This is the first in a series of technical articles we will be publishing.
Ceramic glazes are used to protect, seal and provide decorative colour and different finishes to unglazed biscuit earthenware, creating both functional and attractively coloured ceramic vessels and forms. Glazes are vitreous compounds that can transform a porous pottery form into domestic wares that are impervious, food safe and durable.
Glazing can be both the most exciting and demanding of processes for ceramicists: creative, challenging and transforming.
When a glaze is applied to a piece and it is fired, chemical reactions will change the entire look and feel of a vessel or sculpture. The science behind successful and, hopefully, consistent, wanted and predictable glazing results is a complex mix of components, firing conditions and application technique.
There are five main component types that go to making up a glaze and each have their own purpose. The “ingredients” need to be mixed together accurately to give the desired results, just like the best cake recipe! It is always important to also remember that, when creating glazes to take the necessary safety measures to avoid any inhalation or uptake of toxic materials, such as metallic oxides.
In glazes silica, the glass-former, is included in many natural forms, including of quartz, flint or sandstones, or using manufactured silica oxide. This component provides a glossy translucent finish and can give depth to the finished form.
The melting point of silica is 1710 C which is considerably higher than the working temperature of ceramic kilns, therefore other ingredients must be added to silica to lower its melting point and form a seal on the pottery.
The melting point of silica in ceramic glazes is lowered by using fluxes, which also encourages vitrification, the process of transforming silica into glass. Feldspar Soda and Feldspar Potash are commonly used examples of calcium oxide fluxes used in ceramic glazes; these are obtained from limestone.
Fluxes work in very diverse ways and each have a specific level of reactivity when combined with other glaze components. For lower firing temperatures, for earthenware ceramics, more reactive fluxes are needed to allow a glaze to mature. For higher temperature firings, for stoneware ceramics, fluxes of lower reactivity are therefore required.
It is important that all fluxes reach full maturity during a firing to prevent any leaching, metallic oxides can be toxic in their immature state and therefore not food safe.
Without a “glue” or “stiffener”, glazes would slide off any vertical surfaces of a piece and so a substance must be added to ensure adherence of a glaze to the clay body. Alumina, in the form of a clay, such as kaolin or ball clay, or manufactured alumina hydrate, is included in the glaze helping it to stick to the pottery’s surface. The amount of Alumina needs close regulation to control the fit or move of a glaze, and can be responsible for creating special affects between layers or glazes.
Alumina also helps to disperse fine gas bubbles that can form during firing and can create unwanted pimples in the final surface.
Without the addition of colourants, melted silica is transparent. A wide range of hues and tones can be achieved by using colourants these components, which are mainly metal oxides, need to withstand high temperatures to avoid burning out.
The melting point of a metal oxide may also affect the overall finish of a glaze recipe. Therefore, the temperature of a firing may need to be altered relative to the concentration of a colourant additive and the other ingredients as a whole. Testing glaze recipes both levels of relative concentrations and temperature of firings is a very necessary and sometimes long-winded task in the development of a new glaze. Also, the very nature and composition of the clay body to which the glaze is applied will also have a distinct effect on the outcome as will position of a piece within the kiln during firing. Understanding what minerals create which colours, and how they react together in differing concentrations, is imperative to making the desired colour and finish.
Other additions to a glaze recipe can be made to modify the finish and texture of a glaze. Such components may affect hold, suspension (before application), opacity and feel.
A transparent glaze can be transformed into an opaque one be adding an opacifier, such as Tin oxide or Zircon compounds. These do not dissolve during the melting process and remain as suspended white particles which add cloudiness to the finish. Opacifiers may also affect the hardness and finish of a glaze, as well as colour. Crystallisation or micro-bubbles may also be caused which can lead to marking during use, such as in domestic ware by cutlery. When used in colour glazes opacifiers will reduce the depth of colour. Therefore, minimising the level of opacifier additions will be necessary.
The simple addition of other modifiers, such as magnesium carbonate to make a “crawl” glaze, can create interesting accents to sculptural and textured forms. Many small additions of numerous components are also used to regulate shrinkage, viscosity and other glaze properties.