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Glazes: Materials, Mixing, Testing, Firing
Posted By Jeff Zamek On November 5, 2009 @ 4:04 pm In Ceramics Monthly Master Class,Glaze Chemistry | 4 Comments
In this article, Zamek covers many aspects of glazes. It can be read from start to finish, or you may choose the portion of interest to you with the following links:
Clay Body/Glaze Interaction
Glaze Testing Procedures
Cone Reading Position
Throughout the article, Zamek refers to his recipe for ZAM Gloss Blue.
How many times have you copied a glaze formula, only to find that it didn’t work as expected? It is not unheard of for glazes with the same formula to produce different results. While this may seem like a dead end, it does not have to be.
A high-temperature feldspathic green, transparent, gloss celadon glaze can be obtained with many different glaze formulas. The flexibility to know which formulas will produce the same glaze effect is a function of experience and the ability to interpret glaze tests. Adjusting glaze formulas requires a knowledge of how ceramic raw materials react in various combinations, temperatures, and kiln atmospheres. Taking a course in glaze calculation and raw materials is probably the most efficient way to learn about the “building blocks” of glazes. The lone ceramist in his or her studio, testing a small number of materials, cannot equal the multiplying effect of many students testing glazes with various raw materials and sharing the information. A narrow, limited education in ceramics can yield many areas for failure.
When choosing glaze materials, the cost of the actual material is not the most important factor. Time, labor and a low defect rate should be more important. Every raw material should be considered for its technical and aesthetic benefit to the glaze. Some unique glazes are worth any irregularities of raw materials or difficulties with mixing, storage, application or firing.
The practice of using generic names for very specific raw materials creates challenges in choosing the appropriate ingredients when trying to duplicate glazes. Different ceramics suppliers use different manufacturers or distributors for the same raw materials. Each processor or wholesaler of raw materials can have several different grades of that material. The result is a common name for a raw material that can be different in particle size, chemical composition or trace elements, depending on where it is processed and eventually sold.
The particle size of a raw material is a critical factor in glaze melt. A smaller particle size means increased surface area for a given weight, and melting is more efficient. My ZAM Gloss Blue can drip or run on vertical surfaces if a finer mesh nepheline syenite, flint or whiting is used.
It is important to know the actual mesh size when trying to duplicate any glaze formula. Silica, a major component in any glaze, can be purchased in 60-, 100-, 200-, 325- and 400-mesh particle sizes. The larger mesh numbers indicate smaller particles. Frequently, a glaze formula will not specify a mesh size for silica. In such instances, use 325 mesh. Nepheline syenite, a common high-temperature glaze flux, is produced in 270 and 400 mesh. If the glaze formula does not specify a mesh size for nepheline syenite, use 270 mesh. Coarser mesh whiting can cause the solids in a glaze to sink to the bottom of the glaze bucket. It also can cause a transparent glaze to become semi-opaque when fired due to incomplete melting of the material in the glaze matrix. Unless otherwise noted, use 325-mesh whiting. When ordering any glaze material, always specify the mesh size where applicable.
Problems can occur when potters use inappropriate substitution materials in the glaze formula. If the glaze requires nepheline syenite, a sodium feldspar, it is best not to substitute a potassium feldspar or a lithium feldspar. Clays are grouped as ball clays, bentonites, earthenwares, fireclays, kaolins and stoneware clays. Feldspars are grouped as potash, sodium or lithium. When making a substitution, always use a material from within the same group of clays, feldspars or raw materials.
Some glazes were developed using materials that are no longer in production. For example, Oxford feldspar, a potassium feldspar, is no longer being mined. If you have a container of Oxford feldspar in your studio and use it in a glaze, there might not be a readily available supply when you run out. Before mixing a glaze formula, make sure all of the materials are still in production.
In some instances, continually available materials may subtly change in chemical composition, particle size or organic content over time. All of these can alter the glaze. Often the supplier is unaware of changes in the raw materials they sell. The best course of action, though time consuming and inefficient, is to test raw materials before committing to a production glaze batch.
