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Low Expansion Glossy Clear Cone 6

Code: G1215U
Modification Date: 2015-10-30 04:31:44

A recipe sourcing high MgO (from Ferro Frit 3249) to produce a low expansion glass resistant to crazing on lower silica porcelains.

F-4 Feldspar24.024.4%
Frit 324920.020.3%

Firing Schedule

Rate (F)Temp (F)Hold (Min)Step


The original impetus to create this recipe was to reduce the thermal expansion of the G1214M and G1215W recipes to work better on porcelains (the others craze). Although a clear transparent glaze, the higher MgO content can produce dimples in the surface if fired too quickly. The higher MgO also affects the color response (this glaze does not work with chrome-tin pink and maroon stains).

Keep in mind that you cannot expect one glaze to work well on both porcelain and stonewares. This fires to a very low expansion glass, while good on vitrified porcelains, it may be under too much compression if used on stonewares (for example, if used on the insides of vessels its compression could make the piece brittle and prone to failure, especially when exposed to sudden increases in temperature). G1214W may be a better choice. To know for sure, glaze a piece only on the inside, fire, then drop it on the floor. If it explodes into many many pieces, the glaze is under too much compression.

As of spring 2013 we are altered this recipe to reduce the MgO content and eliminate the expensive frit 3249 (See G1216L and M). This was done because some users experienced a shift toward a silky matte appearance. We were not been able to explain this sudden change, possibly it was due to a change in the frit. However this recipe continue to work for many other people.

Frits work much better in glaze chemistry

Frits work much better in glaze chemistry

The same glaze with MgO sourced from a frit (left) and from talc (right). The glaze is 1215U. Notice how much more the fritted one melts, even though they have the same chemistry. Frits are predictable when using glaze chemistry, it is more absolute and less relative. Mineral sources of oxides impose their own melting patterns and when one is substituted for another to supply an oxide in a glaze a different system with its own relative chemistry is entered. But when changing form one frit to another to supply an oxide or set of oxides, the melting properties stay within the same system and are predictable.

Frits work much better in glaze chemistry

1215U glazes with various Mason stains

G1215U vs. G1215W glaze flow test

G1215U vs. G1215W glaze flow test

These recipes have the same chemistry but the 1215U uses frit to source the MgO and CaO. This demonstrates that it is not just chemistry that determines melt flow. Raw materials are crystalline and have different melting patterns than frits (which have already been melted and reground).

Cone 6 glazes can seal the surface surprisingly early - melt flow balls reveal it

Cone 6 glazes can seal the surface surprisingly early - melt flow balls reveal it

These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.

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By Tony Hansen


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