Glazes ^NEW^
Leigh Mccowin
It's crucial that clay and glazes be matched for firing temperature. Both low- and mid/high-fire clays can bisque at cone 04, but the recommended glaze firing range will be different for each. Glazes need to mature in the recommended glaze firing range for the clay. Fire the glaze too low and it won't mature, too high and it may discolor or flow too much. Fire the clay too low and it won't vitrify enough to be durable, too hot and the clay will soften and distort or may even melt. Also, we always recommend firing test tiles to verify that the glaze is a good "fit" for the clay. Most of the time commercial glazes and clays work together, but it's still possible for them to not have the same rate of expansion and shrinkage, which can result in glaze crazing or shivering.
Many glazes can fire hotter than the specified firing range, but the glaze will typically flow more, and certain colors may fade, darken, or otherwise change in appearance. Consult the manufacturer's guidelines to determine whether the glaze you have in mind can take a hotter firing and be prepared to fire test tiles to make sure you know what to expect before applying to actual ware.
From low fire to high fire, there's hundreds of glazes and underglazes to choose from. Find the best deals on the best colors from Amaco, Mayco, Duncan, Speedball, Spectrum, Coyote, Laguna, Mason Stains, and more!
Is your glaze too glossy? Not the right shade of blue? Knowing your materials and how they interact with each other gives you the freedom to make adjustments and fine tune your glazes to be exactly how you want them.
If you love learning about and discussing glazes, I'd like to invite you to my free social learning Facebook group called Understanding Glazes with Sue. The group is full of videos and discussions about firing, mixing glazes and fixing various glaze issues. Please join!
Measuring specific gravity is the essential first step before you do any flocculating or deflocculating of your glazes. Once you know the water is where you want it, you can ask yourself the 3 questions above to decide if it needs to be flocculated.
:I always check the specific gravity for my glazes at the start of each session. If you flocculate your glaze, what happens when you return to it a month later and want to check the specific gravity again? Can you get an accurate reading from the already flocculated glaze?
I have a question about using flucculant for slip. I am doing my first attempt at using slip for decorating. I saw a video stating that I need a flucculant for the slip. Is this flucculant good to use for slip as well (not only for glazes)? Thank you. Barbara
In grad school one professor was always adding epsom salts to glazes thinking students overthinned them. We would laugh with her, always looking for the Epsom salts. I always suspected the effects of the epsom salts wore off over time because it is soluable and thus the behavior of the glaze was fluctuating as a result. That explained why she was constantly needing to add more.
Glazing renders earthenware impermeable to water, sealing the inherent porosity of biscuit earthenware. It also gives a tougher surface. Glaze is also used on stoneware and porcelain. In addition to their functionality, glazes can form a variety of surface finishes, including degrees of glossy or matte finish and color. Glazes may also enhance the underlying design or texture either unmodified or inscribed, carved or painted.
Historically, glazing of ceramics developed rather slowly, as appropriate materials needed to be discovered, and also firing technology able to reliably reach the necessary temperatures was needed. Glazes first appeared on stone materials in the 4th millennium BC, and Ancient Egyptian faience (fritware rather than a clay-based material) was self-glazing, as the material naturally formed a glaze-like layer during firing. Glazing of pottery followed the invention of glass around 1500 BC, in the Middle East and Egypt with alkali glazes including ash glaze, and in China, using ground feldspar. By around 100 BC lead-glazing was widespread in the Old World.[3]
During the Kofun period of Japan, Sue ware was decorated with greenish natural ash glazes. From 552 to 794 AD, differently colored glazes were introduced. The three colored glazes of the Tang dynasty were frequently used for a period, but were gradually phased out; the precise colors and compositions of the glazes have not been recovered. Natural ash glaze, however, was commonly used throughout the country.