Metallic coloring oxides can differ in metal concentration, particle size and trace-element content. As with other raw materials, there are many processors of metallic coloring oxides. For example, cobalt oxide (Co304) is processed in three grades, 71.5%, 72.5% (ceramic grade) and 73.5%. The percentage represents the cobalt contained in the oxide. Each grade can affect the intensity of the blue that will be generated in a glaze. In addition, the quantity of trace elements in a metallic coloring oxide can influence its effect on the glaze color. For example, zinc oxide (French process) also can contain trace amounts of copper, lead, iron and manganese. Copper oxide also can have trace amounts of magnesium, sodium chloride, lead and other heavy metals. Use the same processor of metallic coloring oxides when ordering materials. When this is not possible, always test the oxide. While slight differences in trace metallic oxide content usually will not cause a radical color change, particle size can affect the look of a glaze. For example, a coarser particle size of cobalt oxide can cause larger blue specks in a glaze than a finer grind of the same oxide.
The point at which the fired clay and glaze meet and fuse together in the ceramic structure plays an important role in the development of the fired glaze. Some clay bodies will draw part of the flux content from the forming glaze during the firing process. This can cause opacity or dry surface textures in the glaze. A light colored clay body, such as a white stoneware or porcelain, can have an intensifying affect on a colored glaze. ZAM Gloss Blue, when applied to a white clay body, will be light blue. The degree to which the clay body matures in the firing can promote or retard glaze maturation. Always consider the clay body.
Every glaze will require different amounts of water, but it is best to use less water in initial mixing. It is easier to add water than remove it. If too much water is used in a glaze containing soluble materials and the excess water is poured off, it can change the glaze formula (solubles leave with the water). Glazes should be run through an 80-mesh sieve three times for final mixing.
Ease of application is especially important in production situations where time-consuming touch-ups mean a decrease in profits. Some glazes will become soft, dusty and fragile when drying on bisqueware. Other glazes will drip and run down vertical surfaces or pool unevenly in horizontal areas. Glazes containing high percentages of clay or light-density materials such as magnesium carbonate can become fragile and loose on bisqueware. This can develop into crawling in the fired glaze. While there are several gums that can improve glaze application, it is often more efficient to choose glazes that do not need additives.
Generally, a spray application will impart a uniform glaze layer as opposed to a dipped or brushed application. However, much depends on the skill of the individual. Dipping the pot into the glaze can result in drips as the excess glaze runs off the surface. Brushing also can result in uneven glaze thickness.
Some glazes are especially sensitive to the way in which they are applied to the piece. Apply test glazes in varying thicknesses to determine the true glaze color and texture. The thinner the glaze application, the more the underlying color and texture of the clay body are likely to be revealed. Often, a thin glaze application can retard the development of color, texture and opacity in the fired glaze. A thick application can cause some glaze formulas to run and drip on vertical surfaces. ZAM Gloss Blue, when applied too thin, will not achieve a rich deep blue color. A great percentage of glazes can be applied slightly thinner than the thickness of a dime or about as thick as three business cards stacked together.
It is amazing that most glazes reproduce accurately with a minimum of additional information; however, it is always best to start a testing program with the knowledge that occasionally a glaze formula will not work as described. We all know of people who obtain a glaze formula and then mix up 30 gallons without considering that it might fail. Experimenting on such a large scale is not a good idea. Eventually, there will be a major glaze and/or kiln problem caused by a glaze failure.
One important, often-overlooked item required for testing glazes is a notebook. Writing down each step in the process and the results from each test is more effective than memory. While there is no single testing procedure that will suit all work habits and objectives, consistency of method will ensure greater accuracy in duplicating glazes. It is important to know if a glaze will run or drip on vertical surfaces during firing. Vertical test tiles should be at least 4 inches in height and 2 inches wide. Tiles also must be of sufficient surface area to approximate the surface area of finished works. Many times, a small test tile will be successful because the weight of the molten glaze when heated is not enough to cause it to run down vertical surfaces. However, when larger areas are glazed, the increased weight of the fluid glaze might cause it to run.
Test tiles should have a smooth edge, a rough edge and any other textures likely to be used under the glaze, including throwing ridges. Some glazes can form razor-sharp edges on the fired clay. The testing stage is the time to find out if this is likely.
A gradual increase from a small test batch to the final large-volume glaze batch is important in ensuring a glaze formula’s reliability. A 500-gram test batch will allow you to glaze several test tiles, which should be placed in a number of different kiln firings. If the test glaze does not need an adjustment, it is often a good policy to mix up a preproduction batch of 4000 grams (makes roughly 1 gallon). This larger batch will allow you to glaze several pieces and place them throughout the kiln. Many kilns do not transfer heat evenly throughout their interior space, and not every kiln fires consistently every time. There are always slight variations. If possible, test in several different kilns. Once you are confident with a glaze, larger batches can be mixed and slowly incorporated into your existing glaze palette.
