Cone 2: Develop a glaze and engobe to make stoneware from terra cotta bodies

Project Name

Cone 2 Stoneware: A New Opportunity

Project Codenumber

UnAssigned

Notes

We are all trying to be greener in our practice of ceramics. Many potters would like to move to low temperatures, but while there are many advantages there is one big disadvantage: Ware strength and porosity, low temperature ware cannot compare with stoneware. So any opportunity that presents itself to reduce energy consumption while maintaining strength is absolutely fantastic. Cone 2 could be that opportunity and a recent industry-wide change has made it more attractive than ever (more on that in a moment..). In hobby and pottery kilns, most of the energy is poured into firing the last few degrees, cone 2 is 140 degrees F lower than cone 6, this is a huge potentional energy saving (reduction in wear and tear on kilns is a big added bonus).

Starting in 2021, even body manufacturer has been forced to move away from white burning talc-bodies (for legal reasons). These have been the mainstay of low fire ware for decades, thousands of glazes are made to fit them. Fortunately dolomite has proven to have the same "thermal expansion increasing" properties as talc, enabling manufacturers to transition. While it has a number of advantages over talc there is one downside: These bodies have high porosity and lower strength. This is being tolerated by most people used to working at low temperature, they prioritize brightly colored glazes over fired strength. But this is not an option for people who expect stoneware or porcelain strength.

However terra cotta bodies are a different situation. While many have contained talc (e.g. 10%), the commercial glaze suppliers have gradually been able to reduce the thermal expansion of their products so that that they fit talcless terra cotta bodies. Terra cotta, while firing red or brown, produces a far stronger product. At Plainsman we have removed the talc from our L215 and L210 and all glazes we have tried are fitting! The beauty of this development is that these bodies now develop stoneware maturity into the cone 2-4 range (the former talc versions became brittle and discoloured when overtired to cone 2 or higher).

Terra cotta bodies respond to the use of engobes much better than white burning bodies and the adherence is much better because vitrification is the mechanism of strength (as opposed to sintering in the white bodies). A white engobe produces a base on which colors can be as bright as on white bodies (actually, even brighter, because engobes can be much whiter than a body because they contain zircon).

In this project we are going to develop a good base glossy and matte glaze and an engobe that fit Plainsman L210, L215 and L4170B casting. These should work on other terra cotta bodies from other manufacturers also. Then potters will be able to transplant the mechanism of their many of their glazes into the cone 2 range.

Pictures

L215 and L210: The good color is at lower temperatures

Top: Plainsman M390 and M350 at cone 6. Bottom: Plainsman L215 and L210 at cone 2. While the latter are still mainly used at cone 06-04 (with commercial bottled glazes), since they no longer contain talc both tolerate higher temperatures much better, by cone 2 they are stonewares. Notice the L215 produces a better red than M390 and L210 a better leather-tan than M350. The first runs of these two were done during March 2022, their code numbers start with the digits shown on these bars: 6983 and 6984. If you would like to experiment with cone 2 stoneware consider making a base transparent glaze by starting with G1916Q. It is expansion-adjustable to deal with crazing or shivering. Just add kaolin and silica at the expense of the frit (e.g. 10 of each) to move it up to cone 2.

Cone 2 Clear (based on G1916Q)

Total:101.00

Auto Unity Formula

Notes

*This is an adjustment to G1916Q, a cone 04 glaze that has proven very practical because it uses common frits and is thermal-expansion-adjustable. This version adds Al2O3 and SiO2 to increase the melting point for use at cone 2 (sourcing them from increased ball clay and adding silica). These additions will increase fired durability and lower thermal expansion also.

The idea here is to create a cone 2 base to which colorants, variegators and opacifiers can be added. It would also be possible, of course, to mix it as a brushing glaze.

Initial firing results are looking good, more photos and information coming soon during March 2022.

Pictures

G1916V first test: Clear glaze on L210, L215 at cone 2

This was not a slow-cool firing.

G1916Q vs G1916V melt fluidity at cone 04

G1916V at cone 3 on L210, L215

G1916V at cone 3 on L210

L215, L210 with G1916V clear + 1% iron oxide - Cone 2

Cone 2 White Engobe for L215, L210, L4170B

Total:122.00

Notes

*This is an engobe recipe that we are working on in March 2022, fired results are not yet available. But it is predicted to work as explained below.

This recipe base (L3685Z1-7) has been working very well, we have documented its firing shrinkage across various percentages of frit at a wide range of temperatures for each (and with different pigments). For any specific use at one temperature with one body, it is a matter of searching the data to find a matching engobe Z-series recipe to match.

A series of tests using this base recipe has shown that the cone 2 firing shrinkage is very sensitive to the percentage of Frit 3110. 5% jumps it to 10% and zero% frit has less than 4% firing shrinkage at cone 2 (we want to match the 6% of the three target bodies, L210, L215 and L4170B). 3% 3110 is also too high. This mix thus uses 2%.

The NZK is sticky and enables good suspending but we found that some bentonite is needed for better adherence in leather hard applications (this is not intended for application to bisque, it will flake off). The silica is needed for matching thermal expansion and the nepheline (or feldspar substitute) seems to be the best filler. The 20% zircon is the secret for the incredibly white fired surface that can produce.

A nice feature of this recipe is that the powder can be shake-mixed in a plastic bag and then propeller mixed with an equal weight of water to create a thick creamy slurry. The slurry can be mixed at high speed to remove agglomerates of NZK (very important), and then Darvan or sodium silicate added (only a few drops) to thin it down to creamy working consistency. This kaolin and the bentonite impart thixotropy that helps hold the slurry in place after dipping.

Pictures

L3685Z5 engobe at cone 03 on L4170B

Incredibly whiter, better than the Amaco white underglaze.
L3685Z5 is the same as L3685Z2 but it adds 3% Ferro Frit 3110 to get a little more fired hardness.