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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 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.

Preparing balls for a melt fluidity test using Veegum

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

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

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. Alberta Slip is a recipe of mined clays and refined minerals designed to have the same chemistry, firing behavior and raw physical appearance (but not plasticity). This is a GBMF melt flow test showing them side-by-side.

Testing a new brand of dolomite

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?

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

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).

Glaze flow tester mold (three pieces). Piece on the left goes on top.

Flow tester master model showing dimensions

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

Variables

Suggested units are cm

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

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