Rutile is titanium dioxide with iron and other impurities left in it, and it is the cheapest way to get a glaze with depth. The speckle comes from titanium crystals that form at the surface during cooling; the tan comes from iron in the rutile interacting with calcium in the base. Both depend on how much the rutile was ground, which is the variable most people never consider.
H 10 · Ø10 / Ø12 / Ø6.5 cm
Base
A cone 6 satin whose colour comes entirely from rutile and a small amount of tin. No stains, no oxides beyond those two: the tan field and the blue speckle both come from the titanium in the rutile reacting with the iron and calcium already in the base.
| Material | Parts | Share |
|---|---|---|
| Custer Feldspar | 30 | 28.8% |
| Silica (SiO₂) | 22 | 21.2% |
| EPK Kaolin | 17 | 16.3% |
| Whiting (CaCO₃) | 16 | 15.4% |
| Talc | 9 | 8.7% |
| Rutile (TiO₂) | 6 | 5.8% |
| Tin Oxide (SnO₂) | 4 | 3.8% |
| Total charge | 104 |
Fired surface
A warm tan field with blue-grey flecks where the rutile has clumped, and a soft sheen that breaks to a brighter tan over ridges. Speckle density depends on how coarsely the rutile was ground.
The grind size of rutile is the whole game
Coarsely ground rutile, around 100 mesh, gives a strong speckle with distinct blue-grey flecks. Finely ground rutile, 300 mesh or finer, dissolves completely and gives a smooth tan with almost no speckle. Suppliers do not always say which they are selling, so sieve a sample of your rutile through 200 mesh and see how much stays on the screen — if a third stays, it is coarse.
For the look in the photograph, you want the coarse material, and you want to use it without sieving the mixed glaze past 60 mesh. Sieving the finished glaze through 80 mesh removes the coarsest particles and half the speckle with them.
Why 4 percent tin
Tin oxide opacifies slightly and pushes the rutile toward blue rather than brown. Without it, the same recipe fires a flat brown-tan with grey flecks; with 4 percent tin, the flecks read distinctly blue-grey and the field warms up. Above 6 percent the glaze starts to look milky and the speckle gets lost in the opacity.
Tin is the most expensive material in this recipe by a wide margin. If cost matters, 2 percent still does something; 0 percent turns it into a different glaze that is worth firing once to see the difference.
Thickness and the break
Rutile glazes want 0.5 to 0.7 mm to show their depth. At 0.3 mm the glaze is thin enough that the flecks sit on a nearly transparent ground and the colour looks washed out; at 0.9 mm it runs at the foot and the speckle merges into a muddy brown.
The break over a ridge or a rim is where this glaze earns its place on a pot. Plan the form so there is a ridge to break over: a thrown line, a trimmed shoulder, a fluted rim. On a plain cylinder the glaze is pleasant; on a form with a ridge it is interesting.
Firing and the cooling that makes the flecks
Fire to cone 6 and cool at 100 degrees per hour from peak to 1000 degrees. Titanium crystals form in that window; a natural cool gives fewer, larger flecks and a slower cool gives many small ones.
This glaze is stable across the firing range: cone 5 gives a matte tan with strong speckle, cone 6 gives the satin surface described here, and cone 7 gives a glossier surface where the flecks begin to dissolve. All three are usable, which makes it a good glaze for a kiln with a known gradient — the same recipe produces three related surfaces across the shelves.
Steps
- Sieve a sample of your rutile through 200 mesh to check whether it is coarse or fine ground.
- Weigh the seven materials, using coarse rutile for speckle or fine rutile for a smooth tan.
- Add water to 1.45 specific gravity and sieve the mixed glaze through 60 mesh only.
- Dip test tiles for one and a half seconds to reach a 0.5 to 0.7 mm deposit.
- Fire to cone 6 with a 10 minute soak.
- Cool from peak to 1000 degrees at 100 degrees per hour to develop the titanium flecks.
- Check the break over a ridge on a test tile and adjust the thickness up if the ridge reads flat.
- Fire one tile at cone 5 and one at cone 7 to see how the surface shifts across your kiln.
Where it goes wrong
- Sieving the finished glaze through 80 mesh. It removes exactly the particles that make the speckle.
- Buying rutile without checking the grind. Two suppliers' rutile can produce two completely different glazes from one recipe.
- Substituting tin with zircon. Zircon opacifies but does not push the flecks blue.
- Applying at 0.3 mm and concluding the glaze is washed out. This glaze needs thickness to work.
| Symptom | Cause |
|---|---|
| No speckle at all | the rutile was fine ground, or the glaze was sieved through 80 mesh. |
| Flecks are grey-brown instead of blue | the tin was omitted or reduced below 2 percent. |
| Glaze ran onto the shelf | applied above 0.9 mm; the rutile makes the melt more fluid than the base recipe suggests. |
| Colour differs between the top and bottom shelves | this glaze shifts across cone 5 to 7; label which shelf gives which surface. |
Recipe and schedule numbers here were recorded on the workshop’s own kiln. A different kiln, a different thermocouple position or a different clay batch will move the result, so treat every firing schedule as a starting point and keep your own log beside it.
Same cone — Cone 6
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- Opening and flooring: how thick the base actually needs to be Stoneware
- Pulled handles: matching moisture and a 6 mm oval section Stoneware
- Rolling slabs to an even 8 mm and joining them so the seam outlives the pot Stoneware