Low Budget Testing of the Raw and Fired Properties of a Glaze
There is more to glazes than their visual character, they have other physical properties like hardness, thermal expansion, leachability, chemistry and they exhibit many defects. Here are some simple tests.
What if you could greatly increase the quality of your fired ware by only small changes? Would you do it? Testing it thoroughly is a key ingredient. But mention the idea of testing a glaze and 99% of us think in terms of dipping a test tile and firing it to see how it looks. Many have learned that there is often little correlation between how a glaze looks on a little test tile compared with how it looks when used on ware, thus they tend to put little effort into testing. However if you already have a glaze in production, or have made an adjustment to an existing one, you are likely willing to expend much more effort to evaluate it fully.
Raw and fired glazes exhibit many properties that both production and end users are knowingly or unknowingly concerned with. These include slurry properties, dry hardness, behavior of the melt, freezing characteristics, hardness and scratch resistance of the fired glass, compatibility of host glaze and added colorants, leach resistance, glaze fit with the body, clarity of the fired glass, etc. Certain factors such susceptibility to material change and varying firing conditions are more difficult to measure.
Most people and companies do not have fancy glaze testing equipment to evaluate these properties. It is to these that this article is directed, there is much you can do to greatly increase your confidence in the true 'quality' of your product. Testing your glaze thoroughly is doubly important if you are using one or two base glazes because, in a sense, you have all your 'quality eggs in one basket'. We 'hoot' a lot about the advantages of focusing your efforts on one base glossy and matte recipe and altering these to make whatever you want. Thus the following tests all assume and depend on the fact that you are testing the transparent base without any colorants, opacifiers, or other additives. Remember that improving the base improves every recipe that is based on it.
Here are some tests you should consider:
Slurry Properties Mix a small pail of the glaze and bring it to the proper consistency (I am assuming you are glazing bisque ware). Most glazes work best when there is enough water to make them fluid and not so much that they do not gel. Typically you should be able to stir a glaze with a stick, pull the stick out, and the slurry should remain in motion only for 2-4 seconds. Measure its specific gravity and record this amount. If in future the glaze gels, settles, or runs excessively check its specific gravity. If it is correct then you need to investigate changes in your water supply or dry materials that could flocculate or deflocculate the slurry. Let the container of glaze sit overnight and check it the next day for any tendency to settle or gel excessively. If it settles incorporate some bentonite in the recipe. If it gels to the point that remixing does not thin it out then replace materials known to be soluble (i.e. Gerstley Borate, Nepheline Syenite) or use distilled water.
Application Properties Dip your dry finger into the slurry and pull it out. Only one or two drips should fall and a good layer of glaze should hold itself in place. For glazes intended for application to bisque ware, dip a thick test tile (2 cm thick) all the way in for two seconds, pull it out and turn it over and watch for any tendency to run and drip. Note how quick it dries. When dry dip it again half way and hold it in for several seconds. Set it out to dry and note any tendency of the glaze cover to crack during drying (this is an indication of too much clay or too plastic clay in the recipe). When the specimen is completely dry note its tendency to 'powder off' in your hands. Will it accept overglaze painted decoration without turning to mud or will wax resist paint on and hang on well (if not the clay content is either too low or not plastic enough). Dip a thin test tile also (5mm thick or less) and apply thick and thin layers as above (the tile should be thin enough so that the clay becomes waterlogged and the glaze thus dries slowly). Note any tendency of the glaze to bubble as it dries, this indicates that the specific gravity of the glaze may be too low.
Crystallization, Clarity, Crawling Fire the thick and thin test specimens made above. The thick one will cool slower, note any tendency for it to devitrify (develop crystals) compared to the thin one. Note any tendency toward crawling on the thickly glazed sections of the specimens, especially near edges. Note any tendency of the glaze to cloud with bubbles in the thicker areas.
Stain Compatibility Using a small brush paint stain ID numbers and color swatches on a flat tile of the target clay body. Glaze the whole tile over with a transparent version of the base glaze you are testing and fire. Note colors that are off and research the required chemistry to explain why.
