Do you have a matte glaze that you wish was a bit glossier? It’s really easy to convert a matte glaze to a glossy glaze, just by adding silica.

Defining the Terms 

Stull Chart: A graph developed by R.T. Stull in 1912, which plots the SiO2 and Al2O3 levels from the UMF of a glaze recipe. The position of a plotted recipe on the graph indicates whether a glaze is likely to be matte/glossy/underfired/ crazed etc. 

Unity Molecular Formula or UMF (aka Seger formula): This representation of a glaze recipe shows the proportions of molecules rather than the weights of materials. In the UMF, the fluxes always add up to 1 (= unity) and each glass former is displayed in its proportion to 1 molecule of flux. 

A Look at the Recipes

If you want to adjust the surface of a glaze from matte to glossy, you can do so by adding silica—as long as your matte glaze meets the parameters of chemistry outlined in this article. The two recipes below (1) show a matte glaze on the left and a glossy glaze on the right. When comparing these two recipes, note that the whiting, nepheline syenite, EPK, and frit are the same, but the silica has 40 more grams (per 100g) in the glossy version than the matte version. That’s literally the only change that was made to the matte glaze to make it glossy. So, if I were to make 100g of this matte glaze and then add 40g of silica, I get a glossy glaze. And that’s exactly what I did to create the test tiles below (2).

1 The base recipes for a matte (left) and glossy (right) glaze. Recipe screenshots courtesy of glazy.org.

2 The three photos above are pairs of test tiles where the only difference between each pair is the addition of silica. In each photo, the matte glaze is the tile on the left, and the matte glaze with added silica is the glossy tile on the right.

The next thing to notice is the recipe totals at the bottom. The matte glaze adds up to 100 and the glossy glaze adds up to 140. I could have normalized the glossy formula so that it also adds up to 100. This would recalculate the totals for each ingredient to make them all a percentage of 100, but I left it this way to make the change obvious when I go back and look at it in the future.

A Look at the Chemistry

Figure 3 shows two glazes (two white dots) plotted on the Stull chart. This chart takes the unity molecular formula (UMF) of a glaze recipe and plots the silica level (SiO2) on the x-axis and the alumina level (Al2O3) on the y-axis. On the Stull chart, the only variables are silica and alumina. Fluxes and boron remain constant throughout.

I have color coded the regions of the chart to make it easier to see the patterns. The green region is where the matte glazes are found, the pink region is where the glossy glazes are found, and the yellow region is somewhere between matte and glossy. The turquoise regions are underfired.

3 This Stull Chart has two white dots, which represent the two recipes shown in figure 1: matte glaze on the left, glossy glaze on the right.

Stull’s original research was done at cone 11, but I have recreated this chart by firing 50 tests with varying silica and alumina values at cone 6 and the patterns are generally the same. My tests also contained boron, which is required to bring the melting temperature down from cone 11 to cone 6. 

The white dots on the chart represent the silica and alumina levels of the matte glaze on the left and the glossy glaze on the right (referring to the recipes from figure 1). As you can see, the matte glaze is plotted in the matte section of the chart, and the glossy glaze is plotted in the glossy section of the chart. Make sense?

In order to understand where these points came from, let’s look at the recipes again, this time with the UMF included above the recipe (4). Check out the values for SiO2 and Al2O3.

In both recipes, the Al2O3 is the same, at 0.46. This makes sense because we didn’t change any of the ingredients that contain Al2O3. We only increased the silica, which is pure SiO2. In the matte glaze, the SiO2 is at 2.17. In the glossy glaze, the SiO2 is at 4.12. More silica added equals higher SiO2. These highlighted UMF values are what is plotted on the Stull chart.

With Al2O3 levels being the same, the points are plotted on the same horizontal line because we are only changing the x-axis (SiO2) values. Any time you add straight silica to a recipe, you will move horizontally to the right on the Stull chart.

4 The UMF is displayed above each recipe, with the SiO2 and Al2O3 levels highlighted. These are the numbers that are plotted on the Stull Chart. Recipe screenshots courtesy of glazy.org.

Another thing to look at in the UMF is where it says SiO2: Al2O3. This is the silica:alumina ratio and it’s given as a numerical value that represents the number of silica molecules for every alumina molecule in the formula.

The matte glaze has a SiO2: Al2O3 ratio of 4.72. This means that for every molecule of Al2O3 in the glaze recipe, there are 4.72 molecules of SiO2. For the glossy glaze, the ratio is 8.98 molecules of SiO2 for every molecule of Al2O3. It’s this ratio that determines whether a glaze is likely to be matte or glossy. As the SiO2: Al2O3 ratio goes up, a glaze will move from matte to glossy. But you can’t keep adding silica forever. Adding silica raises the melting temperature of your glaze. Eventually, the glaze will have too much silica to melt and it will be underfired.

Will adding silica turn every matte glaze glossy? No. As with everything in ceramics, there are some exceptions and nuances to be learned, but there are pretty simple criteria that will tell you if this method is likely to work. All you have to do is take your matte glaze recipe and use glaze calculation software to look at the UMF. My favorite way to do this is at glazy.org. Create a free account first if you don’t have one, and then enter your matte glaze recipe. Glazy will convert your glaze recipe to UMF and also plot it on a Stull chart. If your matte glaze falls into the matte region of the Stull chart, then you have what I call a “Stull Matte” glaze and this method of adding silica is very likely to work.

How to Adjust Your Stull Matte Glaze

Now, to make adjustments to your Stull Matte glaze, all you have to do is add enough silica to move the point on the Stull chart to the right and into the glossy region. You can do this by making a copy of your matte glaze in Glazy.org and then use the + button to increase the silica as you watch the dot move to the right across the chart. Voila! You have reformulated your matte glaze to be a glossy glaze.

If you want your matte glaze to be satin, or only slightly glossy, add only a small amount of silica, for a SiO2: Al2O3 ratio of between 5 and 6. This will move the dot only slightly to the right. If you want it to be glossy, aim for a SiO2: Al2O3 ratio of between 7 and 10. Once your SiO2: Al2O3 ratio approaches 12, your glaze is at risk of becoming underfired. Testing is always required to get the exact result you’re looking for.

Exceptions

If your matte glaze does not fall within the matte region of the Stull chart, then adding silica might not make it glossy. Some glazes are matte because they contain titanium, which crystallizes during the cooling cycle. Adding silica to a titanium matte glaze won’t necessarily make it glossy. It just depends how much titanium is in the recipe as well as where it falls on the Stull chart. 

Other glazes are matte because they’re underfired. An underfired glaze appears matte because it hasn’t melted into glass yet. Adding silica to an underfired glaze definitely won’t make it glossy because adding silica raises the melting temperature. If your matte glaze falls in the glossy region of the Stull chart and does not contain titanium, there’s a good chance it’s underfired.

the author Sue McLeod has been a studio potter since 2010 and works from her home studio full time, researching glazes and teaching three online glaze courses: Glaze Mixing Essentials, Mastering Glaze Consistency, and The Art of Glaze Chemistry. She also runs a large, free glaze-support community on Facebook titled “Understanding Glazes with Sue.” Read more ceramics articles and register for her online glaze courses at suemcleodceramics.com. 

Topics: Glaze Chemistry

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