. This layer is known as an acid-resistant ground. It is typically composed of a waxy substance or other materials that can withstand the corrosive nature of the acid used in etching, effectively acting as a barrier. When the artist applies the acid, it will only affect the areas of the plate that are not covered by this ground, allowing for controlled engraving and the creation of intricate designs.

The other substances mentioned do not serve the same purpose. Oil-based ink, for example, is primarily used in the printing phase to transfer the image onto paper and does not provide the necessary protection needed during the acid application. Gum arabic is often used in various printing techniques and can act as a binder or emulsifier, but it does not have the properties needed to protect the plate efficiently in etching. Wood glue, although an adhesive, does not have any relevance in the etching process as a protective ground. This highlights the specific role that acid-resistant ground plays in preparing the plate for etching.