Kiln bricks, posts, shelves and stacked pots all radiate heat. Therefore, larger kilns have greater thermal mass, and will radiate more heat during their heating and cooling cycles than smaller kilns. Small test kilns are an inaccurate indicator of clay body and glaze reactions when compared to larger kilns. ZAM Gloss Blue can run or drip in a larger kiln due to prolonged heatwork on the glaze. Conversely, it can fire light blue with a satin-matt surface texture in a small kiln.
A kiln fired at a fast rate of heat increase can cause the clay body to remain more porous, causing crazing and a less durable clay. Glazes fired at a very slow rate of heat increase can run and drip due to the extra heatwork acting on the glaze. While there is no perfect rate for ZAM Gloss Blue, I recommended a 75-80°F (23-26°C) heat increase per hour from Cone 06 (1828°F/997°C) to Cone 9 (2300°F/1260°C), for this type of glaze.
Whether the glaze is fired in an electric, wood, natural gas, propane or oil kiln (with or without soda or salt), the atmosphere affects the glaze color, texture and melting capacity. Electric kilns produce clean, repeatable neutral atmospheres. Carbon-based fuels such as natural gas, propane, wood, coal, oil and sawdust can produce oxidation, neutral and various intensities of reduction atmospheres. It is reduction that can be very difficult to reproduce, as one potter’s medium-reduction atmosphere can be another’s heavy-reduction atmosphere. Reduction atmospheres can cause greater melting due to the increased fluxing action of the metallic coloring oxides contained in the clay body and glaze.
The cobalt oxide in ZAM Gloss Blue will fire blue in almost any kiln atmosphere, but there can be variations in the intensity of the color due to the atmosphere in the kiln and the fuel used to maintain that atmosphere. In soda, salt or wood firings, it can run or drip on vertical surfaces or pool in horizontal areas because of the fluxing action of sodium vapor or the alkaline content of the wood ash. Pyrometric cones are also subject to the fluxing action of sodium vapor, giving an inaccurate indication of the kiln temperature. In most instances, the glaze reactions to salt and wood firing are aesthetically positive.
The actual pyrometric cone reading is a difficult piece of information to obtain, because potters read pyrometric cones at different positions. Many potters consider the cone reaching maturity when it bends to the 3 o’clock or 9 o’clock position (bending over halfway in relation to the bottom of the cone pack). Other potters read the cone as being mature when it actually touches the cone pack. ZAM Gloss Blue has a two- to three-cone maturing range and will not change significantly when fired to Cone 8, 9 or 10. Some glazes are very sensitive to slight temperature variations. If you do not get a good glaze result, consider firing half a cone higher or lower.
Glazes with a wide maturing range are desirable, as not every kiln will fire evenly. While the glaze might not look the same at the lower end of the range as it does at the higher end, it should be functional, with a smooth nonpitted surface.
Whenever possible do not use soluble glaze materials. Borax, boric acid, colemanite, Gerstley borate, soda ash, wood ash, Gillespie borate, Boraq, potassium bichromate and pearl ash (potassium carbonate) are the primary sources of solubility in glaze formulas. Other glaze materials such as lithium carbonate, magnesium carbonate, nepheline syenite, strontium carbonate, and some frits can have lesser degrees of solubility but generally they do not interfere with the glaze application or fired glaze effects.
Soluble materials can take on atmospheric water in storage. This can affect the accurate measurement of the materials when weighing them. These materials will leach into the water in the glaze, changing its chemical composition over time, which can result in several glaze defects. As water evaporates from the glaze during application, soluble materials travel in a wicking action, drawing higher concentrations of material to the ridges and edges of the pot. Essentially, in the elevated edges of the pot, the glaze formula is different due to the concentration of soluble materials. This can cause blisters, pinholes, dry surfaces or changes in color.
The use of soluble materials is required in some instances as they contribute distinctive characteristics to a glaze. For example, in Shino (a high temperature viscous, feldspathic glaze developed in Japan more than 400 years ago), the inclusion of soda ash causes the glaze to melt at lower temperatures, sealing in carbon produced during body reduction. The carbon remains in the surface of the fired glaze. When soluble materials are required in a glaze formula, they should be stored in waterproof plastic bags. A conservative approach is to mix only enough material for one glazing session as the stored liquid glaze can change over time.
the author A frequent contributor to CM, Jeff Zamek is a ceramics consultant in Southampton, Massachusetts. For further information, see www.fixpots.com.
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