|Monthly Tech-Tip |
A high temperature reduction glaze made by adding a small amount of iron oxide to a magnesia matte base glaze
Key phrases linking here: bamboo glaze, bamboo matte - Learn more
Usually describes a bamboo colored matte glaze (especially in reduction stoneware). These are normally achieved in a magnesia matte white by adding a small amount of iron (0.5-1%) and some tin oxide opacifier.
These mugs are Plainsman H443. The bamboo glaze on the left (A) has 3.5% rutile and 10% zircopax added to the base G2571A dolomite matte. The one on the right (B) has the same addition but in a base having slightly less MgO and slightly more KNaO. B stains badly (as can be seen from the felt marker residue that could not be removed using lacquer thinner). Why does A stain only slightly? It has an additional 4% Gerstley Borate (GB). GB is a powerful flux that develops the glass better, making the surface more silky. The differences in the recipe provide another advantage: (A) has a much lower thermal expansion and is less likely to craze.
Courtesy of Susan Clarke
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.
Refired to 1950F. The recipe is very flux-heavy (high feldspar) dolomite:spodumene matte, zero silica, 4% tin oxide and less than 1% iron oxide. Sounds like crystallization territory. The plate on the left is the way it normally fired. On the right the way it started firing. The mug on the bottom looked like the plate on the right, but look what happened after refiring at 1950F in oxidation! The tin is likely a catalyst for the crystallization that occurred in the original result. Could be a fragile mechanism. This underscores the need for a period of oxidation at the end of a normal reduction firing.
G2571A - Cone 10 Silky Dolomite Matte glaze
A cone 10R dolomite matte having a pleasant silky surface, it does not cutlery mark, stain or craze on common bodies
GR10-J1 - Ravenscrag Cone 10R Bamboo Matte
Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
Dolomite matte glazes have the potential to be very silky and pleasant to the touch, while at the same time being hard, durable and non-crazed (if they are formulated correctly).
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
|By Tony Hansen|
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