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GLFL - Glaze Melt Flow - Runway Test

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 the right amount of water to enable forming a small ball (dewater it on a plaster batt if needed). Gummed glazes can be very difficult to dewater, if it is not possible to get a plastic mass just dry them till crumbly and add enough water back in so that fragments will hold together. Form balls and dry them to get a 9-10g weight (usually about 12g wet). The GBMF test has more information on forming balls from non-plastic materials (glazes containing clays almost always have enough plasticity).

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 (be sure they are close the same weight). Write the identity (using a ceramic pencil) above the reservoirs 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.

Preparing balls for a melt fluidity test using Veegum

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This method uses 3% Veegum. A 0.01g scale is required (the amount of Veegum needs to be precise since it affects melting). Weigh 11g of the frit and 0.33 of Veegum. Put them in a small ziplock bag, zip it to entrap air and roll the zipper down to inflate it. Shake well to mix. Stir the powder into water (~5-8g) in a small bowl. Pour it onto a plaster slab - it dewaters very quickly (e.g. as little as 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 (but not sticky). Roll it into a ball, label it and dry it under a heat lamp (this could take an hour, the Veegum really holds on to the water). When dry we use a sharp knife to cut off enough to bring it down to 9 grams. One caveat: The Veegum is a melting catalyst, balls will flow noticeably more, this is not an issue if you are comparing the melts of two materials (but it is for tests meant to be absolute).

G1215U vs. G1215W glaze flow test

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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).

Melt fluidity of Albany Slip vs. Alberta Slip at cone 10R

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This is a GLFL test, it employs a slipcast melt flow tester to show the flow patterns of two glazes (or materials), side-by-side. Albany Slip was a pure mined silty clay that, by itself, melted to a glossy dark brown glaze at cone 10R. By itself it was a Tenmoku glaze at high temperatures. Alberta Slip is a recipe of mined clays with added refined minerals that give it a similar chemistry, firing behavior and raw physical appearance. As you can see, the melt fluidity is very similar.

Testing a new brand of dolomite

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Dolomite is a key material for glazes, especially mattes. We were forced to adopt a new brand and needed confidence it was equivalent. Three tests were done to compare the old long-time-use material (IMASCO Sirdar) with a new one (LHoist Dolowhite). The first melt flow tester compares them in a very high dolomite cone 6 recipe formulated for this purpose; the new material runs just slightly more. The second tester is uses the G2934 cone 6 MgO matte recipe 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. These three gave us the confidence to proceed.

Glaze melt fluidity comparison between G2931F and fritted G2931K show the effect of LOI

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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).

Flow tester master model showing dimensions

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This is one of multiple views of the solid plastic original model of a glaze melt fluidity tester.

Old three-piece glaze flow tester mold

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These three plaster mold pieces were made manually by carving the original and casting each of these from that using clay, soap, cottle boards and clamps. Then negatives of each of these were cast using PMC-746 rubber. Plaster is then poured into each to make these working molds. This heavy mold was useful to produce large numbers of testers. But for a potter, hobbyist or educator it was serious overkill. Our new 3D printed version requires no clay, soap, cottle boards, clamps or even the third piece - just pour plaster into two 3D-printed shells.

Our G2934Y matte yellow vs. Amaco SM-63 yellow

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Left: Our G2934Y recipe with 12% Cerdec 239416 yellow stain. Right: Amaco SM-63 matte yellow. This GLFL test compares the melt flow of these two in a cone 6 C6DHSC schedule firing. The SM-63 is melting very well and yet is more matte. Its color and character indicate the presence of rutile. Notice its flow has many pinholes, this indicates the presence of materials having significant LOI (these holes are also evident where layer thickness is greater on the tile). The high melt fluidity (indicated by this melt flow tester) is the mechanism behind the way its color intensity varies with the thickness (note the tile on the lower right). The SM-63 also performs well in a cutlery marking test.

Variables

Suggested units are cm

Characterize the flow noting bubbling, blistering, matting, crawling, etc.

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