From the eighth century, the use of glazed ceramics was prevalent in Islamic art and Islamic pottery, usually in the form of elaborate pottery.[citation needed] Tin-opacified glazing was one of the earliest new technologies developed by the Islamic potters. The first Islamic opaque glazes can be found as blue-painted ware in Basra, dating to around the 8th century. Another significant contribution was the development of stoneware, originating from 9th century Iraq.[6][full citation needed] Other places for innovative pottery in the Islamic world included Fustat (from 975 to 1075), Damascus (from 1100 to around 1600) and Tabriz (from 1470 to 1550).[citation needed]
Raw materials for ceramic glazes generally include silica, which will be the main glass former. Various metal oxides, such as those of sodium, potassium and calcium, act as flux and therefore lower the melting temperature. Alumina, often derived from clay, stiffens the molten glaze to prevent it from running off the piece.[7] Colorants, such as iron oxide, copper carbonate or cobalt carbonate,[7] and sometimes opacifiers including tin oxide and zirconium oxide, are used to modify the visual appearance of the fired glaze.
Glaze may be applied by dry-dusting a dry mixture over the surface of the clay body or by inserting salt or soda into the kiln at high temperatures to create an atmosphere rich in sodium vapor that interacts with the aluminium and silica oxides in the body to form and deposit glass, producing what is known as salt glaze pottery. Most commonly, glazes in aqueous suspension of various powdered minerals and metal oxides are applied by dipping pieces directly into the glaze. Other techniques include pouring the glaze over the piece, spraying it onto the piece with an airbrush or similar tool, or applying it directly with a brush or other tool.
Overglaze decoration is applied on top of a fired layer of glaze, and generally uses colours in "enamel", essentially glass, which require a second firing at a relatively low temperature to fuse them with the glaze. Because it is only fired at a relatively low temperature, a wider range of pigments could be used in historic periods. Overglaze colors are low-temperature glazes that give ceramics a more decorative, glassy look. A piece is fired first, this initial firing being called the glost firing, then the overglaze decoration is applied, and it is fired again. Once the piece is fired and comes out of the kiln, its texture is smoother due to the glaze.
Other methods are firstly inglaze, where the paints are applied onto the glaze before firing, and then become incorporated within the glaze layer during firing. This works well with tin-glazed pottery, such as maiolica, but the range of colours was limited to those that could withstand a glost firing, as with underglaze. Coloured glazes, where the pigments are mixed into the liquid glaze before it is applied to the pottery, are mostly used to give a single colour to a whole piece, as in most celadons, but can also be used to create designs in contrasting colours, as in Chinese sancai ("three-colour") wares, or even painted scenes.
Heavy metals are dense metals used in glazes to produce a particular color or texture.[9] Glaze components are more likely to be leached into the environment when non-recycled ceramic products are exposed to warm or acidic water.[12] Leaching of heavy metals occurs when ceramic products are glazed incorrectly or damaged.[12] Lead and chromium are two heavy metals which can be used in ceramic glazes that are heavily monitored by government agencies due to their toxicity and ability to bioaccumulate.[12][13]
Ceramic manufacturers primarily use lead(II) oxide (PbO) as a flux for its low melting range, wide firing range, low surface tension, high index of refraction, and resistance to devitrification.[14] Lead used in the manufacture of commercial glazes are molecularly bound to silica in a 1:1 ratio, or included in frit form, to ensure stabilization and reduce the risk of leaching.[15]
Barium carbonate (BaCO3) is used to create a unique glaze color known as barium blue. However, the ethical nature of using barium carbonate for glazes on food contact surfaces has come into question. Barium poisoning by ingestion can result in convulsions, paralysis, digestive discomfort, and death.[16] It is also somewhat soluble in acid,[17] and can contaminate water and soil for long periods of time. These concerns have led to attempts to substitute Strontium carbonate (SrCO3) in glazes that require barium carbonate.[18] Unlike Barium carbonate, Strontium carbonate is not considered a safety hazard by the NIH.[19][17] Experiments in strontium substitution tend to be successful in gloss type glazes, although there are some effects and colors produced in matte type glazes that can only be obtained through use of barium.[18]
To reduce the likelihood of leaching, barium carbonate is used in frit form and bound to silica in a 1:1 ratio. It is also recommended that barium glazes not be used on food contact surfaces or outdoor items.[20]
Chromium(III) oxide (Cr
2O
3) is used as a colorant in ceramic glazes. Chromium(III) oxide can undergo a reaction with calcium oxide (CaO) and atmospheric oxygen in temperatures reached by a kiln to produce calcium chromate (CaCrO
4). The oxidation reaction changes chromium from its +3 oxidation state to its +6 oxidation state.[21] Chromium(VI) is very soluble and the most mobile out of all the other stable forms of chromium.[22]