|Monthly Tech-Tip |
This is the L3954B engobe. 15% Mason 6600 black body stain has been added (instead of the normal 10% Zircopax for white). Of course, a cover glaze is needed, but even without it is absolutely coal black (so a lesser stain % is possible). Lots of information is available for L3954B (including mixing and adjustment instructions). Engobes are tricky to use, follow the links below to learn more. L3954B is designed to work on regular Plainsman M340 (this piece), M390 and Coffee Clay, these bodies dry better than porcelains and are much less expensive, coating them with an engobe to get a surface like this makes a lot of sense. This engobe is actually a highly plastic body, but it does not contain enough feldspar to be a porcelain (this is on purpose to match the firing shrinkage of the stonewares). This can be adjusted to fit any stoneware.
This is part of a project to fit an engobe (slip) onto a terra cotta at cone 02 using the EBCT test.
Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way!
Right: SHAB test bars of the white slip and red body enable comparing their drying and firing shrinkages.
Center back: A mug with the white engobe and a transparent overglaze. The slip is going translucent under the glaze because it is too vitreous. Its higher fired shrinkage curls the bi-clay bars toward itself. Reducing the frit will reduce the firing shrinkage and make it more opaque (because it will melt less).
Front: A different, more vitreous red body (Zero3 stoneware) fits the slip better (the strips dry and fire straight).
This is how bad the fit can actually be. In the front is a bi-clay EBCT test strip of a grogged cone 10R sculpture clay sandwiched with a porcelain. After drying this bar was relatively straight. But during firing the porcelain has a much higher fired shrinkage and it pulls the bar toward itself. During cooling, the sculpture clay has a higher thermal expansion and it pushes from its side bending the bar further. This bar is a time bomb, just waiting for a mechanical or thermal stress to bust it into a hundred pieces. Admittedly, putting a thin layer of this onto a piece of heavy ware is not going to bend it. But will it flake off when exposed to stresses (like freeze thaw, being put in an oven, having a hot liquid poured into it, being bumped).
This black engobe, L3954B, is on a cone 6 buff stoneware (at leather hard stage). Because a thin layer works well with this high-opacity engobe it is possible for the slurry to be more fluid, less gelled. An immediate benefit of this is that it dries more quickly (enabling handling of the piece within a few hours). Another benefit is a much more even coverage than would be possible with more a viscous consistency. The thinner layer also means much fewer issues with flaking during drying. However application of the engobe takes patience, waiting for drips to stop and gelling to set in and hold it in place.
L3954B - Cone 6 Engobe (for M340)
Dry and firing shrinkage fitted to Plainsman M390, M340
L3954B engobe page at PlainsmanClays.com
Mason 6600 Black Stain
Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.
Thixotropy is a property of ceramic slurries. Thixotropic suspensions flow when you want them to and then gel after sitting for a few moments. This phenomenon is helpful in getting even, drip free glaze coverage.