|Monthly Tech-Tip |
Alternate Names: Calcined Rutile, Light Rutile, Rutile Ceramic
Because it is ground to minus 325 mesh and calcined, ceramic rutile powder is a light brown (vs. the black raw granular material). It is the recommended type for use in ceramic glazes since it will not produce gases of decomposition.
The theoretical chemistry of pure rutile is TiO2 (however no mineral deposit of rutile is pure). Good quality ores are blended to achieve consistency, lower quality ones can be processed in furnaces to increase purity. However, rutile is a feedstock to make potassium titanate, the rutile commonly used for ceramics in North America (from TAMCeramics.com). Its original container bags are labelled "Weld Rod Titanate" (for use in welding rods). Since they are adding K2O (as a flux) this is a furnace-processed material whose purity and quality are tuned and maintained. The potassium is not a problem in ceramics, it is a flux found in all glazes containing feldspar. The high consistency offered by this grade makes it preferred in ceramics (over pure rutile).
This bag is very small, this material is very dense and heavy. The primary use of this material is obvious: For making welding rods. We can thus assume it will continue to be abundantly available for ceramics. Notice the bag bottom is marked "Ceramic Rutile" (with a batch number). Why would a product intended for making welding rods be used in ceramics? The answer is very interesting.
Left: GA6-C rutile blue glaze on a brown stoneware. The 4% ceramic rutile powder gives the blue variegated effect. Right: We ball-milled our granular rutile and then screened it down to 325 mesh and put that into the same glaze. The results are the same. So if any of your rutile glazes ever lose this effect with a new supply of the material the cause could be that it has not been milled sufficiently fine. Finer rutile powders are browner in color.
Yes, the granular and powdered grades or pure rutile are the same material. But grinding it is very difficult. Commercial ceramic grade powder is minus 325 mesh, the companies doing this obviously have very good grinding equipment. They also have patience because even in this efficient porcelain ball mill, 90 minutes was only enough to get 50% to minus 325 mesh! The color of the powder is a good indication of its quality, the finer the grind the lighter tan it will be.
Rutile variegates glaze surfaces. But it also opacifies at higher percentages. The blue effect is a product of crystallization that occurs during cooling, it is thus dependent on a slower cooling cycle, especially above 1400F. This is GA6-C Alberta Slip glaze with 4, 5 and 6% rutile. At 6% the rutile crystallization has advanced to the point of completely opacifying the glaze. At 5% the blue is still strong, even on a buff burning body. The loss of color occurs suddenly, somewhere between 5 and 6 percent. Rutile chemistry varies from batch to batch. The blue develops differently on different bodies. So do you want to play "at the edge", with 5% in the glaze, in view of these other factors and the finicky firing curve needed. Change in any of which could push it into the blueless zone?
The 80:20 base GA6-A Alberta slip base becomes oatmeal when over saturated with rutile or titanium (left: 6% rutile, 3% titanium; right: 4% rutile, 2% titanium). That oatmeal effect is actually the excess titanium crystallizing out of solution into the melt as the kiln cools. Although the visual effects can be interesting, the micro-crystalline surface is unpleasant to touch and susceptible to cutlery marking and leaching (not as stable or durable as in glazes which are pure amorphous glass). For functional ware, rutile glazes are among the most troublesome to keep consistent, one way of avoiding problems is keeping the percentage as low as possible while still getting the desired variegation (of course that will vary depending on the melt fluidity of the glaze, more highly fluid ones can handle more rutile or titanium).
This is a common problem with these glazes. The visual effect is very compelling but also punishing! Potters experiment with higher bisque firing and soaking during bisque. They try cleaner clay bodies. They employ long hold periods at temperature in the glaze firing. But the problem persists. The solution is actually simpler. These glazes have a high melt fluidity and enough surface tension to hold a bubble static during soaks at temperature (no matter how long you hold it). It is better to cool the kiln somewhat (perhaps 100F) and soak at that temperature. Why? Because the increasing viscosity of the melt overcomes the surface tension that maintains the bubbles. You may need to cool more or less than 100 degrees, but start with that.
These are GA6-C Alberta Slip floating blue (left), AMACO Potter's Choice PC-20 Blue Rutile (center), GR6-M Ravenscrag floating blue (right). The clay is M390. The firing is cone 6, the schedule is C6DHSC (drop-and-hold, slow cool). All of these recipes are descendants and improvements of the 50-year-old original G2826R floating blue. The inside glaze on these mugs is GA6-B. The two on the left develop the blue color because of the slow cool, the one on the right works on fast-cool because it contains cobalt (although it will fire somewhat more mottled). Remember, these work best on dark-burning bodies.
A closeup of a cone 10R rutile blue (it is highlighted in the video: A Broken Glaze Meets Insight-Live and a Magic Material). Beautiful glazes like this, especially rutile blues, often have serious issues (like blistering, crazing), but they can be fixed.
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
2, 3, 4, 5% rutile added to an 80:20 mix of Alberta Slip:Frit 3134 at cone 6. This variegating mechanism of rutile is well-known among potters. Rutile can be added to many glazes to variegate existing color and opacification. If more rutile is added the surface turns an ugly yellow in a mass of titanium crystals.
These mugs are Plainsman H450 fired at cone 10R. Both have a black engobe (L3954N) applied to the insides and half way down the outside during leather hard stage (the insides are glazed with Ravenscrag silky matte and G1947U over the black engobe). The bamboo glazes can thus be seen over the black (upper half) and the raw buff body (lower). The bamboo glaze on the left has 1% iron added to the base G2571A recipe. The one on the right has 3.5% powdered rutile and 10% zircopax added.
The glaze is G191T (a variation of G1916Q). Firing was cone 04 drop-and-hold with slow cool. Sometimes a raw colorant is advisable over a ceramic stain. At low temperatures stains are almost universal. But in this case, the orangey-yellow color that rutile produces merits further testing. On the red body (Plainsman L215) the color is barely perceptible, but on the light Buffstone body it is working well. The variations in thickness highlight contours better than what a stain would do.
A raw TiO2-containing mineral used in ceramics to color and variegate glaze surfaces.
A type of ceramic glaze in which the surface variegates and crystallizes on cooling in the presence of titanium and iron (usually sourced by rutile)
Ruflux 61 rutile data sheet
Metallic based materials that impart fired color to glazes and bodies.
Opacifiers are powders that turn transparent glazes opaque by various chemical and physical mechanisms (and combinations of mechanisms).
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
|By Tony Hansen|
Follow me on