Tenmoku is the family of high-iron glazes that fire to a dark brown or black, and the satin version is the most useful of them because it is neither a mirror nor a dry matte — it has a soft sheen that shows the form of a pot better than a high gloss does. This recipe uses 8 percent red iron oxide, which is enough to saturate the glass without making it metallic.
H 12 · Ø12 / Ø13.5 / Ø6.5 cm
Base
An iron satin at cone 10 in reduction. High iron and a moderate alumina level give a satin surface that is neither glossy nor dry, and the iron breaks to a rust brown on every edge. It is the most dependable dark glaze in this library.
| Material | Parts | Share |
|---|---|---|
| Custer Feldspar | 40 | 40.0% |
| Silica (SiO₂) | 20 | 20.0% |
| EPK Kaolin | 18 | 18.0% |
| Whiting (CaCO₃) | 12 | 12.0% |
| Red Iron Oxide (Fe₂O₃) | 8 | 8.0% |
| Talc | 2 | 2.0% |
| Total charge | 100 |
Fired surface
A deep iron-brown to near-black satin with warm rust-brown breaking on rims, handles and any incised line. Where the glaze is thin, it is a translucent amber-brown; where it pools, it darkens to a glassy black-brown with fine iron crystals.
Iron percentage and what each level gives
At 4 percent iron in this base the glaze is a translucent amber-brown; at 6 percent it is a warm dark brown; at 8 percent it is the deep, near-black brown of this recipe; at 12 percent it goes metallic and can blister and crawl. Eight percent is at the top of the range that still behaves like a glaze rather than a metallic coating.
The colour is a function of both the iron and the alumina. Raising the kaolin from 18 to 22 percent in the same recipe gives a lighter, more orange brown at the same iron level, because the extra alumina keeps the iron from crystallising. If your tenmoku is too dark, add alumina before you reduce iron.
The satin surface and how to lose it
A satin surface comes from a controlled amount of micro-crystallisation at the surface, and it is unstable in both directions. Firing 20 degrees hotter, or holding at top temperature for 30 minutes longer, dissolves the crystals and gives you a glossy tenmoku. Cooling slowly below 1000 degrees grows them further and gives you a dry, matte surface with large visible crystals.
For repeatability, fix the soak: this recipe wants a 15-minute soak at cone 10 and a free cool. Any soak longer than 25 minutes turns it glossy, and a slow cool turns it matte. Write the soak time on the bucket lid.
Breaking, and using it on texture
Iron satin breaks beautifully on every edge and every incised line, because the glaze thins there and the iron is exposed to more oxygen and a thinner glass layer. This makes it the best glaze in this library for throwing rings, faceted surfaces and carved decoration: the form reads in rust brown against the dark body.
On a smooth, untextured surface it is bland. If you are making a plain cylinder or a flat plate, consider the magnesium satin matte from the mid-fire section instead — this glaze needs something to break over.
Firing and atmosphere
Fire in light to medium reduction. Heavy reduction makes the iron go metallic and can produce a metallic sheen that looks like a defect on functional ware; oxidation gives a lighter, more orange-brown colour with less depth. Light reduction from 900 degrees through the end of the firing is what this glaze wants.
The glaze is stable and food-safe at 8 percent iron if it is fully melted and not metallic. Test a fired tile with a drop of lemon juice left on the surface for an hour; if the surface is unchanged, it is suitable for functional use. If there is any metallic sheen, use it on the outside of vessels only.
Steps
- Weigh the six materials and combine them dry, breaking up iron oxide lumps through a sieve.
- Add water and mix for five minutes, then adjust to 1.45 specific gravity.
- Sieve through 80 mesh; iron oxide tends to leave coarse particles that show as specks.
- Dip for two seconds to build a 0.7 mm layer, or pour the interior and dip the exterior.
- Wipe the foot clean; this glaze is fluid at cone 10 and will run on a thin foot ring.
- Load with 1 cm between pieces and on a washed shelf.
- Fire to cone 10 with light reduction from 900 degrees and a 15-minute soak.
- Cool freely; do not damper down unless you want a matte surface.
- Note the soak time on the bucket lid so the next firing matches.
Where it goes wrong
- Soaking longer than 25 minutes. The satin surface dissolves into a gloss.
- Dampering the kiln down during cooling. The glaze turns dry and matte.
- Reducing heavily, which makes the iron go metallic and look like a defect.
- Using 12 percent iron and expecting the same stability. It blisters and crawls.
- Using it on plain, untextured forms where there is nothing for the iron to break over.
| Symptom | Cause |
|---|---|
| Surface is glossy rather than satin | the soak was longer than 25 minutes, dissolving the surface crystals. |
| Surface is dry and matte with large crystals | the kiln was dampered during cooling, growing the crystals too far. |
| Metallic sheen across the whole surface | heavy reduction; reduce less and re-fire a test tile. |
| Glaze ran and stuck to the shelf | applied too thick or the foot ring is too thin for a fluid cone 10 glaze. |
| Colour is orange-brown rather than deep brown | iron below 6 percent, or the kaolin was increased. |
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.