Leaching Add a colorant like cobalt to a transparent version of the glaze being test, make and fire a bowl, fill it with vinegar and leave it in a warm place for twenty four hours. Note any fading of color, reformulate and test again till no leaching occurs.
Hardness Take a sharp concrete nail and attempt to scratch the glaze. Compare its performance against a benchmark glaze that has proven to be durable. Or use a hardened steel file to attempt to remove glaze on the lip or corner of a piece.
Cutlery Marking Using the edge of a stainless spoon or fork attempt to mark the surface of the glaze. If it marks attempt to rub or clean the mark off. If you cannot consider reformulating the glaze to develop a smoother surface or fewer crystals, flux it to melt more, or fire it higher.
Crazing, Shivering Cycle a thin glazed shard of your body-glaze combination between three-minute immersions in boiling water and ice water. Do it three times. Use a black marker to color an area of the surface. Clean the ink off using methyl hydrate. Note any craze lines highlighted by the ink. Note any areas near edges that may be flaking off (shivering). Reformulate the glaze to reduce its thermal expansion (increase it for shivering), fire, and test again.
Glaze Over Solubles Prepare a 12cm diameter disk 5mm thick from your clay and dry it under a fan with the center 6 cm covered. Any soluble salts in your clay body will be amplified on the outer 3cm ring. Break the disk in half, glaze half, and support and fire it resting on the flat. Note any reaction between the soluble salts in your clay and the glaze (i.e. blistering, orange peeling).
Flow Check the flow of the melt using a flow tester.
Fired Volatility Many glazes work well at one temperature but if fired slightly higher or lower they can have a much different surface. This type of behavior is normally only tolerated in very special purpose glazes. Thus you should prepare sample tiles for firing one cone higher and lower than you working temperature. This will help you spot possible trouble in future if kiln firings are not totally consistent.
Microscope Using a $20 30X Lightscope (available at hardware stores) look carefully at the glaze surface for any signs of poor melting (especially in matte glazes) or surface pits that could harbor bacteria. Identify any tiny specks as either impurities or the first stages of crystals. Break a shard and check for a good interface between glaze and clay. Examine any scratches you were able to make with the concrete nail.
Make Functional Ware Mix a couple of gallons of the base glaze and make a variety of functional ceramic shapes, glaze, and fire them. Note any problems like pinholing, crawling, bubbling, devitrification, and blistering.
A batch of fired clay test bars in the Plainsman Clays lab
A batch of fired test bars that have just been boiled and weighed, from these we get dry shrinkage, fired shrinkage and porosity. Each pile is a different mix, fired to various temperatures. Test runs are on the left, production runs on the right. Each bar is stamped with an ID and specimen number (the different specimens are the different temperatures) and the measurements have all be entered into our group account at insight-live.com. Now I have to take each pile and assess the results to make decisions on what to do next (documenting these in insight-live).
Measuring slip viscosity using a paint-measuring device
A Ford Cup being using to measure the viscosity of a casting clip. These are available at paint supply stores. It drains water in 10 seconds. This casting slip has a specific gravity of 1.79 and we target a 40-second drain. That being said, if you are not working in a factory, if will be sufficiently to eye-ball the viscosity as you gain experience. If you are in a high-production situation, the seconds-value that this test produces gives you something to write down in testing records, producing an audit-trail for quality control and problem solving later. One thing to note: A slurry can gel while it is draining, if this happens the value produced is not valid. First adjust the rheology so it maintains viscosity throughout the drain time.
Feldspars, the primary high temperature flux, melt less than you think.
A cone 8 comparative flow tests of Custer, G-200 and i-minerals high soda and high potassium feldspars. Notice how little the pure materials are moving (bottom), even though they are fired to cone 11. In addition, the sodium feldspars move better than the potassium ones. But feldspars do their real fluxing work when they can interact with other materials. Notice how well they flow with only 10% frit added (top), even though they are being fired three cones lower.
Limit Formulas and Target Formulas Glaze chemistries for each type of glaze have a typical look to them that enables us to spot ones that are non-typical. Limit and target formulas are useful to us if we keep in perspective their proper use.