Find and Improve a Cone 6 Clear Ceramic Glaze

How I found a recipe on Facebook, substituted a frit for the Gerstley Borate and added the extra silica it needed to fight crazing. I got a fabulous cone 6 clear.

A. Insight-live

Click here to watch this at or click here to go to our Youtube channel

Kill the Gerstley borate. Fix the crazing. These are some of the most common things people do using Insight software. This can be a fantastic clear glaze if you adjust it to stop the crazing and replace the troublesome Gerstley Borate. The glaze ends up being more durable too. This is a video about how I used my account at to manage a testing project on this glaze and the surprizing things I learned. Click the Youtube link below to watch the video. Following is an outline transcript outline I used to make it.

This caught my attention on Facebook so decided to test it
-Copied from Facebook: create new recipe, import it, name it, code number it as G2926
GB not linking, fix it.

Mixed 2000 grams (mix ticket)
Glazed a mug and fired it
-It is very transparent, didn't craze out-of-the kiln on my porcelain. Good? Not quite.

Appraise it, its materials:
-Need to kill GB (dont have it, it gives amber to clears, causes blisters, gels slurry)
-Click GB (sources boron, a little MgO)
-Duplicate it, edit the copy, name/code#. Replace the GB with Frit 3134, Save and Done

Where are we?
-KNaO higher, B2O3 lower, MgO gone
-Edit and add a little Talc to source MgO, Save and Done

Juggle the recipe to match the chemistry
-Enter calculation mode, set increment at 0.5, enter MgO,

-Set Increment to 2 and add frit to match B2O3
-Al2O3 low, we can add clay, great! But not yet.

-Reduce Nepheline to match KNaO

-Correct B2O3
-Switch back to 0.5 and correct MgO
-Switch to 2 and add kaolin
-Adjust B2O3 again, then SiO2
Retotal recipe

-Glazed a mug: good
-A flow test to compare with original GB version (will attach it here)
-Did 300:icewater on a mug, crazed: expansion too high
-Another to compare to a standard I already know. Running too much.

Will accept silica.
Add 10% silica and retotal again
This was tested. Result was excellent

Do white variation (will be harder, flow even less, lower expansion)
-Print recipe for 2000 g

The lessons are:
-Test your glazes thoroughly
-Replace the problem materials
-Make sure they fit your clay
-Keep a good audit trail of your tests, use code numbers and pictures. Do it at

Improving a clear by substituting frit for Gerstley Borate

Improving a clear by substituting frit for Gerstley Borate

Melt fluidity test showing Perkins Studio clear recipe original (left) and a reformulated version that sources the boron from Ferro Frit 3134 instead of Gerstley Borate (right). The later is less amber in color (indicating less iron) and it melts to very close to the same degree.

Use a low silica porcelain to craze test your glazes

Use a low silica porcelain to craze test your glazes

Cone 6 transparent glaze testing to fit Plainsman M370: Left and right: Perkins Studio Clear. The far left one is a very thick application. Center: Kittens Clear. The porcelain for all is Plainsman P300. Why? Because P300 is much more likely to craze the glaze because it has a lower silica content (about 17% and only kaolin whereas M370 has 24% silica plus the free quartz that comes with the 20% ball clay it also contains). If a thick layer works on P300 it is a shoe-in to fit M370. If it also passes the oven:icewater test.

How can you test if a different brand of tin oxide will work?

How can you test if a different brand of tin oxide will work?

This is a melt fluidity test comparing two different tin oxides in a cone 6 transparent glaze (Perkins Clear 2). The length, character and color of the flow provide an excellent indication of how similar they are.

This cone 6 transparent looked good, but I still improved it alot

This cone 6 transparent looked good, but I still improved it alot

The green boxes show cone 6 Perkins Studio Clear (left) beside an adjustment to it that I am working on (right). I am logged in to my account at In the recipe on the right, code-numbered G2926A, I am using the calculation tools it provides to substitute Frit 3134 for Gerstley Borate (while maintaining the oxide chemistry). A melt-flow GLFL test comparison of the two (bottom left) shows that the GB version has an amber coloration (from its iron) and that it flows a little more (it has already dripped off). The flow test on the upper left shows G2926A flowing beside PGF1 transparent (a tableware glaze used in industry). Its extra flow indicates that it is too fluid, it can accept some silica. This is very good news because the more silica any glaze can accept the harder, more stable and lower expansion it will be. You might be surprised how much it took, yet still melts to a crystal clear. See the article to find out.

An example where adding silica really helps a glaze

An example where adding silica really helps a glaze

The flow on the left is an adjusted Perkins Frit Clear (we substituted frit for Gerstley Borate). It is a cone 6 transparent that appeared to work well. However it did not survive a 300F oven-to-icewater IWCT test without crazing on Plainsman M370. The amount of flow (which increases a little in the frit version) indicates that it is plenty fluid enough to accept some silica. So we added 10% (that is the flow on the right). Now it survives the thermal shock test and still fires absolutely crystal clear.

Do your functional glazes do this? Fix them. Now.

Do your functional glazes do this? Fix them. Now.

These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.

Adding silica will fix crazing, right? Not here.

Adding silica will fix crazing, right? Not here.

G2926B (center and right) is a clear cone 6 glaze created by simply adding 10% silica to Perkins Studio clear, a glaze that had a slight tendency delay-craze on common porcelains we use. Amazingly it tolerated that silica addition very well and continued to fire to an ultra gloss crystal clear. That change eliminated the crazing issues. The cup on the right is a typical porcelain that fits most glazes (because it has 24% silica and near-zero porosity). The center one only has 17% silica and zero porosity (that is why it is crazing this glaze). I added 5% more silica to the glaze, it took that in stride, continuing to produce an ultra smooth glossy. It is on cup on the left. But it is still crazing just as much! That silica addition only reduces the calculated expansion from 6.0 to 5.9, clearly not enough for this more severe thermal expansion mismatch. Substituting low expansion MgO for other fluxes will compromise the gloss, so clearly the solution is to use the porcelain on the right.

Out 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.

  • (URLs) Overview Video

  • (Glossary) Crazing

    Crazing refers to small hairline cracks in glazed surfaces that usually appear after firing but can appear years later. It is caused by a mismatch in the thermal expansions of glaze and body. Most ceramics expand slightly on heating and contract on cooling. Even though the amount of change is very s...

  • (Oxides) SiO2 - Silicon Dioxide, Silica
  • (Recipes) G2926B - Cone 6 Whiteware/Porcelain Transparent Base Glaze

    A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.

    2014-02-06 - A cone 6 transparent general purpose base recipe developed at Plainsman Clays by Tony Hansen (see link to go there below, it contains technical and mi...

  • (URLs) YouTube channel for Tony Hansen

In Bound Links

  • (Materials) Gerstley Borate - Plastic Calcium Borate

    Colemanite, Calcium Borate, Borocalcite

By Tony Hansen

Feedback, Suggestions

Your email address


Your Name


Copyright 2003, 2008, 2015, All Rights Reserved