In both recipes, the Al2O3is 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 = higher SiO2. These highlighted UMF values are what I plotted on the Stull chart.
With Al2O3levels being the same, the points are plotted on the same horizontal line because we are only changing the x-axis (SiO2) values. Anytime you add straight Silica to a recipe, you will move horizontally to the right on the Stull chart.
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 Al2O3in 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 temp of your glaze. Eventually the glaze will have too much Silica to melt and it will be underfired.
What you will generally find is:
SiO2:Al2O3 = 5 or less —> matte glaze
SiO2:Al2O3 = 5 to 6 —> “satin” glaze (between matte and glossy)
SiO2:Al2O3 = 6 to 12 —> glossy glaze
SiO2:Al2O3 = over 12 —> underfired glaze
Will adding Silica turn every matte glaze glossy?
No, not every matte glaze will turn glossy with the addition of Silica. As with everything in ceramics, there are some exceptions and nuances to be learned,but there’s a 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 (Unity Molecular Formula). My favourite 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 the 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
All you have to do is add enough Silica to move the point on Stull 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 just use the + button to increase the Silica as you watch the dot move to the right across the Stull chart. Voila! You have re-formulated your matte glaze to be a glossy glaze.
If you want your matte glaze to be satin, or only slightly glossy, just add a small amount of Silica. This will move the dot only slightly to the right on the Stull chart. Typically a SiO2:Al2O3 ratio between 5 and 6 will give you a satin glaze. If it’s too matte, add some Silica. If it’s too glossy, remove some Silica. Testing is always required to get the exact result you’re looking for.
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.
If your glaze isn’t a “Stull Matte” glaze
If your matte glaze does NOT fall within the matte region of Stull, 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 might, it just depends how much titanium is in the recipe as well as where it falls on the Stull chart. Doesn’t hurt to try it.
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 Stull and does not contain titanium, there’s a good chance it’s underfired.
Look at the images below. The matte glazes are in the 1st photo. See how the glazes are slightly runny? And then the glossy glazes in the 2nd photo aren’t runny at all. This shows how increasing Silica raises the melting temperature of a glaze.
