There are many ways to alter the appearance of the crystalline glaze after the piece has been finished. If the glaze is too dark and the crystal can be seen only under bright light or with very close examination, acid etching may help brighten the crystal and create greater contrast between the crystals and the ground color. Even if there is no back ground and the piece is covered with a solid patch of crystals, this acid bath process can make the crystalline structure easier to see and render a wider range of colors within the crystal. Some glazes are formulated with an excess of copper, cobalt, and other metal oxides and is intended to be acid etched, as with Jose' Mariscal's "Galaxy Glaze" formulations. Galaxy Glazes - before acid etching - appear to be black and the crystals and very difficult to see but after etching, it reveals an amazing array of crystals and other secondary crystalline structures.
The acid we used in this demonstration was Sodium Bisulphate, which we bought at a pool supply store (used to balance the PH level in pools). If you want to be on the safer side, vinegar is another acid choice that can be used but instead of hours, it may take days to achieve the desired results. If you want faster results, a stronger acid such as Muriatic Acid can be used, but this is more dangerous and etches very quickly and lead to over etching.
The amount of etching time can vary from glaze to glaze and depends greatly on the strength of the acid. In this example, we etched in Sodium Bisulphate for about an hour. When we first started etching, we would etch a piece for 15 minutes, wash and dry, record the results and then etch for another 15 minutes and repeat until we got the desired result - often we would go just one more 15 minute segment of etching and ruin the piece but for the next piece, we would know when to stop.
As you can see in the before and after photos here, the crystals are much easier to see and appreciate and the color contrast is much greater.
Should you decide to give this technique a try, it is important to begin with short intervals of acid etching - once the piece is etched, the effect cannot be reversed - it is possible to over etch the piece. If the piece is over etched, the crystal begins to loose definition and the surface becomes a bit chalky - much like sand blasted glass. In the case of a glaze with very high copper content, the background can shift from a dark green or black to a white with a silky/satin surface quality. If you get the piece wet after it has been etched, the color will temporarily return to a darker color and the white disappears , once dry, it returns to the lighter colors and white surface. This is important to point out to a customer, if they are not aware of this color shift when wet, they might see this as a defect and return the piece - we have had this happen.
Check out the video below - we demonstrate the acid etching process - it is magical to watch the transformation. Let us know what your thoughts are and if you have any questions.
Before Acid Etch - stronger colors, toward the bottom, the color becomes darker and we loose the contrast between crystal and ground.
After Acid Etch - lightens the color and increases the contrast between crystal and ground.