Subsituting Gerstley Borate in Floating Blue

Use Desktop Insight to explore ways of calculating substitutes for Gerstley Borate in the popular Floating Blue cone 6 glaze recipe while maintaining or improving the other raw and fired properties of the glaze.

D. Desktop Insight

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Details how to substitute Gerstley Borate for another boron source in the popular Floating Blue glaze recipe. The lesson demonstrates that the most practical way to deal with the GB issue is on a glaze-by-glaze basis, formulating an approach that maintains the chemistry of the glaze, evaluates the sources of boron in the light of the chemistry of GB but also its physical properties and their importance to the working properties of the glaze. The boron sourcing frit (Ferro Frit 3134) is tried first and proves to be unsuitable, then ulexite is employed and found to be an ideal substitute.

Lesson 6: Substituting Gerstley Borate in Floating Blue, when frits are not suitable, ulexite, using phantom, static and added status with colorants and retotaling and static materials

Substituting Gerstley Borate in Floating Blue
Welcome. I am going to talk about substituting Gerstley Borate for another material in the popular floating blue glaze recipe.
Also, when frits are not suitable, ulexite, using phantom, static and added status with colorants and retotaling and static materials.
Gerstley Borate has been a popular ingredient in raw glazes for many years. At we author a website about this at
Recently it has gone through various perplexing cycles of becoming unavailable, then available again. More frequent changes in its behavior have accompanied this. The artware and pottery worlds have thus been absorbed in the pursuit of a substitute.
There is an better alternative. You can use INSIGHT to remove GB from your glazes and supply the lost oxides from other materials.
Floating blue glaze recipe
I will demonstrate replacement using the popular Floating Blue cone 6 glaze (this recipe has its own detailed page at I will replace the GB with another boron sourcing material.
I am going to be mentioning some techniques using INSIGHT that might not be totally clear unless you have watched some of the previous lessons.
Enter and duplicate the recipe
I have made sure the Lessons materials table is selected.
And have keyed floating blue into recipe 1 (I have left out the iron, cobalt and rutile, I am going work with the colorless base only).
Now I am going to click here to duplicate it into recipe 2 and make sure I have both recipes set to RO Unity calculation.
Replace Gerstley Borate with frit 3134
I have also removed the Gerstley Borate from recipe 2 by making sure its line was selected and clicking the line delete button.
I have also selected the next blank line in the recipe and added the same amount of Frit 3134.
Now if I click the MDT button to open the materials dialog you will notice that …
With this frit is a good choice
This frit contains no alumina, thus I can source alumina from kaolin to suspend the glaze. I need to think about this because it was the natural claylike nature of Gerstley Borate that kept the original glaze in suspension.
But this frit is not working
At first this frit seems like it could work. But it brings a lot of extra sodium so the nepheline, which is currently supplying the bulk of this, has to be dropped to compensate. This will create a balance that makes juggling the materials to match the oxides quite complex.
Notice I have also checked the KNaO box.
Turkish Ulexite is better
A better solution is to choose a material of more similar mineralogy and chemistry to Gerstley Borate. This is the substitutes page at
Turkish Ulexite is imported into North America and other continents in large quantities for use the fiber glass industry.
If I click here I will be taken to a detail page at the Digitalfire Reference Database.
Let's try this ulexite.
Replacing GB with ulexite
I am going to replace the frit with 20 units of ulexite. Notice I have cleared the Label and am about to click Update.
Planning a strategy
Notice that the CaO and MgO are now lacking and the KNaO is high.
If I add materials to source the former, the latter should be pushed downward. Why? Because this is a unity formula, INSIGHT recalculates the fluxes to total one, if I increase one the other amounts drop.
Notice also that the SiO2 and Al2O3 are high. Increasing the total amount of flux is going to force their amounts downward for the same reason.
Matching CaO, MgO with whiting, talc
I have added talc to source MgO and Whiting to source CaO. I played with the amount of each (by incrementing and decrementing) to get the closest formula match on a recipe amount multiple of 1.
Notice the KNaO is just a little high…
Fine-tuning the new recipe
So now I will decrement the nepheline by 2 to match it up and then increase the kaolin by one to bring the Al2O3 back up to match.
Amazingly the silica does not need to be changed to adjust the SiO2.
Next, I am going to put the colorants into the recipe.
Assigning coloring oxides as phantom
Notice what happened here. When I entered this material I just typed “cobalt ox” assuming INSIGHT would find it in the materials database and expand the name. But this asterisk indicates that it did not find it. How do I know that? I can click here and this dialog explains what the status characters mean.
To get the name to look right I will edit this blank and update the line.
But that is not really a problem, I do not want coloring oxides in the formula anyway. So I am going to click this checkbox for these two lines and update also.
Using static colorants when retotaling a recipe
OK, these materials are not affecting the calculated formula anymore.
One more problem I want to deal with is this total. I would like to have a total of 107 and designate the coloring oxides as added materials.
But if I just recalculate to 107 the coloring oxide amounts are going to change. The solution is to first mark the three lines with static status. Static lines are ignored during retotal. Notice the new status characters.
Defining colorants as Added materials
I have chosen retotal from the Calc menu and specified 100 and then used Round Amounts in the Calc menu.
Here is the new total and the recipe. I should mention that the static status is not remembered with the recipe.
However added status is. I am going to set each of them as added using this checkbox, thus when I print the recipe they will be separated as such.
Fired samples: Improvements still needed
Take another look at the chemistry.
This glaze is very high in sodium and potassium and its calculated thermal expansion is likewise very high, that means crazing. However the high boron content appears to counteract this somewhat in practice. You can use INSIGHT to substitute some of the KNaO for another flux, but that might affect gloss.
Here are the fired samples. The Ulexite version has the same character, it just needs a cobalt/iron change for a little more color and a flow test to see if fluidity needs adjustment. The glaze slurry seems to work well also.
The End
This is yet another example of a problem that can only be effectively dealt with on the chemistry level.
Many people have struggled with this problem but doing it this way was quite simple, don’t you agree?
That is the end of this lesson.

