Changing the color of Crystalline Glazes after the Initial Firing
I employ two techniques or "tricks" to alter the colors of my crystalline glazes. Both involve taking a completely finished piece and re-firing it. The re-fire temperature must be fairly low -- if I were to take the temperature too high, I would run a risk of melting and destroying the crystals that were formed in the initial firing.
The piece to be re-fired is completely cool, and it goes into dedicated post-fire kilns which are also completely cool. The finished pieces are truly subjected to two completely different, distinct firings.
This is my reducing kiln (see below). If it looks low-tech, that is because it is. A 30 gallon trash can is lined with 2" of high-temperature ceramic fiber insulation that is held in place with wired-on homemade ceramic buttons. Flames from a natural gas raku-style burner enter a 3-1/2" diameter hole near the bottom. Exhaust gasses exit through a 5-1/2" hole in the lid that is partially covered with a brick which serves as a damper. Temperature is monitored with a battery powered digital pyrometer in the side. Reduction is monitored visually by how much smoke exits the flue.
The bottom 10" of the kiln acts as a firebox. The stackable (usable) area of the kiln is only 13" diameter by 14" high. It is hard to maintain an even temperature from top to bottom in such a small kiln. I intend to acquire a larger reduction kiln with thicker insulation.
High-fired ceramics are very susceptible to thermal shock, much more so than unfired clay is. This means that I must take the secondary firing very slowly, and it often takes longer than the initial firing.
Crystalline glazes are particularly suited to these techniques because crystalline glazes begin melting much sooner than typical high-fire glazes. Again, I do not want to heat my glazes to a point where I begin ruining the preexisting crystals -- but I do want to soften the glazes enough so that chemical reactions can take place in them. Under the right circumstances, these chemical reactions affect and change some of the colorants in my work.
"Striking" or "Striking the Color"
Striking is a glassblowing term that merely means re-heating a piece. I don't really understand the science or chemistry of it, I just know that it works, often with very dramatic results. How great the change is depends on the colorant. Some colorants change a lot, and others hardly change at all.
The temperature that I usually strike at is the exact same temperature that applied gold lustres mature at, so I often combine striking the color with applying gold rims and bands to my work.
Reducing refers to firing a piece in an oxygen-starved atmosphere. As the gas burns it consumes all of the oxygen in this kiln, creating the reducing atmosphere. Oxygen that is chemically combined to the glaze is drawn out -- as the oxygen is drawn out, the metal colorants in the glaze literally migrate through and are deposited on the glaze surface. Like striking, reducing alters the color of some, but not all glaze colorants. Sometimes pure metallic reflective surfaces are created.
Striking, and especially reducing, are extremely unpredictable and notoriously un-reproducable techniques. Results range from incredible to incredibly disappointing. It can be very frustrating, or very rewarding. In spite of the high failure ratio, I keep going back to these tricks because they are responsible for many of my very best pieces.
MORE: This page is still under construction. In the near future I intend to add before and after pictures of struck and reduced pieces. I will also add links to more information about striking and reducing. I will make a note on my "New Work" page (also known as my blog or kiln-log page) when these additions have been made.