Lesson 2 - Creating a Matte Glaze

Learn to convert a glossy glaze into a matte by comparing its chemistry with a target matte formula. Alter the chemistry in such a way that the thermal expansion does not rise and it maintains good physical application and suspension properties.

D. Desktop Insight

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Takes you on a detailed tour of how to start with a glossy glaze, compare its chemistry with a target formula in the light of what kind of chemistry matte glazes have, then alter the chemistry while at the same time watching that the thermal expansion does not rise and that the glaze maintains good physical application and suspension properties. The chapter shows you how to research the topics of fixing crazing and the chemistry of matte glazes and how to choose the right frit to supply the boron. This lesson teaches many valuable tips on both using Insight and manipulating glaze recipes.

Lesson 2 Transcript: Turning a Glossy Glaze Into a Matte
Learn the chemistry differences between glossy and matte glaze and convert a cone 6 glossy into a matte. This process will expose you to many other issues.

Creating a matte glaze
Welcome to Lesson 2, I am going to create a matte glaze.
We are going to deal with many basic concepts in both ceramic chemistry and using INSIGHT itself.

Close both recipes, then open
mWhat makes a glaze matte. Many factors can be at play, but the major one is the chemistry.
To demonstrate I am going to adjust the chemistry of a glossy glaze to make it fire matte. I still have the recipes open and the Lessons materials database selected from the last lesson so I could start by clicking the Close Both Recipes button and key in the glossy recipe.
But instead I will just open a glossy recipe I already have as a starting point. I will select recipe 1 and choose Open from the File menu.

Recipe open dialog
INSIGHT displays the Recipe Database window. To find the one I want quickly I am searching the word 'gloss' here.
Notice I have checked the See Notes/Recipes box to make this dialog show me more detail about each line as I click it.
Here it is. This is the 20x5 recipe that we have used for years as a starting point for adjustment projects like this.
I will click the Open button.

Copy recipe 1 to recipe 2
Now I will duplicate recipe 1 into recipe 2 and then make changes to 2 and compare. This is a way that INSIGHT is often used, you will really see how it excels at this as we go along.
In the dialog that displays I will click the Copy Right button (notice I can also move a recipe to the other, add it, or swap them).

Side-by-side recipes
The Recipe List should now look like this.
As before, notice the recipes share the same material label but have separate columns for amounts.
Now I will click the Recipe Details tab.

Recipe details tab
Notice I have entered ‘Cone 6 Matte Base’ into the Description blank for recipe 2. I also assigned a new code number in this 1214 series to write on test samples.
There is a web page about this glaze, I am going to paste the URL here.
In future, anyone looking at this glaze will be able to click this button to read all about it here.

Opening a target formula
I am going to compare the formula of the G1214M recipe with a target formula to rationalize what changes might matte the glaze.
I will click green Open Target button.
In the recipe database window INSIGHT is showing me only the ones that contain the word TARGET in the code column.
I will click the Roy-Hesselberth one for cone 6 and then click open.

Target formulas and KNaO
The Formula List should now look like this. I adjusted the column widths a little to display them correctly.
Notice that the target formula weaves itself into the list and also displays oxides that may not be present.
I have unchecked KNaO to demonstrate that this target combines the K2O and Na2O into KNaO so it does not appear beside them.
I will click it again, now that looks better.

Learn about matte glazes at Digitalfire.com
You can learn what makes glazes matte searching for ‘matte base’ in the google search on digitalfire.com home page.
It dynamically pushes the search results downward on the page. I clicked here in the Digitalfire Reference section to get it to show more.
This is the one I want.

Matte glaze mechanisms
Matteness is the result if the glaze micro surface is not glassy smooth. This smoothness can of course be impeded by improper melting, but of course you won’t get a durable glass. Here are some correct ways to do it:
Glossy glazes with a significant amount of boron (e.g. 0.2 or higher) form matte surfaces at a point after complete melting but before smooth-out.
Al2O3 stiffens the glaze melt and keeps it from running off the ware. High alumina thus prevents the glaze from ‘leveling out’.
High silica glazes are glossy so it stands to reason that low silica ones with high end Al2O3 would more likely be matte. Now we know why people talk about the silica:alumina ratio, it can be a measure of gloss, a ratio below 5 is a typical goal.
In well melted glazes if some oxides like CaO, SrO, BaO amounts are high-end or higher than limits a micro-crystal mesh can form on the surface during cooling and it scatters light and looks matte.

