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To get the best results from a glaze, it is important to control all of the factors affecting the glaze appearance. Measure the ingredients accurately, and mix the glaze thoroughly so that it is homogeneous and properly suspended, with none of the heavier materials settling out. Screen the glaze through a filter, such as an 80-mesh sieve, to remove any oversize particles. Apply the glaze evenly on the piece, and dry the piece thoroughly before placing it into the kiln. Determine the correct firing cycle, and fire a test piece to ensure that the glaze is working the way it is expected to.
But having done all of this right, if the glaze is not applied to the proper thickness, the results can still be unsatisfactory.
Suspension AdditivesWhether the glaze is applied by brushing, dipping or spraying, the glaze thickness can be controlled by keeping track of two fundamental variables: viscosity and specific gravity (SG). Initial control over these variables is provided by the materials chosen and the suspension system of the glaze. Some of the most common materials used for glaze suspension are bentonite, veegum, carboxymethylcellulose (CMC) and, in some cases, calcium chloride or epsom salts.
In general, up to 0.5% (dry weight) of bentonite and/or veegum is used for suspension. CMC is added to glaze suspension systems to slow down the drying time of the glaze. The CMC also hardens the surface of the unfired glaze and makes it less prone to damage prior to firing. Therefore, a spray glaze—which should dry very quickly—requires only a small amount of CMC, such as 0.03%, to help toughen the surface.
Dipping glazes need to dry a little more slowly to allow the glaze enough time to flow evenly over the piece. Dipping glazes generally range from 0.05% to 0.5% CMC, depending on the circumstances. Brushing glazes require a comparatively long time to dry in order to be able to spread the glaze evenly over the piece. This can mean the use of up to 2.0% of CMC to achieve the necessary drying time. Experimentation and record-keeping are the only sure-fire ways to determine the best suspension system for a particular application.
Specific GravityAside from the actual glaze materials, the other important ingredient is water. The proportion of water to dry glaze material has the most obvious effect on the viscosity and SG of the glaze.
Specific gravity is the density of a liquid measured in grams per cubic centimeter. Water has a SG of 1.0, which means that 1 cc of water weighs 1 gram. To calculate the SG of a glaze, divide the weight of the glaze by the weight of an equal volume of water (i.e., weigh 100 cc of glaze and divide by 100 to obtain the SG of the glaze). This can be done easily using a scale such as an Ohaus triple beam balance and a plastic measuring cylinder cut off at the 100 cc mark. Fill the cylinder to the top, weigh it (remember to subtract the weight of the cylinder), and divide by 100 to obtain the SG.
In general, the SG of a spray glaze should be as high as possible while maintaining good spraying characteristics. Who wants to waste their time spraying more water onto the piece than necessary? In addition, the more water used, the slower the glaze will dry.
Depending on the glaze materials used, a spray glaze can range in SG from about 1.70 to over 2.0. For a dipping glaze, a combination of viscosity and SG is required that will result in the correct thickness of glaze being deposited on the piece in one dip application. The usual range of SG for a dipping glaze is 1.50 to 1.70, the lower end of the range being for clear glazes and the higher end for opaque glazes. Various other factors will also affect the determination of the appropriate SG of the glaze. These factors include whether the piece to be dipped is greenware or bisque, the bisque temperature, whether the piece is made from pug clay or casting slip, the moisture content of the piece, and anything else that affects the ability of the piece to absorb glaze.
Brushing glazes usually have SG in the range of 1.45 to 1.60. Clear glazes are at the low end of that range because thinner coatings usually produce a better appearance. In an opaque glaze, the higher the SG the fewer brush coats will be required to obtain opaque coverage. However, depending on the skill of application and the fluidity of the glaze, at least two or three coats must be used to get even coverage and hide the effects of the brush strokes.
ViscosityGlaze viscosity (or thickness) is measured in terms of the number of seconds that a given volume of glaze takes to flow through a hole of a certain diameter. The easiest way to measure viscosity is by obtaining a viscosity cup, also called a Zahn cup, from a hardware or paint store, along with a stopwatch. The viscosity cup is a small container with an open top and a bowl-shaped bottom, with a small hole to allow the glaze to flow out. The cup has a handle so that it can be dipped in the glaze. Fill the cup to the brim, and measure the time it takes for the stream of glaze to break at the bottom of the cup and only drips remain. The number of seconds is a reliable measure of the viscosity of the glaze.
Spray glazes usually have a viscosity in the 20 to 40 second range, though this will obviously vary depending on the size of the hole in the cup. These glazes are kept thin so that they are easy to spray, but if they are too thin, settling out of the glaze will occur. If this happens, either less water or more suspender should be used to increase the glaze viscosity.
Dipping glazes have somewhat higher viscosities in the 40 to 60 second range, although many of the factors mentioned previously with respect to SG may also call for viscosities outside this range. Brushing glazes are generally very thick and will have readings in excess of 90 seconds because of their high CMC content.
By maintaining specific gravity and viscosity within a narrow range, it is possible to control the thickness of glaze that is deposited on the piece. This is especially important with respect to dipping glazes, for which there is little control over the number or thickness of coats being applied. Dipping glazes also tend to thicken as they are used because water is sucked out of the glaze into the bisque as pieces are dipped. Thus, it is necessary to monitor viscosity and SG on an ongoing basis, and not just in the initial setup of the glaze.
Achieving Consistent ResultsThe control of glaze thickness is essential for the production of consistent, high-quality results. Glazes that are applied too heavily or too lightly are the cause of a number of glaze defects, including pitting, crawling, color variation and rough spots.
The simplest way to check glaze thickness is with a thickness gauge, which can be purchased through an instrument supply catalog. Hold the gauge on the surface of the glaze. When the button is pressed, a needle penetrates the glaze to the bisque surface and displays a reading of coating thickness in either millimeters or thousandths of an inch.
Testing pieces to determine the range of glaze thickness that will produce the desired glaze results can save many hours of unnecessary troubleshooting. Once the appropriate thickness range has been determined, it can be correlated to a range of viscosity and SG values that produce the correct thickness. Maintaining viscosity and SG within the desired range can usually be achieved by adding small quantities of water. Additives can also be used to achieve the desired glaze characteristics, such as calcium chloride for thickening and sodium hexemetaphosphate for thinning.
Making pottery is a technical science, as well as an art. Achieving the proper glaze thickness on a consistent basis through the control of viscosity and specific gravity is one of the keys to avoiding many common glaze defects and creating the perfect piece.