Making Glazes – Theory And Practice
Written by Peter Smith & Dianne Peach
There are two reasons: cost and control.
The ingredient costs of a glaze may only be 10-20% of the price you pay for a commercial glaze. That’s a real incentive to get started.
Then, getting the glaze precisely the way you want it, and have that result repeatable, is only possible if you control every aspect of it: from formulation to its viscosity or flow characteristics on the bisque ware; or from the surface colour and its texture to the firing temperature.
Cost and control, the two “C”s of roll-your-own glazes.
GLAZE THEORY 101
Glazes are needed because at normal kiln temperatures, the clay body does not vitrify into an impervious structure. Instead it “sinters”, where the edges of the particles in the clay undergo a chemical change and some melting. Under the intense heat of the kiln, these edges fuse together – or sinter – into a matrix-like structure that gives fired clay its rigidity and hardness.
Glazes have both decorative and functional properties. Glazes are mineral oxides mixed together so that at kiln temperatures, they melt into a glass-like surface that protects the clay, making it water-proof and serviceable. Many glazes are formulated to be resistant to organic and non-organic materials that may be stored in them.
Glazes are also beautiful.
Traditional Celadon and Chun glazes subtly enhance the form of traditional ceramic ware. Majolica glazes, perfected in the 15th century, provide a superb platform for decorative art on the clay surface. Delicate and difficult crystalline glazes are a stunning tribute to the potter’s craft while lustre glazes add striking coloured and metallic highlights in a third firing.
Glazes consist of mineral oxides mixed in proportion to produce two things: glass, and other materials that give rise to characteristic colour and surface properties.
“Most glazes have 3 main parts – the blood, bone, and flesh. Here’s how they work:*
* Fluxing agent or ‘lifeblood of the glaze’ – causes the glaze materials to melt and flow together in the kiln firing
Refractory or ‘bone of the glaze’ – resists heat and melting, providing structure and strength to the glaze body*
Glass Former or ‘flesh of the glaze’ – creates complexity, depth and unique qualities.”
Putting this in plain language, fluxes are there to make the glaze melt at the required temperature.
Refractory minerals act as stiffeners, preventing the molten glaze from running off the pot.
Glass formers are some of the about seven materials that will cool to form a glass-like surface. For example, potters use Silica which melts at 1,700°C and Boron, which melts at 2,076°C. Boron though has some surprising properties that also make it a flux that can lower the glaze melting point dramatically.
Glaze ingredients are not mystery materials. They all come from the earth and you might find a rock or two that you can grind up and fire into a beautiful glaze.
Making a glaze is like baking a cake, you need a recipe, the ingredients, tools to mix it, some decorative elements and finally, an oven to bring them all together.
It’s just a process, and if you follow a few simple rules, your home-made glazes will give your work a unique touch and give you an amazing sense of satisfaction.
BASIC SET OF INGREDIENTS
Getting started with mixing your own glazes can be as simple as finding a good recipe for the kind of glaze you have used before and ordering the materials to mix up a batch.
While there are a lot of common materials used in low, mid and high fired glazes, each has its own needs. Let’s assume you are starting out with mid-fired work.
US glaze researcher Sue McLeod published a getting started list of materials that can be bought for around $180. Australian product names and codes (Walker) have been added to help you if you want to look them up.
- Silica (BA770) – 5 kg – $11
- EPK (Kaolin BA331) – 5 kg – $15
- Frit (KMP4124) – 2 kg – $26
- Gerstley Borate (BA390) – 2 kg – $20
- Nepheline Syenite (BA67) – 5 kg – $20
- Potash Feldspar (BA350 )– 5 kg – $8
- Whiting (calcite BA170) – 5 kg – $9
- Talc (BA850) – 5 kg – $13
- Dolomite (BA310) – 5 kg – $9
- Bentonite (BA110) – 2 kg – $13
- Copper Carbonate (BA250) – 250g – $22
- Red Iron Oxide (BA510) – 500g – $10
You should buy your ingredients from a respected pottery supplier. Some minerals can be more or less contaminated with geologically closely related but unwanted elements.
SOME TOOLS YOU’LL NEED
There are three categories of things you’ll need to buy to get started: storage vessels, mixing equipment and safety equipment.
Starting with the raw ingredients, you’ll need plastic containers with lids to store them, with a set of labels to write the name, product code, date of purchase and original quantity. Good glazing technique is about writing things down and labelling tests, so get used to doing it now.
To store the mixed glazes, you’ll need some sturdy, lidded buckets – some 20 litres ones and some of 5 litres to start with.
To weigh out the glaze ingredients, you’ll need a set of accurate scales. Good quality kitchen scales will do nicely, but for more accurate measurements, then proper triple beam mechanical scales or a set of electronic scales will increase your precision.
To mix the glaze ingredients with water, you can use a wooden paddle or a stirring rod either hand-held or attached to an electric drill or paint mixer.
The mixed glazes then need to be sieved to ensure no large particles have found their way into the mixture. An 80 mesh sieve is a good starting point, and then you’ll need a stiff brush to force the glaze liquid through the mesh, or you can look at a rotary sieve to make the job easier and faster.
Last but not least is the safety equipment you will wear EVERY time you handle glaze materials. A half-face respirator with P2 dust particle filters, a set of protective goggles, full-length rubber gloves and a waterproof apron are the minimum requirements. Remember that glazes and glaze chemical can be toxic when breathed in and some can be dangerous on exposed skin.
GLAZY – A GLAZE RECIPE RESOURCE
One of the largest and most respected glaze recipe databases is called “Glazy”. You can use it to look up recipes of all kinds by glaze type, firing temperature, kiln atmosphere, colour and material surface.
These have been uploaded either by the database creator, Derek Au, or by users around the world, including an increasing number of Australian potters.
Glazy has a calculator that allows you to find the quantities required for any batch weight, a huge materials database, an analysis engine that looks at the oxide breakdowns of the ingredients and their ratios.
Charts showing the glazes behaviour at various temperatures are provided along with important safety information about the glaze.
In future articles, we will look at the various ways of applying glazes to your work and how to best fire them in your kiln, to produce either highly predictable results, or for ways of leaving your glazes open to chance environmental factors in the kiln.
Happy glazing, go ahead and try, you know you want to. And be safe!