Remove the silica
Notice these. This means there is a trace of the oxide, less than 0.00.
Now we are going to increase the Al2O3 to the upper end of its range and reduce the SiO2 to the lower end of its range.
The first obvious step is to remove the silica in recipe 2 to reduce SiO2 in the formula as much as possible. I can do that by editing the line to zero or clicking this button.

The SiO2:Al2O3 ratio
Notice how much the SiO2 has dropped and how much lower the Si:Al ratio is. Again, this ratio is an indicator of gloss in alumina matte glazes and it needs to be around 5:1 for this type of glaze for a good matte. Thus I need to bring it down more.

How to find the greatest Al2O3 supplier
I will select Al2O3 in the Formula List and click the Supplier button. INSIGHT searches for the recipe material contributing the most Al2O3 and selects the kaolin in the Recipe List.
As you can see, I have doubled the amount of kaolin in recipe 2 to 40.
Remember how you know what oxides are supplied by a material? Just double-click it in the recipe list to bring up the materials dialog for that material.

Watching out for side effects
The silica:alumina ratio is now much lower, maybe even too low. This would very likely be a matte.
However there are a couple of problems:
First, 40 parts kaolin provides the needed Al2O3 but would likely shrink and crack the glaze during drying on the ware.
Second, the thermal expansion if this recipe is high, it will likely craze. How do I know? Because in my experience, 7.5 is too high an expansion for bodies I use.

Warding off high expansion
To find articles on crazing at the Digitalfire Reference Library follow the procedure I am doing here (after you log in). At the articles home page this section on thermal expansion is especially interesting. I found an article that says substituting high soda/potash for another lower expansion flux is the biggest single thing you can do to kill crazing (I guess I knew that because I wrote the article).

Test feldspar exclusion with phantom checkbox
What would happen if I removed the feldspar, the biggest contributor of sodium and potassium? There is a way to check without changing the recipe.
I will make sure recipe 2 and the feldspar line is selected and click the Phantom check box and then click Update (this tells INSIGHT not to include feldspar’s chemistry in the calculated formula).

Status column characters
Notice the asterisk in front of the feldspar.
I will click this button. INSIGHT explains what the symbols it displays in front of recipe lines mean. INSIGHT now ignores the feldspar completely in chemistry calculations, behaving as if it could not find chemistry information for it in the materials database.

Fine tune B2O3/SiO2 to finish
Notice the SiO2 has dropped considerably and the KNaO is down a lot in exchange for calcia. Remember, I want low silica for matting and low KNaO to prevent crazing. This looks good, I will zero the amount of feldspar in recipe 2.
To finish I would reduce the frit to match B2O3 back up and increase the silica in the recipe to bring the SiO2 up to the minimum 2.5 target.
However, as noted, 40 kaolin in the recipe is likely too high because the glaze will shrink excessively during drying, I could use a mix of calcined and raw kaolin to solve the problem. That is a lot of trouble, there is a better way.

We have the wrong frit!
Now the feldspar is gone and I have changed the kaolin amount in recipe 2 back to '20'.
I will also uncheck the Phantom checkbox for this line and update so recipe 1, which contains the feldspar, will calculate correctly again.
Now I will select the frit line and double-click it and to open the Materials dialog.

Finding a better frit
Notice that this frit contains no Al2O3.
I will click frit 3124 (it is just above 3134, so I need to scroll the list up). Notice that frit 3124 has significant Al2O3.
It is interesting to note that if you add enough Al2O3 to Frit 3134 to equal the Al2O3 in Frit 3124 the overall chemistries will almost match. As you can see using a frit with no alumina is a little illogical for making a matte glaze if it necessitates the use of too much clay to source alumina.

Switch the frits and appraise
I have closed the Materials dialog, selected the Frit 3134 line for recipe 2 and zeroed it.
I also selected the next blank line in recipe 2 and entered ‘Frit 3124’ with an amount of ‘20’.
The formulas look like this now.
Notice the B2O3 is down but the Al2O3 is up without any extra kaolin yet.

Re-match B2O3 first
I have increased Frit 3124 to match the B2O3 first. To do this I selected the Frit 3124 line and made sure Recipe 2 was selected.
I clicked the Increment arrow repeatedly. Each time INSIGHT incremented the amount on the selected recipe line by the amount shown here and updated the calculated formula for recipe 2. I kept doing this until the B2O3 in the two formulas matched (it did at 31 parts of frit).

Match Al2O3
That took the Al2O3 in G1214M to 0.36, I am targeting at least 0.45. The silica should be the minimum 2.5 target. That gives a silica:alumina ratio of 5.5.
I selected the kaolin line in recipe 2 and clicked the Increment button until Al2O3 for recipe 2 reached 0.45 (this took the kaolin to 27).

