Titanium dioxide’s ability to crystallize is what makes it different from other opacifiers. Here are few tests and recipes to help you start your own experiments with it in the glaze lab.
Defining the Terms
Diffraction: The bending or spreading
of light through an opening or around
Opacity: The quality of lacking transparency or translucency.
Titanium Dioxide (TiO2): An opacifying oxide that causes crystals to develop in the glaze. Other sources of titanium include rutile and ilmenite, both of which contain iron oxide.
The Titanium Effect
Titanium dioxide (TiO2) is an opacifying oxide that develops crystals in glazes. Increasing the percentage of titanium dioxide will affect a glaze’s opacity. Small amounts of titanium dioxide, up to 1%, dissolve in a melted glaze. When the titanium is increased to just 2%, it separates from the glaze when cooling, creating microcrystals in the glaze surface that diffract light.
As a single additive, titanium dioxide is refractory and makes the glaze hard. With the addition of coloring oxides, titanium dioxide has an influence on the color’s appearance. When 3–4% titanium dioxide is added to a constant amount of a coloring oxide, for example 1% cobalt carbonate (CoCO3), the blue is brightened. Increasing the amount of titanium dioxide to 4–6% makes colors mottled and variegated. With additions of titanium from 10–15%, the colors are quite opaque and can be dull, and the surfaces can become patterned and uneven with a shimmer.
Titanium dioxide’s ability to crystallize is what distinguishes it from the other opacifiers, tin and zircon. Each of those is a straightforward additive for opacity and uniform whites. Tin tends to produce softer whites, and zircon makes brighter and harsher whites.
Put and Take Testing Method
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, because the added material will change the viscosity (thickness) and specific gravity (density) of the slurry. This method saves a lot of individual glaze cups 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%.
Crystal Matte (3)
Titanium dissolves in molten glaze, but the majority of it separates and forms crystals upon cooling. The surface of a glaze becomes a crystalline matte by the formation of microcrystals, which diffract light resulting in an opaque glaze. It may produce crystals that can be seen with the naked eye and as spotted patterns, which are uneven. Higher levels of titanium dioxide can reveal uniquely shimmering surfaces.
Color Response (4–7)
The amount of titanium dioxide added to a base with a coloring oxide will influence the characteristic of the colorant. Generally, a small amount of around 1% titanium dioxide boosts and brightens colors, moderate amounts between 2–6% mottle and variegate the colors, and higher amounts dull the colors (4A, 4B).
One of the only times cobalt is not blue is when titanium dioxide is added. The resulting color can be pleasantly green (5A–E).
The combination of copper carbonate and titanium dioxide in a strontium matte base replicates patinated bronze. Pinnell Weathered Bronze is an iconic glaze that combines these oxides with strontium and similar effects can also be reached with other glaze bases (6).
Titanium dioxide’s refractory property is good for making some interesting oil-spot looks when 10–15% is used. With high levels of titanium dioxide in a cover glaze base, this look can be achieved, whether the bottom glaze is the same base as the cover glaze or it is another glaze that has a different melt than the cover glaze (7A–C).
All tests were fired to cone 6 in oxidation on light-colored stoneware. The firing schedule is 212°F (100°C) per hour up to 2228°F (1220°C), with a 10-minute hold, and natural cooling down to 1652°F (900°C) with a one-hour hold, followed by a natural cooldown.
A variety of surfaces and color responses are possible depending on the glaze base, coloring oxides, and the percentage of titanium dioxide added. Both gloss and matte bases were used for testing purposes, but testing incremental amounts of titanium dioxide added to a glaze you already use can also reveal new, interesting glaze surfaces. As usual, there are many variables with almost everything associated with glazing. Testing with your own materials, mixing, application methods, and firings will be necessary to find your own gratifying results with titanium dioxide.
the author Alisa Liskin Clausen is an American-born ceramic artist with a BFA from Syracuse University. She has lived and worked as a potter in Denmark since 1997, focusing her work on cone 6 oxidation glaze development from local materials.