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A Low Cost Tester of Glaze Melt FluiditySection: Glazes, Subsection: Adjustment, AdaptationDescriptionThis device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems. ArticleThere are many complex and expensive instruments designed to observe and measure the goings-on in firing kilns. Generally this type of equipment is expensive and measures absolute physical properties that can be quantified easily. However glaze melt flow is like clay plasticity, it is more subjective and not so easy to quantify. It is best measured comparatively, that is, one specimen directly compared with another. Fortunately such tests can be done using inexpensive methods and devices. I would like to submit a general purpose testing method for many glaze melt properties that is both inexpensive and easy to use. So many factors related to the melting, solidification and physical properties and defects of fired glaze surfaces are related to melt viscosity. Thus a test that provides information about this has the potential of being very valuable. Before going on, I will give credit where credit is due. This is not an original idea. I have seen this device described in industry literature to compare melt properties of nepheline syenite and feldspar. Also, I was sent a very nice dual-flow mold by Hugh Nile at Sterling China (it had the initials IMC embossed on it). I am aware that other industries also use similar devices. However I want to take it to the next level by clearing documenting its advantages and a procedure to use it. I have made a rubber master mold of the one described herein and can making working molds for others. If you would like one please see the bottom of this article. Testers that do not work wellSmall or steep angle testers: Although I have messed with smaller sizes in the past I have now seen the light. They just do not work as well. You need a large enough reservoir, and long enough flow ramp at a shallow enough angle to get repeatable and sensitive tests. Inclined tile testers: Some companies prepare a lump of the glaze to be tested and glue it to one end of a tile using a slurry made from the same material. While this will often work it is problematic with compounds that shrink a lot or those lacking dry hardness. The former could crack off and the latter may crumble off. I'll leave it to your imagination what might happen if pieces or the whole sample rolls into contact with a kiln element.  Large and small glaze fluidity flow testers The Dual-Flow Large TesterThis is shown in the picture. It is 13.5cm high while standing (5.5 inches). The long runway is at less than a 45 degree angle for extra sensitivity (there are actually two orientations for two different angles). One of the big advantages of the dual tester is that it can be employed for side-by-side testing of two specimens (e.g. one alongside a benchmark). It is amazing how close you can match the melt fluidity of two materials using this method. This device is cast in a plaster mold using a mix refractory enough to resist warping if walls are cast thin (in production situations flow testers should be made from the same clay that ware is made from but if such is too vitreous you can reduce the feldspar content somewhat. See below for more information on the slip recipe. I usually bisque fire these testers for extra strength. The reservoir accepts a 10-12 gram ball of material that you can just drop right in. These balls are easy to make by dewatering the glaze or material slurry on a plaster surface to the right working consistency and then rolling the ball in your hands, drying it and shaving material off to achieve the right weight. (thus the glaze does need to have enough plastic ingredients to enable this workability or you need to add some bentonite to impart it). I have defined a procedure for this test in the testing area of this site. As noted in the procedure there, for repeatable results it is important that your testers be the same thickness, made from the same clay, fired at the same rate of rise and to the same temperature, and the ball sample must be the same dry weight each time. In case you are not yet clear on how this tester is used: Two glazes are compared by dropping dried balls of each into the reservoirs at the top and the whole thing is fired to the desired temperature (with a tile below to catch any glaze that runs right off the end of the runway). During the firing, the glazes flow down the runway according to melt development, melt surface tension (and other factors like bubble development). What this tester can show you about glazes: 
 Raw Materials Testing Most companies can readily test clay materials for use in bodies and glazes using physical testing methods that require a minimum of equipment. But it is not so obvious how to compare and test fluxing materials like feldspar for consistency. One can just trust the particle size and chemistry information provided by the manufacturer for each shipment and compare numbers. But what is the actual relationship between these numbers and the consistency of product on a production line? Can you trust the numbers anyway? The tester is an elegant simple alternative. It accurately shows melting power, color and impurities, you need to see two feldspars side-by-side to see how sensitive it is (see pictures at bottom for an example). Product Development Many ceramic products are tuned to melt to a certain extent to achieve their function. For example, an engobe needs to have a stiffer melt than a glaze, but much more maturity than the underlying body. Likewise, a ceramic printing ink must have a specific degree of melt fluidity, enough to adhere or melt to a smooth hard surface, but not so much as to bleed into the covering or underlying glaze. Melts used for bonding purposes likewise need to develop enough glass to bond, but not so much that fired geometry cannot be maintained. A standard and a test can be evaluated side-by-side using this tester. If the melt is not fluid enough, then it can be fired higher, or a percentage of frit can be added. Taking Photos Since these fired testers are quite large, storing them for future reference can be a problem. Taking a picture of them and scanning it onto the computer for archival purposes makes more sense. Make them at least twice as large as the ones shown here and they should still take less than 100kb of memory. You may find that making the testers from an off-white, grey or even tan body might be better to prevent washed-out results when taking photos. Also, have plenty of side lighting so that gloss is highlighted. Slip RecipeA good starting recipe is #L2540, it is 50% ball clay, 25% feldspar and 25% silica. This does not cast quickly but the pieces have good green strength and the clay will vitrify around cone 10-11. For a more refractory mix replace some of the feldspar with kyanite, calcined alumina or some other non-plastic high temperature material. You will need to know how to mix and deflocculate a clay slip, search in this library for the word "deflocculation" for an excellent article on understanding the casting slip mixing process. Getting a Tester If you would like a mold of the flow tester shown here, use the email form on this page and ask for a quote. Plainsman Clays has the master rubber mold and can make plaster working molds from it for $100 each plus shipping. If you need alot of these we recommend you buy one plaster mold and use it to make a master rubber block mold so that you can make as many plaster working molds as you need. We can contact them on your behalf to arrange it and get a quote. Delivery time will likely be a month. Out Bound Links
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Pictures Two runs of Alberta slip plus 20% frit 3134 in a flow test comparison at cone 6.  Flow tester used to compare feldspars  Custer feldspar vs. G200 feldspar  A comparative flow test of Custer, G-200, high soda and high potassium feldspars.  This Nepheline Syenite flow test did not demonstrate much of a difference in melting at cone 9 between 270 and 400 mesh materials?  Glaze flow tester mold  Flow tester mold  Flow tester side view  Example of a cone 10 transparent that is running severely on a flow tester, but does run on actual ware. The glaze is cutlery marking (therefore lacking hardness). This, the running and likely leaching are due to extremely low SiO2, Al2O3 content.  Example of a calculated Cornwall stone substitute blend (left) vs. Cornwall stone itself. The two are mixed with 20% Ferro Frit 3134 and fired at cone 5.  | 
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