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Glaze Melt Flow - Runway Test - GLFL

This test procedure was employed in the Foresight Ceramic Database and now is available for those having an account at Insight-Live.com. Accumulating test data using the variables defined in these procedures enables us to create tools that enable you to compare the physical properties of materials and recipes.


Melt fluidity is very difficult to judge from simple observation on glazed ware or samples. Knowing the melt fluidity of a glaze is very important in maintaining consistency, duplicating and understanding it. Knowing how a material melts is important to knowing how to use it in your process. This test requires a special mold to cast flow testers.

This test is done to COMPARE the melt fluidity of glazes, not to establish an absolute value. While not absolute, it is valuable to compare the melt patterns of two materials, see many subtle and obvious differences and understand their issues.

Mix the powdered material with water and prepare a thick paste that can be formed into a small ball (dewater it on a plaster batt if needed). Form 12 gram balls and dry them. 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 flint, 25 feldspar is a good starting point.


LEN - Length (V)

Suggested units are cm

NOTE - Note (V)

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

Melt flow tester used to compare feldspars

Melt flow tester used to compare feldspars

Fired to cone 10 oxidation. Although feldspar is a key melter in high and medium temperature glazes, by itself it does not melt as much as one might expect in this GLFL test.

G1215U vs. G1215W glaze flow test

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

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

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.

Testing the new brand of dolomite

Testing the 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

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

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

Flow tester master model showing dimensions

Flow tester master model showing dimensions

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

Out Bound Links

In Bound Links

  • (Articles) 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.

  • (Tests) GSPT - Frit Softening Point
  • (Tests) BWIW - Boiling Water:Ice Water Glaze Fit Test
  • (Tests) GBMF - Glaze Melt Fluidity - Ball Test
  • (Glossary) Melt Fluidity

    Glazes become fluid when they melt, they are molten. The fluidity (or viscosity) of this melt needs to be considered, especially when troubleshooting problems. While two different fired glazes may appear to have melted a similar amount (even on a vertical surface), one may be radically more fluid th...

By Tony Hansen

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