What is Underglazing of Ceramics?
The first ceramics based workshop I have in mind to host this year will be an underglazing workshop, tentatively slated for October 2023 followed by an overglazing workshop probably the following spring around April or May. Keep an eye on my 'Events' menu for workshop dates or join my email list.
What is underglazing? This is a process where pigments, mixed with clay or other silica based ingredients that will fuse when fired in a kiln, are applied to a ceramic bisque to color it. Underglaze by itself can be fragile, especially when raw and unfired. It is usually covered over with a clear glaze to protect it and bring out the richness of the pigments. This is a decorating technique used by Hagen Renaker and many other animal ceramics manufacturers the world over. It is more durable than over glazing, generally less toxic, and requires a higher heat to set, usually the same temperature low fired ceramics are bisque fired at - typically Orton cone 04. Many underglazes can be fired at much higher temperatures and will eventually, when hot enough, melt and become glossy glazes in their own right.
I like to use underglazes because you have very precise control of the finest of details. Unlike overglazes, underglazes DO NOT MOVE around when fired. Overglazes to some degree will melt, blur or bloom softly and it is very difficult to get extremely precise, fine layers of super sharp, crisp detail - especially with repeat firings. Overglazes float on top of the clear glaze or sink into it unless applied directly to non-porous bisque. Don't get me wrong - I use over glazes as well and love their richer colors. I think together, using both under glazing with over glazing allows me the greatest possible flexibility to achieve my desired goals. More about the benefits of over glazes in a future article!
But underglazes do not necessarily retain the super saturated colors that overglazes give to finished pieces. They are less 'what you see is what you get' than over glazes. Underglazes as a rule however are usually less toxic than most over glazes and thus safer for airbrushing. You still want to wear suitable respiratory protection due to the silica content. More about that in an upcoming post.
Underglazes can be masked, etched with individual hair details, layered, and even have very crisp fine hair by hair painted details. I prefer underglazes when I want to set precise white markings, white hair ticking, fine edges to pinto or appaloosa patterns, and even for some hand painted detail work.
The primary drawback to underglazes is that the colors you apply don't look exactly like what you'll get on the finished piece. Colors are paler and sort of washed out until the clear glaze is fired over them to bring out their true hues. Every kind of clear glaze will effect the final color differently - different levels of gloss or matte, or even changing from one brand to another can affect the final color you will get after firing.
Also, the oxygen level in the kiln during firing, as well as the exact temperatures at the location of the fired piece, can cause the underglaze colors to shift hue. So the results can sometimes be unexpected. Testing, trial and error, and note taking are encouraged with using underglazing. When you have access to a kiln it is standard procedure for ceramists to do many test tiles with underglazes with both hand painted and airbrushed layers, paired up with various clear glazes to see how they turn out under your particular firing conditions.
Do you need better reds in your browns that keep firing out too dull or even greenish? Place those pieces higher on shelves in the kiln, pull out your peep holes and maybe even prop your kiln lid ever so slightly open for more oxygen - but beware, it'll take longer to fire that load and cost more electricity to do so. The same /can/ be true for some over glazes that also require more oxygen when firing. This is also true of blacks - adding some red or red brown can help you not to have cold or greenish hues in firing. Remember I mentioned trial and error and note taking? ;)
For our purposes we'll start with masking a bisque and then airbrushing underglaze over the piece. This is why you will need to acquire and practice with using an airbrush and learn the safety precautions so not to breathe in the silica dust it generates. Underglazes can be applied in layers using a brush by hand or a sponge but will not give smooth coats without texture or streaking. There are both opaque and transparent forms of underglazes. I mostly use opaque underglazes but transparent ones do have their uses in adding additional layering.
Below is an example of a bone china bisque with raw, unfired black underglaze with detailing and clean up shown in progress. Airbrushed over the piece it can then be carefully detailed (remember it is very fragile and easy to scratch or scuff!) and once detailed, it is fired to set the first layer. It is not unusual for me to fire pieces several times before I apply the clear glaze. It is also not unusual for me to have to trim my fingernails before working on a piece. I am all the time scratching my underglazes where I don't mean to, or gouging my freshly slip cast greenware. The things we do for ceramics!
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