Q: I want to etch halftone images into silver and gold, like the heads of coins. How deep can I etch before I undercut the little dots?
A: If you’re using a halftone image, your etch depth will be very little, before you start to lose the dots. A true photoetch, using UV-sensitive resist, exposed through a high quality Kodalith negative, is going to give you the crispest results and the best etch. You can get an etch depth that is clearly a three-dimensional surface, but it’s not likely to resemble a coin. Among other things, a coin is modelled, rather than being a flat image with a recessed indeterminate background.
It also depends on what metal you’re etching. Most etching you’ll do on silver or gold is going to tend to want to undercut at least slightly as it etches, which quickly limits how deep you can go with a fine line or halftone image. Proprietary etching technologies used in the printing industries, however—on copper especially—can leave an almost vertical wall. This sort of thing requires, among other things, a good spray-etching machine for best results, which pretty much means send the image and metal to a professional photoetcher for processing, rather than doing it yourself.
One technique often used by custom jewelers is to take the drawing to any decent graphic arts firm and have the image etched on zinc or magnesium sheet stock, as used in printing plates. This generally gives an etch which slopes away from the image, instead of undercutting. Tiny areas simply stop etching at a certain depth, when the opposing slopes meet at a v-groove shaped bottom, but wider areas will be cut away to about a millimeter or a millimeter and a half or so, the sloping sides meeting a reasonably uniform and almost flat bottom of the etched recess. Now, you don’t normally make jewelry out of zinc or magnesium etched plates, but you can make a rubber mold of the thing and then cast the objects in the metal of your choice.
The normal etching sequence, above, produces what I call the “precision” surface, your photo image, at the top of the metal being etched, where it’s then prone to wear and tear and excessive polishing and the like. You can also reverse this, with the precision image surface forming the recessed background of the piece, the raised portions being no specific height off the background, nor even necessarily flat, but simply raised away from the image. To do it, take your etched zinc plate (you had the image etched in reverse, a mirror image, when you had it etched) and make a sandwich with the plate and a piece of soft casting wax sheet (the pink works well) A single layer of Saran wrap acts as a separator. The wax is backed with a sturdy metal sheet, like a mold plate, and both it, and the zinc plate are then backed up with something that won’t bend, like some scraps of plywood, and then the sandwich is crunched together in a vise, which forces the wax into the etched depressions of the zinc plate. When removed, and the Saran wrap is gingerly lifted, the precision surface that was the outer surface of your zinc plate will now be the background of the wax image. When cast, the result is often quite pretty, and tends to be a less mechanical looking piece. If you use a cold vulcanizing press to crunch the sandwich, you can leave out all that reinforcement, using only another piece of Saran wrap on the outside of the wax to prevent it from sticking to the platens…
Another trick is to take your image—coarsely detailed ones work best for this, such as simple lettering or a logo, rather than very fine lines and halftones—and have it made into a rubber stamp. Be sure to specify that the stamp be made to stamp the mirror image, which will mean that looking at the rubber stamp itself you see the right image. Specify that you do not want the rubber stamp actually assembled into a stamp, but just want the rubber piece. If it’s already made up, a razor blade can separate it from the handle.
The stamp rubber can be bent and deformed some, and then simply added to a wax model and burned out and cast along with the wax. Be sure there are no air bubbles in the joint between the rubber and the wax, or you’ll get investment in there when you vacuum the investment. Because the rubber is flexible, you can use this technique to put, for example, precisely lettered printing around the shoulders of a ring or contoured surface. Burnout time will need to be maybe a little longer for the rubber to burn out completely—and be very sure you’ve got decent ventilation in your shop.
by Peter W. Rowe M.F.A., G.G.