Match SiO2
I have selected the silica line for recipe 2 and nudged it up until the SiO2 reached 2.5, this turned out to be 4.0 silica. Later this bit of silica in the recipe will make it easy to fine-tune the amount of gloss.
Notice again that there is more CaO and less KNaO now. This will help drive down the thermal expansion and it will encourage matting. How do I know that? I can double-click the CaO oxide line and click the Info button in the Oxides dialog.

Learning about CaO
INSIGHT takes me to a page where I can read all about this oxide. Notice there are links to the other oxides here.

I now have a glossy on the left and a matte on the right. The silica and alumina are about where I want them.
The ratio is good and
The thermal expansion is the same, this is actually quite an achievement. It is actually possible to take the expansion a lot lower than this by sourcing MgO from dolomite or talc (or better yet a frit) at the expense of CaO.
But this should not be necessary, this should fit just about any body I use. I could increase the CaO even more to grow more calcium silicate micro crystals on cooling to get an even better matte, but this would require care to be sure the glaze was not soluble.

Add a static opacifier
Matte glazes look best when they are opaque. I have selected the next blank line in Recipe 2 and put in 4 parts Zircopax and then clicked the Static checkbox and the Update button. Why 4 parts? From experience, but you might like to do a line blend.
Notice the line has an ‘=’ sign here. This is the static attribute, it tells INSIGHT not to change its amount when retotaling the recipe, forcing others to compensate. For example, the others would total to 96 if I retotaled to 100 now.
What is the total for this recipe? Look in the Calculated Items list, it is 86.0.

Recalculate total using static status
I am recalculating the recipe to a total of 104 (100 for the recipe plus 4 opacifier). I am using the Retotal dialog from the Calc menu. After clicking OK notice that the Zircopax amount has not changed. Notice also that the other amounts are rounded off to the nearest tenth. I did this by choosing Round Amounts in the Calc menu.
You might like to mix this recipe up and try it at cone 6, you will see that it is a matte.

Recalculating total using added status
I want to show you something else while we are here.
I could have set Zircopax to Added status by checking this instead of phantom (notice the A in the status column).
But notice what happens when I recalculate the total to 100. These numbers total 100 but the Zircopax gets recalculated to 4.88, and thus the whole recipe to 104.88.
The zircopax was 4 parts out of 86 before, so it has be to 4.88 out of 104.88 now to be the same proportion.

Comparing the analyses
Now I have selected Analysis calculation type for both recipes. Remember, analyses compare oxide amounts by their weights rather than numbers of molecules. Some engineers like to look at glazes this way.
Notice the comparison, remember the matte is on the right. There is 8 less silica, 50% more alumina, half the KNaO and much more CaO. These are all characteristics of a true matte glaze. An added bonus: Chrome-tin pink stains require at least 10% CaO, both of these work really well with them.
Mole% calculation is different again, it compares the percentages of numbers of molecules. Some engineers, like Richard Eppler, argue that this is the most realistic way to compare glazes.

You may have some questions:
How can I get away with CaO that is higher than the limit?
-This is often acceptable if MgO is near zero, if there is plenty of B2O3 to melt things well. Remember also, chrome tin colors require high CaO and matte glazes work best with high CaO.
-Since these recipes look so different now you might think it is a stretch to say that the matte is an adjusted version of the glossy? But remember we are working on the oxide level, this matte formula is an adjusted version of the other.

Glaze solubility
High calcia glazes can have leaching problems, but we have high alumina and plenty of boron to help melt it, these are stabilizing influences. My initial goal was appearance and glaze fit but it would be easy to add a coloring oxide like cobalt and then do a vinegar leaching test as an initial indicator. But the solution will still be a matter of understanding oxides and their interplays.
You might think I am taking a cavalier attitude toward the limits? But remember that limit formulas are about what melts well, not what does not leach. If I exceed a target and get good melting, that is great. Glazes within limits are less likely to leach but those outside can also be durable. Also, mattes are considered special-purpose glazes, so don't expect them to have typical chemistries.

Out Bound Links

In Bound Links

  • (Recipes) G1214Z - Cone 6 Silky Matte

    This glaze was born as a demonstration of how to use chemistry to convert a glossy cone 6 glaze into a matte.

    2003-06-10 - Please click the article link below for more information. This recipe is adjustable in that you can raise and lower the silica to increase and decr...

  • (Glossary) Matte Glaze

    A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. I...

  • (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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