Titanium dioxide (TiO2) is an opacifier and unlike tin oxide and zircopax, it develops crystals in glazes. Titanium dioxide glazes with at least 2% titanium develop beautiful microcrystalline surfaces. In today’s post, an excerpt from the May 2019 issue of Ceramics Monthly, Alisa Liskin Clausen presents a few tests and recipes to help you start your own experiments with titanium dioxide glazes in the glaze lab. –Jennifer Poellot Harnetty, editor.
In a series of increment tests, a single 100-gram batch of a base glaze is used for increasing additions of titanium dioxide. The first 1% increment of titanium is added, blended, sieved, and a tile is dipped into the glaze. The next 1% titanium increment is added, repeating the same procedure above, until the batch reaches 15% added titanium dioxide.
Where a coloring oxide is tested with titanium dioxide, the percentage of the colorant remains constant while the amount of titanium dioxide increases. The coloring oxide, say 1% cobalt carbonate, is added to a new 100-gram batch of base glaze. To that base, incremental amounts of titanium dioxide are added as above. The first incremental increase of 1% titanium is added to the same batch of glaze. The test is still 1% cobalt carbonate, but now with 1% titanium. The next test may have an added 2 grams of titanium, so now the test is 1% cobalt carbonate and 3% titanium.
Water is adjusted during the additions to the titanium dioxide glaze, because the added material will change the viscosity (thickness) and specific gravity (density) of the slurry (learn how to make a hydrometer to test specific gravity.). This method saves a lot of individual glaze cups (compared to triaxial blends) and materials because many tests can be made from one cup. It is, however, not quantitatively accurate, because each time a tile is dipped in a test cup, a little bit of the original 100 grams is taken, skewing the relative percentage of additives. Despite this, it works well to reveal the characteristics of titanium for these tests, and to help narrow down the combinations that could be further explored.
Below are the properties of titanium dioxide, with glaze tests to demonstrate the characteristic being discussed. It is beneficial to hold (or soak) glazes containing titanium while firing down from top temperature to give optimal time for a crystalline matte surface to develop.
Crystal Formation and Opacity (1)
Starting at an addition of 2%, titanium dioxide will start to crystallize. White and blue streaks appear in the fired glaze with titanium additions at 3–4%. This glaze becomes completely opaque with upward of 5% additions of titanium dioxide.
Opalescent Blue and Yellow Mattes (2)
In addition to reduced transparency with the incremental increases of titanium dioxide, there are also several changes in color. This base is tested with 1–15% titanium dioxide. Typical streaky blues occur at 4–6%, complete opaqueness by 8%, and matte greenish yellow and yellow colors at 10–15%.
To see more test results with this titanium dioxide glaze, check out the May 2019 issue of Ceramics Monthly.