|Monthly Tech-Tip |
These are GA6-C Alberta Slip floating blue (left), AMACO Potter's Choice PC-20 Blue Rutile (center), GR6-M Ravenscrag floating blue (right). The clay is M390. The firing is cone 6, the schedule is C6DHSC (drop-and-hold, slow cool). All of these recipes are descendants and improvements of the 50-year-old original G2826R floating blue. The inside glaze on these mugs is GA6-B. The two on the left develop the blue color because of the slow cool, the one on the right works on fast-cool because it contains cobalt (although it will fire somewhat more mottled). Remember, these work best on dark-burning bodies.
The body is red-burning Plainsman M390. The firing was dropped and soaked at 2100F for 30 minutes and then dropped at 300F/hr to 1400F. This really helps to produce a dazzling defect-free surface in the GR6-M glaze. These are, of course, mix-your-own recipes and the pieces were dipped to get even coverage.
The insides are GA6-A Alberta Slip cone 6 base. Outsides are Ravenscrag Floating Blue GR6-M. The firing was soaked at cone 6, dropped 100F, soaked again for half and hour then cooled at 108F/hr until 1400F. The speckles on the porcelain blue glaze are due to agglomerated cobalt oxide (done by mixing cobalt with a little bentonite, drying and pulverizing it into approx 20 mesh size and then adding that to the glaze slurry).
The dipping glaze version of G2917 was used on the left, we make that by the bucket and it drains and dries in seconds after dipping bisque ware. We also make a brushing glaze version in our studio (it is not sold). That was used on the outside of the mug on the right. Of course, it is much slower to apply but there are some advantages. First, it was easy to control the thickness to maximize the variegation this glaze gives with thin and thick, revealing the throwing lines better. And where thicker application was needed (e.g. at the rim) it was easier to achieve that. Near the foot ring, it has been applied in a thinner layer. And the brush strokes do impart a more handmade look that is also nice.
Floating Blue - Substituting Gerstley Borate
A popular cone 6 glaze that employed Gerstley Borate, it was very troublesome to use and to fire. Much work has been done to create alternative recipes.
Rutile Blue Glazes
A type of ceramic glaze in which the surface variegates and crystallizes on cooling in the presence of titanium and iron (usually sourced by rutile)
Rutile mineral ground to a very fine particle size (e.g. 325 mesh) contributes titanium and iron that colors and variegates ceramic glazes.