Fixing a Settling Glaze Slurry

Learn to reformulate a glaze that is settling in the bucket. Al2O3 and KNaO are sourced by the feldspar, we will source them from kaolin and frits instead.

D. Desktop Insight

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Change the recipe of a glaze to increase its clay content (and thereby improve its suspending properties) while maintaining the same chemistry (and therefore fired results).


Welcome. In this lesson we are going to fix a problem with a settling glaze.
And if you are not sure of what that is, it refers to: A ceramic glaze that does not stay in suspension, it settles out in the bucket and it is very difficult to stir it back up.
Here is the glaze that I have been supplied.
I am going to copy it, then go into Insight, choose Interpret Clipboard from the menus and click OK.

A ceramic glaze is not a solution, it is a suspension. Simplistically, it is a bunch of rock powder stirred up in water.
Normally that powder should sink to the bottom immediately after you stop stirring.
But the clay in the mix holds all the other powders in suspension.
But at least 15% kaolin or 10% ball clay is needed for this to work.
This glaze has 2.5% bentonite, but no other clays. It needs kaolin.

Before continuing, notice that line containing the frit.
Insight has put an asterisk in front of it to signal us that it could not find this in the materials database.
To figure out what this frit is we will go to the Digitalfire Reference Library and search for it...
Notice it says it is a low alumina, high CaO boro-silicate frit.
At the bottom it links to Ferro Frit 3134 as an alternative.
So in Insight I will click the frit line and enter 3134 in the Materials List Lookup blank and click Update.
Notice that Insight has left the line label as frit 4108, but it is looking up the chemistry of frit 3134 to do the calculation of the chemistry.

How do we proceed?
In the other lessons we learned the best way is to copy it right to recipe 2, we can alter that and compare with the original on the left.
However in this case I tried that and it does not work!
The reason is that there is not very much Al2O3 in the glaze.

I will demonstrate that by opening a target formula for cone 6 to compare.
0.25 is the minimum and we have 0.23.
If I click the Supply button above the formula list, it will show me which material in the recipe is supplying the most Al2O3, it is soda feldspar.
The potash feldspar is going to be contributing a similar amount as well.
There will be almost no alumina coming from any of these other materials.
I will double-click the frit, it has zero alumnia, so the feldspars are the only two materials supplying alumina.

I am going to have to pretty well completely remove those two feldspars and start from scratch in supplying the boron from another frit that has alumina and the boron we need.
I will click recipe 2 and choose a calculation type of no-unity.
Whenever we build from scratch using the supply button we do this.
To start I have the frit 4108 line selected and I want to source boron from it. So I will click the Supply button.
I will check boron and then click Done. This matches up the B2O3.
It has supplied some of the KNaO that I needed (notice I have KNaO checked so Insight combines K2O and Na2O).
I am not going to be able to supply them individually and match them up, but as a total I can.

To supply KNaO I am going to introduce another frit. But which one.
I want one that has alot of K2O and Na2O but very little boron or alumina.
I will click the MDT button to see the materials database.
In frit 3110 we find almost no Al2O3 and a tiny amount of boron.
Notice it has lots of sodium and potassium as well.

I will close this dialog, select a blank line at the end of the recipe and enter frit 3110 in the Materials Lookup field and click Update.
I will click the Supply button. I cannot check the box for K2O and Na2O, but I will check just Na2O.
Notice I have 0.24 KNaO but I need 0.32. So I will increment the frit amount in the recipe and watch it until I get to 0.32.

Notice that in supplying the KNaO I have overshot the B2O3.
I will have to juggle the amount of these two frits, reducing the other and increasing this until I get a match.
I am not being really precise right now, an approximate match is sufficient.

Now I will supply the amount remaining for the CaO, I already have 0.27.
I will check the whiting line and then use the Supply dialog to source it.
I will supply the BaO from Barium Carbonate.
The only ones left to match are alumina and silica.
The alumina is very low, I will be able to supply all of that from Kaolin.
I will add EPK to the recipe, my experience is that it suspends glazes well.
In the supply dialog I will choose Al2O3 and click done.
To match the SiO2 I will choose Silica in the recipe and do likewise.
All the oxides now match.

Because I have been working in no-unity calculation type it totals more than 300.
I will retotal it to 100. The old recipe totalled 102.5 because of the 2.5 added bentonite, I am not using it here.
Notice also that in the old recipe there was copper carbonate and tin oxide.
I have ignored those, I just want to match the base glaze, they can be added later.
I will retotal it now using the ReTotal item in the menu.

Notice that I have 16 parts kaolin, that should suspend the glaze well.
If more suspending power is needed, some of that bentonite could be added back in.
Now notice that the numbers no longer match in the formula. Why?
If I set the calculation type back to RO Unity, they will again match.

Most people do not think chemistry impacts the raw properties of the glaze, the way the glaze slurry works, for example.
But we have created a different set of materials to source the same chemistry, and that set of materials has physical properties that are better.
Also, normally when we do major surgery like this it is to good to see if there are any other problems that could be resolved at the same time.
For example, barium carbonate powder is not the safest thing to use.
If you could find a barium sourcing frit that also had low alumina, you could work it into this recipe also.

Out Bound Links

  • (URLs) Tutorial Videos at Digitalfire

  • (Glossary) Boron Frit

    This term is very generic, referring of course to frits that contain boron. Unfortunately that is 80-90% of available frits! Boron frits may have 1% boron or 50% boron. Even though the boron in the frit is no longer in the borax form it is still customary to refer to such as "borax frits". Since man...

  • (Glossary) Suspension

    In traditional ceramics, glazes are suspensions, not solutions. They are mixes of insoluble mineral, frit and/or stain particles that have been added to water to form a liquid useful in the ceramic process. That suspension is what confronts us in the bucket or tank, learning how to assess and contro...

In Bound Links

  • (Troubles) Powdering, Cracking and Settling Glazes

    Powdering and dusting glazes are difficult and a dust hazard. Shrinking and cracking glazes fall off and crawl. The cause is the wrong amount or type ...

  • (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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