Special effects glazes are those which give extraordinary effects due to the glaze composition or the firing technique. The unique state of these glazes shows through in the colour variations, crazing or textured surfaces, crystal formations inside the glaze, or broken and mottled colour combinations.
Breakthrough Glaze
As the title suggests, this is exactly what occurs during the firing process. This is a combination of glazes which are applied to the ware, one after the other. The base glaze breaks through the top glaze, resulting in a variety of mottled, coloured effects. It has been found that Albany slip causes the glaze to bubble and boil due to the volatile gases which are released during the firing process.
This process usually occurs in the Cone 2 to 5 temperature range, thus during a firing which goes above this temperature, the results will vary. The bubbling will cease, and even out to form a normal, smooth glaze. If a less fluid glaze is on the top of the base glaze, the bleeding and breakthrough will occur in a variety of mottled colour combinations. Boiling glazes are very similar in composition to breakthrough glazes, with a wide firing range (cone 018 – 10).
Floating Glazes
Colours in a floating glaze give the impression of a deep pool of water, seeming to float on the surface of a dark background of great depth. These glazes are fickle and unpredictable. Even the mineral content of the water used can alter the desired effect, as well as firing in the same kiln with bisque ware or glazes containing colouring oxides. For these reasons distilled water must be used, and the wares fired separately.
Crackle Glaze Effects
Tension develops in a fired piece when the glaze and clay body expand and contract at different rates during the firing and cooling process, which results in the crackle which makes this a popular and sought after glaze. The crackle patterns can be large or fine, hairline cracks, depending on glaze composition or the clay body to which the glaze has been applied.
To emphasise the crackle patterns, the vessel can be rubbed with any form of dark colourant, such as ink, tea or coffee, colouring oxide, or water soluble paint. If the piece has recently been fired, the crackle will be more emphasised, whereas if the ware has stood for around a week or more and is consequently rubbed, the result will be a smaller crackle. Note that wares glazed in this way are not to be used for food purposes, as the crackles could result in food poisoning.
Flowing Glazes
These glazes make use of a flowing action during the firing process. Their composition is that of a formula with a high fluxing agent, and low stabiliser, such as alumina. The flux melts more rapidly, flowing down the side of the vessel during firing, creating an unusual effect. Flowing glazes can be used under, between or over a second glaze.
Crater Glaze
Crater glazes are so named because of the crater-like texture that they form on the surface of the fired vessel, reminiscent of craters on the surface of the moon. They are produced by the addition of compounds which liberate gases during the firing cycle. As these volatile gases attempt to escape through the surface of the glaze they create bubbles which rupture the surface and then ‘pop’.
If the firing and cooling process is done rapidly, the depressions do not heal or smooth over, but leave crater like cavities on the surface of the vessel. These effects are best gained by using a darker, more heavily grogged clay, which allows the natural colourants of the clay to bleed or show through.
Beaded Glazes
During the first cycle of the firing process, a planned crawling takes place. This occurs when the glaze has moved away from certain portions of the vessel, exposing the clay body beneath it. This is in fact a fault (crawling), but is also promoted as a desired decorative effect. There are a number of causes of crawling, the main one being that the high plasticity and viscosity of the glaze results in excessive shrinking and ‘beading’.
Special effects glazes create an interesting option for the potter, especially when considering that they are not part of everyday ceramic manufacture. While many of these glazes are not suitable for use on wares meant for food consumption, certain of them are safe to use for functional purposes and can create unique and beautiful ceramic wares. The sculptural ceramicist will especially find a fascinating and diverse new world waiting when investigating this avenue.
Resources:
Parmalee, Cullan Ceramic Glazes Industrial Publications (1951)
Hamer, Frank and Janet Potters Dictionary A & C Black Publishers (1991)
Chappel Potters Complete Book of Clay & Glaze Watson-Guptill Publications (1976)
Mellissa Bushby - Mellissa Bushby is an author, illustrator and ceramicist. She studied Fine Art for four years, and her newest book release is January ...
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