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Mixing and Using Glazes
Glazing can be a tricky business, therefore it is a good practice to test any glaze that is new to you by preparing and firing several test pieces. The best kind of test piece is one that will resemble the pots that you intend to glaze. One method is to dissect an entire pot into pie slice segments and use the segments as your test items. Be sure to use the same application technique (brushing, dipping, spraying or pouring) that you plan to use as well. While preparing the test piece, make notes about the consistency of the glaze. For instance, does the glaze appear to be the viscosity of milk, light cream or pancake batter. Notice how the glaze builds up on the piece - if you want to, you can scratch a line of glaze from the bisque. If for instance, the resulting furrow is the thickness of a U.S. dime, the application thickness may be too heavy. (We will examine more scientific methods of measuring viscosity and specific gravity later in this article.)
The signs that an application thickness is too heavy are many: transparent glazes may appear cloudy, rolls of glaze can run to the foot or off the pot and the glaze may crackle, peel or crawl. The main sign that a glaze is insufficiently thick is that the surface will have an pebbly orange peel texture. Generally, glazing too heavily is a common problem, glazing too lightly is not.
Mixing Your Glaze
Prepared liquid glaze should be poured through an 80 or 60 mesh sieve into a container to remove any lumps. Mix the glaze vigorously using a whisk or similar tool. If the glaze has been sitting idle for some time, it will likely be in a state where some of the water has separated (it will be seen to be floating on top of a denser glaze layer) and the glaze will need to be mixed thoroughly to make it homogenous. Use several test pieces to assess the thickness. Remember that is far easier to add water to a glaze than it is to attempt to remove it, so add water with care!
Powdered glazes are a little more work to prepare, but can be worth the effort because powdered glazes almost always cost less. Minnesota Clay USA powdered glazes carry a safety label that instruct the user about specific safe mixing practices. More broadly, for powdered glazes it is important to: protect yourself from inhaling ceramic powders, preventing skin absorption of soluble materials and avoiding eye injuries caused by powders or splashes from liquid glazes. A properly fitted NIOSH certified respirator is necessary for adequate protection against ceramic dusts. Pay attention to the safety labels on ceramic materials, they will help to inform you of any other precautions you should take.
Mixing from powdered glazes can be as high or low tech as you like. However, the greater amount of agitation that you may bring to your glaze during mixing, the less lumpy the glaze will be - and that will allow it to go through a sieve more easily. If you can dedicate a blender to this task, so much the better. Just remember not use it for frozen drinks any longer! One technique that will allow you to reduce dust significantly is to add water to your powdered glaze in a water-tight heavy gauge plastic bag and mix by massaging the bag.
Measuring Specific Gravity
A hydrometer is a tool that can help you to control the consistency of your glaze. A glaze hydrometer is a glass tube with a weighted end and a scale printed vertically along its length. When inserted into the glaze slop, it will come to rest at a particular point along the scale. The resulting reading, (between 1.000 and 2.000 and normally about 1.450 for glazes) can help you to fine tune your glaze set-up and make your glaze more uniform from batch to batch. Note: It can be fairly easy to get a false reading using a hydrometer. For instance, a damp hydrometer (one with lower friction properties and greater weight) is unlikely to give you the same reading as a dry hydrometer (one that has higher friction properties and lesser weight). Make sure that you follow the same process each time you use it. Also, Springtime can bring changes to water quality and this change is sometimes reflected in the viscosity of your glazes. When this happens, hydrometer readings can stray significantly from the norm, and changes in glaze consistency will frequently occur.
If you have an accurate gram scale, there is a method of measuring specific gravity that is more reliable. It has been called: "pint weight" by potters for a long time. Pint weight is simply the measure in grams of a known volume (in this case, one pint liquid, not including the weight of the pint container) of glaze. Once you know how much a pint of glaze should weigh for proper application qualities, then that figure is used as a target weight for successive batches of glaze.
To be a little more exact, specific gravity (s.g.) is expressed as the ratio of grams (weight) per milliliter (volume). Water has a specific gravity of 1 because 100 grams of water has a volume of 100 milliliters. Since glaze contains not only water but solids as well, it will always have a specific gravity of more than one.
Follow these steps to determine the s.g. of a glaze. First, find an empty container like a 6 ounce juice can that is tall and narrow. Punch a small hole in the center of the lid. Place the container and lid on the gram scale. Use the tare bar to zero the scale to the combined weight of the lid and container. Fill the container to the brim, so that when the lid is applied, a small amount of water trickles through the hole in the top. Sponge the water on the lid away and weigh the container. The resulting weight in grams will also be the volume in milliliters. For example, if the water weighs 250 grams, the volume is 250 milliliters. Now pour the water out and dry the container and lid. Fill the container with your glaze (in a consistency that you have identified as proper for your application technique) and weigh it as you did with the water. Let's say that this weight is 350 grams. To find out the specific gravity of this glaze: divide the weight (350 grams) by the volume (250 milliliters). In this case, the resulting specific gravity is 1.4 grams per milliliter. Keep a record of this figure for future batches of this glaze.
Application Techniques
There are four major glazing techniques that potters use: dipping, pouring, brushing and spraying. The last, spraying is the least common probably because it requires a significant investment in equipment, (i.e. compressor, spray gun, dust and mist booth, filtering gear.) This is an area of glazing that requires more discussion than is possible here so our focus will be on the other three techniques.
Brushing tends to be very popular because it requires the least volume (as little as 4 ounces) of glaze. Normally three or four coats are applied to get appropriate coverage. Try to overlap coats as you work around your piece and try not to let any one area become too dry - when you apply a wet coat of glaze to a completely dry layer of glaze, you run the risk of the top layer peeling away from the pot as it dries. Brushing can be difficult to achieve an all around even application thickness, sometimes a small amount of food dye can be added to a glaze to assist in making it contrast with the clay that it is to cover.
Dipping and pouring require a more glaze (at least one quart), but the resulting even application can make the investment worthwhile. Transparent glazes generally require a thin coating, while opaque glazes normally work well applied more generously. If dipping one glaze over another, it is best to apply the second glaze before the first has completely lost its wet sheen. This will allow the two coats to dry together and help to prevent peeling. Glaze tongs work well if you want to dip a pot in one or more glazes. Be sure that the glaze is of sufficient volume to submerge your piece entirely without hitting the sides of the glaze container. Try to locate the points of the glaze tongs on an area of the piece that is both structurally strong and away from decoration. The four snake bites that the tongs leave can be dabbed with a brush loaded with the same glaze.