Wax covering and phosphoric acid. Also used to etch glass.
In the old days the acid was kept in a wax bottle.

Barbara Jouvernaux wrote:

>Hello everyone,
>
>I recently picked up a lovely tiger cowry engraved with the Lord's Prayer. I
>understand that these were quite popular in the early 1900s and acid was
>used for the etching process.
>My question is: how is the engraving done? I presume that the pattern or
>lettering is somehow covered with a protective layer so that the acid burns
>away the area around it. What acid is used and what is used for the
>protected areas?
>I appreciate any information or links to websites which could shed more
>light on the subject.
>
>Thanks and happy shelling,
>Barbara Jouvernaux
>Walton 2273
>NEW ZEALAND
>
>
>