Materials Monday - Glaze Vocabulary
- , by Haley Stanaford
- 4 min reading time
This post is kind of a sequel to the Raw Material Vocab post. Let’s talk glazes! Here’s some common vocabulary you may here when discussing glazes that you’ll need to know.
Food Safe
Food safety is very important! It refers to whether the ceramic piece is safe to eat and drink out of. Many factors can cause something not to be food safe including but not limited to certain glaze ingredients and glaze defects (more on that next). Commercial glazes will always say on the packaging whether they are food safe or not, if used in the correct conditions!
Glaze Defect
Glaze defects are strange things that happen to a glaze after firing. These defects make your wares not food safe! Some common ones are crazing, crawling, pinholing, and shivering. You can read more about defects and how to stop them here.
Flux
Fluxes are ingredients that make the glaze melt. More flux in a glaze causes more melt and the ability to melt at lower temperatures. Less flux in a glaze means no melt, therefore no food safe ceramics!
Glass Former
Glass formers are ingredients that, well, form glass in glazes. Glazes are essentially glass structures that melt to seal the ceramic and make it food safe. The amounts of glass formers in a glaze compared to other materials make a glaze have different finishes like gloss or matte.
Low Fire
Low fire glazes are glazes that will work best at Cone 06-04 which, according to the Orton Cone Chart posted by Skutt, is 1828-1945°F. When using low fire glazes, make sure to pair them with low fire clay bodies for the best fit.
Mid Fire
Mid fire glazes are glazes that will work best at Cone 5-6 which is hotter than low fire glazes. The Orton Cone chart says cone 5-6 is 2167-2232°F. When using mid fire glazes, make sure to pair them with mid fire clay bodies for the best fit and vitrification in your clay body!
High Fire
High Fire glazes are glazes that work best at Cone 10. Sometimes these glazes require different kinds of atmospheres in the kiln to get the desired results. Electric kilns are normally only able to reach a cone 6 and can only do oxidation firings. Gas kilns however, can reach a cone 10 and are able to do oxidation and reduction firings. Reduction firings refers to the lack of oxygen in the firing, causing the elements in the glaze to chemically react with the elements in the clay in a different way than if oxygen was present. Reduction firings cause different kinds of looks to glazes. The Orton Cone Chart says cone 10 is 2345°F. Make sure to use a high fire clay body when using high fire glazes and firing to cone 10 or your pieces will turn into a puddle!
Deflocculant
Deflocculants are additives that cause the particles to push away from each other, making the mixture appear thinner and pour smoother. This is especially important in casting slip and terra sigillata. You can purchase deflocculants like sodium silicate and darvan commercially.
Flocculant
Flocculants are the opposite of a deflocculant, making the particles grow closer together and appearing thicker. A common flocculant used is epsom salt.
CMC Gum
CMC Gum can be used as an additive to glazes to make them stick better. This is present in all commercial glazes, helping the glaze adhere to the pot while brushing it on. CMC Gum solution can be purchased in a bottle.
Specific Gravity
Specific gravity is a measurement of liquids to see the volume of the material. The specific gravity of water is 1.0 and in most glazes is 1.4-1.6, meaning the glazes are heavier than water. This is an important thing to monitor when mixing your own glazes, as it will help you know how much water to add in to keep different batches consistent. Specific gravity can be measured with a hydrometer or by filling a graduated cylinder (make sure the scale is zeroed to account for the weight of the cylinder) with the glaze to 100mL and measuring the weight in grams and then dividing by 100.
You can read more in depth about each of these categories, and more, in digitalfire’s glossary.
Haley Stanaford is currently a Post Baccalaureate student in ceramics at the University of Kansas where she is currently taking a Clay and Glaze Formulation taught by Colby Charpentier in addition to her studio practice. She also is a Sales Representative at Bracker’s Good Earth Clays in Lawrence, Kansas.
