Many fluid glazes will do magic things (e.g. variegate) with the addition of rutile (usually less than 5%). The effects are often amplified when other colorants are present (especially iron). The classic rutile effect happens when a glazes melt runs in rivulet patterns. Employment of this effect is common across a wide range of stoneware temperatures in both oxidation and reduction. Rutile can produce vibrant blues (cobalt is the most common way to make blue but it is very expensive and does not produce any of the effects of rutile on its own).
Variegation and phase separation with about 5% rutile
The glaze is a dolomite matte fired to cone 10R. High fire reduction is among the best processes to exploit the variegating magic of rutile.
The rutile mechanism in glazes
2,3,4,5% rutile added to a 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.
Alberta Slip rutile blue on a porcelain (left) and buff stoneware (right)
The recipe is GA6-C. These are from the same firing (slower cooling is needed to develop the rutile effect).
Alberta Slip Rutile blue glaze too thin on a dark body
This mug has thin walls and was bisque fired to cone 04 (so it had a fairly porosity). As a result the glaze went on thinner when it was dipped. This was not evident at the time of glazing but at firing the thinner sections produced the brown areas.
6% rutile is too much in this cone 6 oxidation glaze
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. Even 5% is too much.
Crystallization of Rutile at cone 6 completely subdued? How?
These glazes are both 80% Alberta Slip, but the one on the right employs 20% Ferro Frit 3249 accelerate the melting (whereas the left one has 20% Frit 3134). Even though Frit 3249 is higher in boron and should melt better, its high MgO stiffens the glaze melt denying the mobility needed for the crystal growth.
MgO can destroy the rutile blue variegation effect
The rutile blue variegation effect is fragile. It needs the right melt fluidity, the right chemistry and the right cooling (during firing). This is Alberta Slip GA6C recipe on the right (normal), the glaze melt flows well due to a 20% addition of Ferro Frit 3134 (a very low melting glass). On the left Boraq has been used as the flux (it is a calcium borate and also melts low, but not as low as the frit). It also contains significant MgO. These two factors have destroyed the rutile blue effect!
Tin oxide can stop the rutile variegation effect dead in its tracks!
This is Alberta Slip (GA6C) on the left. Added frit is melting the Alberta Slip clay to it flows well at cone 6 and added rutile is creating the blue variegated effect (in the absence of expensive cobalt). However GA6D (right) is the same glaze with added Tin Oxide. The tin completely immobilizes the rutile blue effect, it brings out the color of the iron (from the rutile and the body).
Rutile blue glaze effect completely lost! A temporary solution.
Left: 4% rutile in the Alberta Slip:frit 80:20 base. This glaze has been reliable for years. But suddenly it began firing like the center mug! Three 5 gallon buckets of glaze (of differing ages) all changed at once. We tried every combination of thickness, firing schedule, clay body, ventilation, glazing method on dozens of separate pieces with no success to get the blue back. Even mixed a new batch, still no color. Finally the 'crow bar' method worked, 0.25% added cobalt oxide (right mug). It is identical ... amazing. It is not the same mechanism to get the color and it is not exactly the same, but worked while we figured out the real issue: the firing schedule (the secret turned out to be cooling, soaking, then slow cooling to 1400F).
Alberta Slip Floating Blue (left) plus 4% spodumene (right).
GA6-C (left) and GA6-E (right) at cone 6 oxidation. The E version adds 4% spodumene onto the 4% rutile in the C (the base is 80% Ravenscrag Slip and 20% frit 3134). This glaze requires slower cooling. It looks the best on dark bodies.
How much rutile can a glaze take before it becomes unstable?
The 80:20 base Alberta slip base becomes oatmeal when over saturated with rutile or titanium (left:6% rutile, 3% titanium; right:4% rutile, 2% titanium right). That oatmeal effect is actually the excess titanium crystallizing out of solution in the melt as the kiln cools. Although the visual effects can be interesting, the micro-crystalline surface is often susceptible to cutlery marking and leaching. This is because the crystals are not as stable or durable as the glass of the glaze.
Blisters in a reduction fired rutile glaze
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.
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