|Monthly Tech-Tip |
Melt fluidity is very difficult to judge from simple observations on glazed ware or samples. Knowing the melt fluidity of a glaze is very important in maintaining consistency, duplicating and understanding it. Information on how a material melts is important to know how to use it in your process. This test requires a special mold to cast the flow testers.
This test is done to COMPARE the melt fluidity of glazes (or materials), not to establish an absolute value. It compares the degree and character of two melting materials, showing many subtle and obvious differences.
Mix the powdered glaze or raw material with water and prepare a thick paste that can be formed into a small ball (dewater it on a plaster batt if needed). The GBMF test has more information on forming balls from non-plastic materials (glazes clay-containing materials almost always have enough plasticity). Form 12-gram balls and dry them. Some materials, such as feldspars, will not melt enough to flow so they need a flux (e.g. we add 15% Ferro Frit 3195 when comparing feldspars at cone 6-10).
Place the balls from the two materials to be compared in the flow testing device (make sure they are the same weight). Write the identity (using a ceramic pencil) above the reservoir of each. Fire the device in the lean-back position to the desired temperature (with a tile below to catch overrun). Take a picture and upload it to your account at insight-live.com.
Flow testing devices: Make your own or buy one from PlainsmanClays.com.
Use a white burning not-too-refractory casting body to make the testers. 50% ball clay, 25 silica, 25 feldspar is a good starting point.
Here is how to prepare a melt-flow ball for the GBMF test or the GLFL test: Weigh 11g of the frit and 0.33 of Veegum (the latter plasticizes the frit and imparts dry hardness). Put them in a small ziplock bag. Zip it to entrap air, roll the zipper down to inflate it and shake well to mix the Veegum and frit. Stir the powder into plenty of water in a small bowl. A brush works best for quick delumping and cleaning out the bowl when pouring on to a plaster slab. It dewaters very quickly, after about 10 seconds, just as the water sheen is gone, peel it up with a rubber rib. Smear it back down and peel it up every few seconds until it is plastic and formable (not sticky). Roll it into a ball, label it and dry it under a heat lamp (could take an hour, the Veegum really holds on to the water). When dry use a sharp knife to cut off enough to bring it down to 10 grams.
These two glazes have the same chemistry but different recipes. The F gets its boron from Ulexite, and Ulexite has a high LOI (it generates gases during firing, notice that these gases have affected the downward flow during melting). The frit-based version on the right flows cleanly and contains almost no bubbles. At high and medium temperatures potters seldom have bubble issues with glazes. This is not because they do not occur, it is because the appearance of typical glaze types are not affected by bubbles (and infact are often enhanced by them). But at low temperatures potters usually want to achieve good clarity in transparents and brilliance in a colors, so they find themselves in the same territory as the ceramic industry. An important way to do this is by using more frits (and the right firing schedules).
Dolomite is a key material for glazes, especially mattes. When you are forced to adopt a new brand it needs to be tested. Here, three tests were done to compare the old long-time-use material (IMASCO Sirdar) with a new one (LHoist Dolowhite). The first flow test is a very high dolomite cone 6 recipe formulated for this purpose; the new material runs a little more. The second is G2934 cone 6 MgO matte with 5% black stain; the new material runs a little less here. The third test is the high dolomite glaze on a dark burning clay to see the translucency and compare the surface character. They are very close. It looks like it is going to be OK. Does your supplier test new materials when they are forced to switch suppliers?
These recipes have the same chemistry but the 1215U uses frit to source the MgO and CaO. This demonstrates that it is not just chemistry that determines melt flow. Raw materials are crystalline and have different melting patterns than frits (which have already been melted and reground).
Albany Slip was a pure mined material, Alberta Slip is a recipe of mined materials and refined minerals designed to have the same chemistry, firing behavior and raw physical appearance.
This is one of multiple views of the solid plastic original model of a glaze melt fluidity tester.
Suggested units are cmNOTE - Note (V)
Characterize the flow noting bubbling, blistering, matting, crawling, etc.
A Low Cost Tester of Glaze Melt Fluidity
This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.
Glaze Melt Fluidity - Ball Test
A test where a 10-gram ball of dried glaze is fired on a porcelain tile to study its melt flow, surface character, bubble retention and surface tension.
Boiling Water:Ice Water Glaze Fit Test
Ceramic glazes that do not fit the body often do not craze until later. This test stresses the fit, thus revealing if it is likely to craze later.
Frit Softening Point
In ceramics, this is the temperature at which a glaze or glass begins to flow, ceasing to exhibit the properties of a solid.
Glaze Leaching Test
Simple tests to evaluate the stability of a ceramic or pottery glaze against leaching metals in food or drink.
Tests conducted on glaze batches used in production (as opposed to tests conducted on the materials used to make those glazes).
Ceramic glazes melt and flow according to their chemistry, particle size and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.
|By Tony Hansen|
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