Granite Glaze Tests - 2024 Year in Review

Testing of glazes using the Mt Vision Granite began Q1 2023 and continues to date, the results of which are documented in the blogs found on this site.

The final tests of 2024, conducted in September, and presented below, drive a brief overview of conclusions to date. In general, we are happy!

The tests cover 3 base glaze variations and 2 colorants + no colorant. They are summarized below:

Base Glaze

Testing began initially in 2023 using a base glaze of 70% milled granite and 30% Wollastonite. This remains a viable starting point. Wollastonite adds more Silica and, more importantly, Calcium as a flux, making it a Calcium dominant glaze. It provides a functional glaze fired at Cone 6.

By itself the base glaze produces a honey matte glaze on white clay as the granite naturally contains iron oxide. It's generally less interesting on dark clay, but not always. The same is true when adding Cobalt Carbonate as a colorant. On white clay a nice grey-blue matte glaze is readily achieved. Adding Iron Oxide produces a melted glaze nearly every time and looks good on any clay. The micas in the granite provide interesting effects (figure 1). Sieving the micas out (120M instead of 80M or 60m) makes the glaze more creamy and better melted.

A common variation has been adding 10% Silica to the base glaze. It tends to give more results stable than the base glaze, though in this test there is not a lot of difference between it (figure 2) and base glaze test (figure 1). In fact it's a bit more matte, which is generally okay, especially when using Cobalt Carbonate as a colorant. In general it improves the glaze in the presence of Iron Oxide or Cobalt Carbonate, but not so much when there is no colorant.

Experiments using Zinc Oxide as an auxiliary flux does move the needle from a matte glaze to a glassy glaze. This took some time to realize and we are just beginning to look closely at it. Crazing occurred in some cases (figure 3). The Iron Oxide test shows comparatively significant movement. The addition of Zinc does also melt the micas without sieving.

Last quick test

Figure 4 shows the results of a last test I snuck in just before year end. A 2025 goal is to play with combinations of Cobalt and other colorants. As I had left over glaze from the tests above, I added ~1% Iron Oxide to each of the Cobalt based tests shown in Figures 1-3 (T44-8, -6, -4).

The first thing to notice is the blue turned to green across the board! This solves one of 2024's great mysteries where some batches of granite produced blue and others produced green. Now we know it's due to variations in the iron content of the granite! Phew. What luck.

And these are nice glazes, if you like green. The iron oxide adds a bit more flux to all examples, making them all a bit richer.

Summary

In summary the following concluding observations are offered:

• The base glaze is great when not adding colorants and produces a yummy honey matte when sieved to 120M and applied to a white body clay. Adding Silica or Zinc produces different effects that are worthy, but the base matte version is the prettiest.
• When using Cobalt Carbonate adding 10% Silica produces a nice matte that often feels prettier than its glassy version, obtained by adding 4% zinc instead. It's nice to have a choice. Muting the cobalt will be an interesting experiment and we saw at the last minute adding iron oxide can produce greens instead of blues.
• When adding Iron Oxide as the main colorant, it's a good glaze almost always, but adding 10% Silica seems to be the sweet spot. Adding Zinc is not necessary, the iron oxide is clearly providing color and flux.
• Generally speaking, removing the micas through sieving improves the melt and reduces any particle roughness in the final glaze (which can be sanded). But the micas do add a nice effect to the final glaze, it's a trade-off.
• Iron Oxide acts as a better flux than Cobalt Carbonate, all else being equal.

What's Next?

Lot's of tests are not covered by the above.

• It should be noted we are trying to find a glaze that uses as much Granite as possible, which seems to be in the 60% - 70% range when firing to Cone 6.
• Very quickly it was realized variations in rock selection for milling, the milling process and the final batch size from milling can change the glaze from glassy to matte. Later, it was realized matte was the more common outcome and the search was on to return to the elusive glassy (something that has not yet been definitively worked out apart from adding Zinc Oxide or Iron Oxide). A lot of work went into this problem over the course of the year.
  • We established stable results between the test and main kiln and at different locations in the main kiln, ruling out these parameters as a cause.
  • We discovered through testing that rock selection for milling, the milling process and the final batch size from milling affects the glaze. Learning more about this will be ongoing, but the bigger the milling batch size, the longer stable results can be obtained, even if they change between batches.
  • And we saw variations of iron oxide in the granite batches will affect the glaze color, especially when using Cobalt as the colorant.
  • We looked at sieving the glaze with different mesh sizes. 60M is fine unless something specific is desired. Sometimes it's nice to remove the micas. But it does not solve the glassy to matte issue.
  • Each new batch of milled granite will need to be retested to determine its particular characteristics.
  • We learned to accept a certain level of non-reproducibility is our reality and not our enemy. We have some control, but not total. Embrace it.
• We played a little with Copper Oxide and Titanium Dioxide and should do more as the results were good.
• We need to perform a proper biaxial test for the addition of Silica and Zinc and a combination thereof to ensure we have their sweet spots.
• A cream-white version of the granite would be nice, and so would a black.
• Finding an additive to soften Cobalt would be interesting to find
  • Zinc Oxide 2-5% - Zinc oxide interacts with cobalt to produce a more transparent and softer blue tone, often referred to as a "robin's egg blue."
  • Magnesium (Talc or Dolomite) - Magnesium can soften cobalt’s intensity and create more muted, pastel-like blues.
  • Rutile / Titanium Dioxide 1-3% - These materials introduce variation and complexity, often creating softer, more variegated effects with cobalt. Rutile in particular can add warmer undertones and break up the uniformity of the blue.
  • Tin Oxide / Zirconium Silicate 5-10% / < 15% - These opacifiers can tone down the intensity of cobalt by making the glaze more opaque and muted.
  • Iron Oxide 1-2% - Adding iron oxide can shift cobalt’s blue toward more complex shades, such as teal or gray-blue. It reduces the starkness of pure cobalt. As we say, it moves the blue to green.
  • Manganese Dioxide 0.5-2% - Manganese can blend with cobalt to produce richer, more subdued blues with a hint of purple or gray.
• Experiment with other fluxes instead of or in addition to Calcium
• Review all notes and add TODO's to this list.

In conclusion, we have a quiver of stable glazes using the granite, therefore we have achieved success. Yea! Now it's time to expand the quiver!