How I found a ceramic glaze recipe on Facebook, substituted a frit for the Gerstley Borate, added the extra SiO2 it needed and got a fabulous more durable cone 6 clear.
In this video I will show step-by-step how I killed the troublesome Gerstley borate material and fixed the crazing in the popular cone Perkins 6 glaze recipe. I also show how I used my account at Insight-Live.com to manage a testing project on this glaze and the surprizing things I learned and unexpoected benefits the recipe accrued on improvement. Following is an outline transcript 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
-Switch back to 0.5 and correct MgO
-Switch to 2 and add kaolin
-Adjust B2O3 again, then SiO2
-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 insight-live.com.
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.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Glaze chemistry is the study of how the oxide chemistry of glazes relates to the way they fire. It accounts for color, surface, hardness, texturem, melting temperature, thermal expansion, etc.
YouTube channel for Tony Hansen
Insight-Live.com Overview Video
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.
|Oxides||SiO2 - Silicon Dioxide, Silica|
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.
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.
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.
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.
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 insight-live.com. 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.
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.
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.