I often receive messages asking why glazes haven’t come out as expected. There are many variables that can affect the appearance of a glaze; glaze consistency, glaze application thickness, firing temperature, and firing time. We will have a look at some of the most common reasons why glazes sometimes don’t come out as expected.
The most important point is to keep your glaze thickness consistent. I like mine to be thinner than single cream but thicker than milk. Once you have established the consistency that works for you, measure the specific gravity by weighing 100ml glaze and dividing the weight in grams by the volume in ml. The specific gravity of my glazes is usually around 1.4 to 1.5. Every time you make up a new batch of glaze, make sure you add the same amount of water to achieve the same consistency. You can vary the application thickness by immersing your pots in the glaze for longer or dipping several times. If over time your glaze starts to look thin and settles quickly in the bottom of the bucket, it may have become deflocculated. You can correct this by dissolving Epsom salts in warm water and adding a teaspoon of the solution to your glaze. It should immediately thicken as the particles clump together and become flocculated again.
Many glazes are affected by firing either too low or too high. Kilns sometimes fire hotter than the temperature displayed. This can affect surface texture, fluidity and glaze colour. The only way to find out the actual temperature inside your kiln is by using pyrometric cones. These measure the heat work and depend on temperature in the kiln as well as heating rate and soaking time at top temperature. Use three cones placed at a slight angle in a pad of clay. The middle cone should be the one you’re aiming at, with a lower temperature cone on the left and higher temperature on the right, so for example I use cones 7,8 and 9. The two lower cones should bend over so that the tip touches the base but the guard cone should remain standing and determines whether the kiln has over fired. If all three cones bend over, set your kiln to a lower temperature or decrease the soak time at top temperature. I usually soak for 15 minutes.
Inconsistent glaze thickness and firing temperature are the two main reasons why glazes don’t come out as expected. More information can be found my books Colour in Glazes and Special Effect Glazes.
Earthenware glazes are made mostly of frit, which is a kind of man-made feldspar. Borax, soda and other water soluble materials can be made insoluble by heating together with silica. The resulting glass is ground up and used together with clay and silica to make low temperature earthenware glazes. Frits are also used to lower the melting temperature of mid-range glazes. The raw mineral borates are found in dried up lake basins in Turkey and California.
Opacifiers can be added to make the glaze opaque. Tin oxide is the most effective opacifier but zirconium silicate is less expensive.
A respirator mask should be worn when weighing dry materials. The dry powders are added to water and left to slake before sieving.
Stoneware and porcelain glazes are made up of at least four ingredients: silica, feldspar, whiting and clay. This is what they look like in their raw mineral forms. Silica is ground flint or quartz, usually from sand or sandstone. These are large pieces of milky quartz from a vein in igneous rock.
Silica has a high melting temperature. In order to melt in a kiln, it needs a flux. The main flux in stoneware and porcelain glazes is feldspar, found in granite, an igneous rock composed of the minerals feldspar, quartz and mica. This piece of granite from Devon has both muscovite (silver, sparkly) and biotite (black, crystalline) mica. The dark vein is mafic rock, high iron and magnesium. Granite forms when molten magma cools and solidifies. The darker minerals solidify first, then the feldspar and finally the quartz, often in large veins running through the rock. Slow cooling, deep in the earth’s crust results in large crystal size.
Clay contains both alumina and silica and is added to increase viscosity to prevent the glaze from running off the pot when molten in the kiln. Clay also helps to keep the heavier quartz and feldspar suspended in water in the glaze bucket.
Calcium carbonate added in the form of whiting is an extra flux which helps to stabilise the glaze and can also produce a matt glaze surface. Whiting is ground up chalk or limestone.
I have been asked to teach several glaze courses in 2017.
A new pottery school is opening in Forest Row in Sussex. I will be teaching glazes there 0n 17-19 February 2017. Contact Katrina Pechal on 0789 444 7938. You can visit the school when they open in December, courses start in January with tutors including Stephen Parry and Ruthanne Tudball.
The founder Katrina Pechal says, “ I am saddened to see so many high-level ceramics courses around the country closing. My days training at Camberwell School of Art were inspiring, having lessons with famous potters like Takashi Yasadu, Colin Pearson and Ewen Henderson among others was a great privilege and I want the next generation of potters to have the same opportunities. I have noticed a recent increase in the popularity of pottery, thanks in part to the BBC’s “The Great Pottery Throw Down. I feel this is a perfect time to move to a bigger site with better facilities where I can help students to train and develop their craft”.
Forest Row School of Ceramics will be based in the Rachel Carson building at Emerson College, set in beautiful grounds within walking distance of the village. It offers outdoor space, which Katrina needs for a kiln site (a kiln-building workshop run by Joe Finch, Potter and author of Kiln Construction (a brick by brick approach), is already planned for early next year). Emerson also has more classrooms Katrina can utilise as she expands plus affordable student accommodation it needed.
Forest Row is three miles from East Grinstead in Sussex, situated next to the idyllic Ashdown Forest yet within easy access of the M23/M25 and a short drive from Gatwick Airport, the South Downs and the creative hub of Brighton.
I will also be teaching colour in glazes at West Dean College on 5-9 March (the course is now full but you can try the waiting list), and demonstrating throwing and glazes all weekend at the Scottish Potters’ Spring Workshop at Tulliallan on 10-12 March.
For anyone in London there are glaze courses at the Morley College in February and April but I will not be teaching them.
Exciting news: I will be teaching a glaze course in Belgium in 2018.
If you can’t come on a course, you can still learn from my books.