A (very) basic understanding of glaze and glaze running.
This post is specifically introducing the tools, techniques , and ways you can mitigate glaze running on top of addressing some misconceptions people have about glaze running. Lets start out with the basics. You shape clay (either by sculpting or on the wheel), it goes through the drying phases; Leather soft, leather, Leather hard, bone dry. When if becomes bone dry clay it is ready to go into the bisque kiln (usually fired at cone 04 or so). After the bisque the water has been driven off by the kiln heat so you have a much more handleable body that will soak up liquid and anything floating in the liquid. This is where glaze comes in. Glaze is very simple in that it’s a suspension of minerals floating in water. You can buy these minerals either online, at a pottery shop, or from a mineral company. Under the heat of the kiln the minerals start to melt at a specific temperature, essentially becoming molten glass at high temperatures and cooling down after the firing process to what we know as finished glaze. One of the materials that compose glaze is a material called silica. Silica is one of the main glass formers in glazes. But in the heating process this goes from the material silica (usually a type of powder) to a glass. This means if your bottoms are not clean or do not leave room to run while turning into glass under the heat of the kiln will make it stick to the kiln shelf and ruin your work. Let’s start off with what glaze is so we can get a better understanding of how these materials turn into glass then move on to the tools that help mitigate glaze running off your pots, ultimately ruining them. Here is an example.
Then we either mix, blend, or sieve the glaze till all the small minerals are well mixed.
This is what glaze is. Minerals floating in water.
The water is just the carrier of the minerals. Your pot absorbs water. The minerals are left on the pot once the water is absorbed.
This simple understanding puts you ahead of a lot of glaze knowledge already. After this it’s business as usual. Just evenly glaze the pot with a brush.
The Glazed pot is then put into the kiln a second time. This is called the glaze fire. This is where the real explanation starts.
When glaze is introduced to heat at a specific temperature (based on it’s mineral composition) it will turn into molten glass and starts to move down the pot. Glaze cannot touch anything but the pot in the kiln. This means your feet (where the kiln shelf touches the pot) cannot have glaze on it or it will stick. Ruining both the piece and the kiln shelf.
In reality you should always test a glaze on a small tile before trusting it on product. But because glazes often runs to some degree. people have made items to help with the problem of running. Lets go over these tools.
These are the most basic types o tools that can help you mitigate running. But these some with one big warning. None of these will save you if you do not clean the bottoms of your pots where the kiln shelf and the glaze would touch.
I want to make this clear from a teachers point of view. None of these tools or methods overtake knowing your glaze and cleaning your bottoms as a measure of keeping your work and kiln shelves safe. Remember to leave plenty of space on the bottom of your pots and do not trust these tools to save you over knowing your glaze and cleaning your bottoms.
