Modification Date: 2016-11-12 22:14:20
A cone 10R dolomite matte having a pleasant silky surface, it does not cutlery mark, stain or craze on common bodies
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A standard Plainsman Clays cone 10R dolomite matte glaze used for many years. It came from another recipe that employed calcium carbonate to supply the CaO. Wollastonite was introduced instead, it not only supplies the same amount of CaO but contributes significant SiO2 also (the silica was reduced to compensate).
While many glazes of this type do not have a proper balance of MgO and do not melt to create a stable glass, this one has a very silky surface yet resists staining and cutlery marking (the vast majority of others have some cutlery marking problems). An alternative recipe using Ravenscrag Slip is also available. It has very similar chemistry.
This bamboo glaze (made by adding 4% tin and 1% iron oxide (left) and 0.5% iron oxide (right) to the G2571A base recipe) amplifies the iron speckle in the body beneath (Plainsman H443). Unfortunately this looks good and but is not very functional. The body is quite porous (5% water absorption), this is necessary for the mottled brown visual effect.
This is the G2571A base dolomite matte recipe. The specimen on the left adds 4% tin and 1% iron oxide. The one on the right has 4% tin oxide and 0.5% iron oxide.
This is the G2571A glaze recipe. It has proven reliable and functional over many years on a wide range of clay bodies in the Plainsman Clays studio. Actually, a better brown color can be achieved using manganese dioxide.
You add up to 5% manganese dioxide. The base recipe is G2571A. The clay body is a buff burning stoneware having iron speckle. The quality of the surface is excellent and it is durable.
GR10-J Ravenscrag silky matte (right) and G2571A matte (left) on a dark burning iron speckled stoneware at cone 10R. Surfaces have identical feel (the chemistries are very close). The former fires a little darker color because of the iron contributed by the Ravenscrag Slip.
GR10-J Ravenscrag silky matte (right) and G2571A matte (left) on a buff stoneware at cone 10R. Surfaces feel identical, the slightly darker color is due to iron content in the Ravenscrag. The former was formulated to mimic the latter using as much Ravenscrag Slip as possible yet still maintain the same chemistry.
Make cone 10R bamboo colors using the GR10-J Ravenscrag silky matte base recipe (right) and adding 1% iron (left), (0.5% centre). These samples are porcelain. This iron addition also works using the G2571A matte base recipe.
This is G2571A cone 10R dolomite matte on an ironware body made from native North Carolina clays. Few glazes have the pleasant silky feel that this has yet are still functional. The feldspar content in the body has been tuned to establish a compromise between the warmer color low percentages have with the higher strength that higher percentages impart. Careful porosity tests were done and recorded in an account at insight-live.com. The objective was to bring the body close to 3% absorption.
Left: An example of G2571A cone 10R magnesia matte. Right: with 10% added zircopax (zirconium silicate). The zircopax version is a very bright pasty white compared to the original.
This is G2571A cone 10R dolomite matte glaze with added 1% cobalt oxide, 0.2% chrome oxide. The porcelain is Plainsman P700, the inside glaze is a Ravenscrag Slip clear. This recipe can be googled, it has been available for many years and was first formulated by Tony Hansen. This base is very resistant to crazing on most bodies and it does not cutlery mark or stain. It also has very good application properties.
GR10-J Ravenscrag dolomite matte base glaze at cone 10R on Plainsman H443 iron speckled clay. This recipe was created by starting with the popular G2571 base recipe (googleable) and calculating a mix of materials having the maximum possible Ravenscrag Slip percentage. The resultant glaze has the same excellent surface properties (resistance to staining and cutlery marking) but has even better application and working properties. It is a little more tan in color because of the iron content of Ravenscrag Slip (see ravenscrag.com).
Opacifying a cone 10 reduction magnesia matte glaze. On the left: G2571A dolomite matte, a popular recipe (from Tony Hansen). Right: 10% Zircopax has been added. Both are on a buff stoneware (H550 from Plainsman Clays).
This is an example of cutlery marking in a cone 10 silky matte glaze lacking Al2O3, SiO2 and having too much MgO. Al2O3-deficient glazes often have high melt fluidity and run during firing, this freezes to a glass that lacks durability and hardness. But sufficient MgO levels can stabilize the melt and produce a glaze that appears stable but is not. Glazes need sufficient Al2O3 (and SiO2) to develop hardness and durability. Only after viewing the chemistry of this glaze did the cause for the marking become evident. This is an excellent demonstration of how imbalance in chemistry has real consequences. It is certainly possible to make a dolomite matte high temperature glaze that will not do this (G2571A is an example, it has lower MgO and higher Al2O3 and produces the same pleasant matte surface).
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By Tony Hansen