Reducing the Firing Temperature of a Glaze From Cone 10 to 6

Learn the chemistry differences between cone 10 and 6 glazes and how to make a glaze melt at a lower temperature without introducing other problems like crazing.

D. Desktop Insight

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A key lesson because it explains the difference between cone 10 and 6 glazes, demonstrates how to evaluate frits to choose the best one to source boron to a glaze, how to determine how much frit to add to increase the amount of B2O3 in the formula by a specific amount, then how to juggle other materials to restore the rest of the chemistry (caused by the increase in other oxides introduced by the frit). It also demonstrates how to make a melt fluidity tester and how to find out which materials in a recipe are contributing a particular oxide to the formula.

Melting temperature is directly related to chemistry, learn what the chemistry differences are between a typical cone 10 and 6 glaze and how to use INSIGHT to make the conversion

Reducing the Firing Temperature of a Cone 10 Glaze to Cone 6
Welcome. In this lesson we are going reduce the firing temperature of a glaze from cone 10 to 6 and end up with a surprise.
I will research glaze melting at, talk about the differences between the two glaze types, boron sourcing and limits, watching thermal expansion when adding flux and unlinking a recipe from the recipe database.

Find an article at Digitalfire Reference Library
Tobegin I will choose Digitalfire Reference library from the INSIGHT Help menu.
On this page I will click Articles.
That takes me here, a long list related to ceramic chemistry, mineralogy and physical properties.
Then I will choose “Find” from the browsers Edit menu (in this case Firefox) and look for the words “firing temperature”. Notice Ctrl-f is the shortcut to find a word on a web page, this works in Internet Explorer also).

Increasing boron is the key
This article, “Reducing the Firing Temperature of a Glaze From Cone 10 to 6” says that boron is the magic oxide, increasing its proportion is the best way to move glazes down a few cones. It discusses some of the side effects of adjusting melting temperature that I will not cover here, I will just demonstrate the mechanics of increasing boron since this is the most important.
This one, “A Low Cost Tester of Melt Fluidity” looks interesting. I will click it.

Glaze melt fluidity tester
This is a very interesting article about a melt fluidity tester you can use to compare two glazes. It also has information about adjusting glaze melting temperature.
Notice this picture. This is a comparison showing the same glaze formula, the one on the left sources MgO and CaO from a frit, the one on the right from talc and calcium carbonate. As you can see melting temperature is more than just the chemistry, frits are a good idea for melting glaze better, no question about it.

Open a target formula for cone 6
To start I am going to open a target formula for cone 6 using this button above the formula list. I will select the Roy-Hesselberth one and click open.

Open 1214M cone 6 recipe in area 1
Here it is.
Then I want to open the 1214M cone 6 recipe that comes with INSIGHT in recipe 1. I will click here and enter enough of the name here to limit the long list of glazes I have stored in INSIGHT. I will click the line and click Open.

Open 1947U cone 10 recipe in area 2
Next, I will select recipe 2 and open the 1947U cone 10 recipe that comes with INSIGHT. I will enter enough of the name here to limit the long list, click the line and click Open.

The main difference: Cone 6 has boron
Now we have a cone 6 formula on the left, a cone 10 on the right and a cone 6 target. Let's look closer.
There is nothing really different about the fluxes, except this cone 10 has a little zinc. But zinc is common in glazes at all temperatures.
The alumina and silica in the cone 10 glaze are higher, but notice in the target that cone 6 glazes can have alumina and silica as high as this cone 10 glaze. Of course, in general, cone 6 glazes have more fluxes, and therefore, less silica and alumina.
The big difference is here, B2O3, or boron.

Why is boron better than other fluxes
Amazingly many people use frits and materials like colemanite and gerstley borate without ever knowing about this magic oxide these materials contribute.
It is a low expansion, low melting glass, that is exactly what we need to reduce the firing temperature of a cone 10 glaze.
Without boron we would have to try and do it with zinc, sodium and lithium, the other fluxing oxides do not melt at cone 6. But these three oxides have so many issues. Sodium is high expansion, zinc conflicts with many colors, lithium can only be used in small amounts or it causes glaze defects. Plus they just do not melt the way boron does.

Comparing a popular frit formula: 3124
But look at this typical frit. It sources so many other oxides besides boron. If we add this to a glaze is it not going to throw the chemistry completely out of whack?
That is why there are so many kinds of frits. Look at the oxides our cone 10 glaze has. The frit does not contribute anything that is not already in the glaze.
If I can use the amount of frit needed to give me a specific amount of boron, I should be able to adjust other materials in my recipe to realign the other oxide amounts.