Floating Blue at cone 6 using Boraq Gerstley Borate substitute

Floating Blue at cone 6 using Boraq Gerstley Borate substitute

Floating Blue is a popular cone 6 glaze recipe used by the pottery community. Gerstley Borate is a material commonly used in recipes as a melter. The recipe produces a variegated surface but is difficult to replicate since its fragile mechanism makes it susceptible to variations in thickness, firing schedule, clay body and material supplies.

Something is definitely wrong. What is it?

Something is definitely wrong. What is it?

An example of how a glaze that contains too much plastic has been applied too thick. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and calcine material. Enough raw is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. The Alberta Slip website has a page about how to do the calcining.

Something is definitely wrong. What is it?

Original floating blue close up

A much better Cone 6 Floating Blue

A much better Cone 6 Floating Blue

GR6-M Ravenscrag Cone 6 Floating Blue on Plainsman M340 buff stoneware. This glaze also has this variegated visual character on porcelain. Because it has the GR6 base recipe (which is publicly available at, the slurry has very good working properties in the studio, it is a pleasure to use. This is an excellent showcase for the variegating mechanism of rutile.

Out Bound Links

  • (URLs)

  • (URLs) Tutorial Videos at Digitalfire

  • (Glossary) Material Substitution

    Material substitution is a constant and ongoing part of any ceramic enterprise that is using clay and/or glaze recipes. Often lack of availability, quality issues and price are the motivating factors. In addition, when recipes need to be used in another locality where the same raw material brands or...

In Bound Links

  • (Materials) Ulexite - NaCaB5O9·8H2O - Sodium calcium borate

    Television Stone

  • (Materials) Gerstley Borate - Plastic Calcium Borate

    Colemanite, Calcium Borate, Borocalcite

  • (Recipes) G2587 - Floating Blue Cone 5-6 (Gerstley Borate)

    A classic recipe from David Shaner. It is a brilliant highly variegated blue.

    2004-03-12 - This glaze has its own page at This is a good example of a glaze that works well using Boraq 2 as a substitute for Gerstley Bo...

  • (Materials) Ferro Frit 3134 - Leadless and low alumina high calcia borosilicate frit


  • (Typecodes) 1: GBS - Gerstley Borate Substitutes
  • (Glossary) Digitalfire Insight

    A desktop application for Windows, Linux, Macintosh that you download and install. Insight is a classic glaze chemistry calculator. -It interactively converts recipes to formulas and back. The main Insight window shows side-by-side recipes and their formulas, you can make chemistry changes to one...

By Tony Hansen

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