I will use frit 3134: It has no alumina
But I am going to use Frit 3134 instead, it has fewer oxides (so it is simpler), more boron, and it is the one already in the cone 6 recipe I have opened. It also has no alumina, that will enable me to source Al2O3 from kaolin to suspend the glaze better.
In a minute you are going to see why this frit is so popular.

Increase B2O3, reduce KNaO
I have selected recipe 2, selected the Frit line, then clicked the increment button to increase the amount two-at-a-time until the B2O3 matches the cone 6 recipe.
Now I will match the KNaO numbers. The frit is adding this so I need to reduce the feldspars contribution.

Kill the ZnO
Then next thing I have done is zeroed the amount of zinc in our former cone 10 recipe using this button.

Unlinking the recipe from the database
Before going any further we need to do something. I have clicked the Recipe Details tab.
I am editing the G1947U recipe, it is code number 232 in the database. If I click the Save button I am going to wipe out that recipe and replace it with this.
Then I will choose Duplicate Recipe in the File menu. This will remove the link between this recipe and database record 232. Now when I click the Save button INSIGHT will create a new database record and link this to it and save the recipe there.

Fine tuning the rest of the recipe
Next I will update these blanks, including the date. Mostly I removed information from the 1947U recipe. Then I will click Save.
Now back at the formula, notice the oxides. There are only minor differences between our adjusted cone 10 recipe and the cone 6 one.
I will match up the KNaO by reducing the feldspar, the Al2O3 by increasing the kaolin, the SiO2 by reducing the silica.
Now you know why this frit is popular, it has a balance of oxides similar to most glazes, so adding it to supply boron does not disrupt the chemistry too much.

The new recipe is the same as G1214M!
OK, I have done the adjustments. The cone 6 recipe and our adjusted cone 10 one have the same chemistry.
But check this out. The recipe of the adjusted cone 10 glaze has almost equal parts of the same 5 materials also. Isn't that interesting.
That means that chemically, the standard Digitalfire G1214M cone 6 transparent recipe is just our standard 1947U cone 10 transparent plus 0.2 molar parts of boron and no zinc.
The amazing thing is I did not realize this until composing this lesson.

Noticing cost, thermal expansion
But when I open the original 1947U in area 1 to compare, you would never know this, they look quite different.
So the glaze on the left is the original, the one on the right is our adjustment to make it melt at cone 6.
Notice it raises the thermal expansion, but not by much. The more you adjust glaze chemistry to reduce the melting point the more of a challenge it is to keep expansion, and thus crazing, down. The main reason is the lower silica and alumina. But again, the low expansion boron is the secret to keeping the expansion down.
Notice also that the cost is up, that is expected since frits are more expensive. You can tell INSIGHT the costs of your individual materials using the Overrides/Typecodes dialog.

The End
So, the secret to reducing the firing temperature of a cone 6 glaze is to add boron and leave the rest of the oxide amounts unchanged. There is no way you can do that without ceramic chemistry.
The article that we found at the Digitalfire Reference Library a few minutes ago says that sometimes it is better to identify the color, opacification and variegation mechanisms and then transplant these into a cone 6 base glaze of similar properties to the cone 10 one.

Out Bound Links

  • (URLs) Tutorial Videos at Digitalfire

  • (Oxides) B2O3 - Boric Oxide
  • (Glossary) High Temperature Glaze

    In functional ceramics this term generally refers to glazes that mature at cone 8 and higher. At these temperatures natural minerals like feldspar, calcium carbonate, etc can be compounded to create glazes that will melt well without the need for powerful (and troublesome fluxes) like zinc oxide, bo...

  • (Glossary) Medium Temperature Glaze

    In functional ceramics this term generally refers to glazes that mature from cone 4 to 7. At these temperatures it is difficult to compound glazes that will melt well without the need for powerful melters like zinc and boron. Thus a medium temperature glaze contains mostly the same kinds of ingredie...

  • (Glossary) Flux

    On the theoretical glaze chemistry level, a flux is an oxide that lowers the melting or softening temperature of a mix of materials. Fluxes are interactors (they often melt poorly on their own but react strongly with high melting materials where Al2O3/SiO2 predominate). There are less than ten commo...

  • (Glossary) Borate

    The term 'boron' refers to the oxide B2O3. 'Borate materials' thus contain B2O3, they source it to glass-building during melting in the kiln. Boron is actually the potter's friend (because of his electronic-controller-equipped kiln) while as the same time it can be a scourge in industry (because the...

  • (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...

In Bound Links

  • (Articles) Reducing the Firing Temperature of a Glaze From Cone 10 to 6

    Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.

  • (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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