|Monthly Tech-Tip |
Modified: 2020-12-15 14:36:00
Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe. It can be found among others at http://ravenscrag.com.
|Ravenscrag Slip 1000F Roast||39.00|
|Ferro Frit 3134||20.00|
|Ferro Frit 3124||10.00|
|Red Iron Oxide||2.00|
David Shaner's cone 6 floating blue has been used for many years by thousands of potters. However the base (the clear to which the colorants are added) is 50:30:20 Nepheline Syenite:Gerstley Borate:Silica. It has serious issues (including slurry gelling because of the partially soluble Gerstley Borate and Nepheline Syenite, consistency and supply issues with Gerstley Borate, susceptibility to blistering and higher thermal expansion because of the high feldspar content). As a consequence most users thus have had a love-hate relationship with the recipe.
This employs an alternative base recipe into which the iron, cobalt and rutile have been transplanted. The GR6-A 80:20 Ravenscrag:Frit base has been conditioned with a further 10% frit addition to get more crystal development and variegation. This base eliminates the solubility and consistency issues and produces a glaze of lower thermal expansion.
Although more expensive to make than the GA6-C Alberta Slip Rutile Blue (because of the cobalt), this one produces the rich blue without needing the slow-cooling C6DHSC firing schedule (although it still benefits with a more defect-free surface). The recipe originally had 80 parts raw Ravenscrag powder but we have adjusted it to 40:39 raw:roast (according to instructions at ravenscrag.com). Feel free to tune the raw:roast mix to get the exact slurry properties you want. More roast makes the slurry shrink less, dry faster and more powdery; more raw makes it shrink more and dry harder.
For mixing instructions please see the master recipe, GR6-A.
Cone 6 oxidation. GR6M Ravenscrag version is on the left. The Alberta Slip version (GA6C) is more fluid, but that also means it will run more during firing and blister more if too thick or on re-firing. Generally, the Alberta Slip version appears better on dark bodies and the Ravenscrag one on lighter burning clays. The Alberta Slip version gets its color only from Rutile (and thus requires a special drop-and-hold firing scheduel), the Ravenscrag one produces blue in any firing schedule (although the color will be better in the drop-and-hold schedule).
GR6-M Ravenscrag Cone 6 Floating Blue on Plainsman M340 buff stoneware. This glaze also has this variegated visual character on porcelain. Because it has the GR6 base recipe (more information at ravenscrag.com), the slurry has very good working properties in the studio, it is a pleasure to use. This is an excellent showcase for the variegating mechanism of rutile.
The body is red-burning Plainsman M390. The firing was dropped and soaked at 2100F for 30 minutes and then dropping at 300F/hr to 1400F. This really helps to produce a dazzling defect free surface. These are, of course, mix-your-own recipes and the pieces were dipped to get perfectly even coverage.
Both have been applied at moderate thickness on Plainsman M325 (using a slurry of about 1.43-1.45 specific gravity, higher values end up getting them on too thick). The Ravenscrag version highlights contours better (the edges are black because of the black engobe underneath). It also produces the blue color whether or not the kiln is slow cooled to 1400F (although a faster cool is less blue). But the Alberta Slip version has zero cobalt so is less expensive to make. It produces a deeper color over the black engobe underneath the upper section of the pieces. Both of these produce a wide range of effects with different thickness, bodies and firing schedules.
Originally popularized by James Chappell in the book The Potter's Complete Book of Clay and Glazes. It is loved and hated. Why? The high Gerstley Borate content makes it finicky. But the magic ingredient is not the GB, it is the rutile, Rutile makes the cobalt and iron dance. This recipe actually produces a number of different mechanisms of variegation. Color and opacity vary with thickness. Small rivulets of more fluid glass flow around more viscous phases producing micro-areas of differing colors and opacities. Titanium crystals sparkle and calcium-borate creates opalescence. Bubbles of escaping gases (from GB) have created pooling. Small black speckles from unground or agglomerated particles of iron are also present. Surprise! This is actually Ravenscrag Floating blue. All the visuals, none of the headaches.
M340 stoneware fired to cone 6 (drop-and-hold schedule). The L3954B engobe fires deep black (it has 10% Mason 6600 black stain instead of the normal 10% Zircopax). It was applied inside and partway down the outside (a much less messy process than using a black clay body). They were bisque fired and glazed inside using the base GA6A Alberta Slip amber clear (using Frit 3195). The outside glaze is Alberta Slip Rutile Blue (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers). To learn more about how to make the engobe and start making black pots click "Product Data Sheets" at PlainsmanClays.com and go to the section on Medium Temperature.
GR6-M Ravenscrag cone 6 Floating Blue (center) on Plainsman M340, a buff burning body. On the left is a version having 80:20 Ravenscrag:Frit 3134 (no extra 10% Frit 3124). On the right is GR6-M on porcelain (where the floating effect has been largely lost). It appears the effect benefits from the iron it finds (albeit not much) in the stoneware body.
Here it is fired to cone 8 where the melt obviously has much more fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course it is running alot more, so caution will be needed.
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).
These are from the same firing, glazed at the same time and are the same thickness. The floating blue effect is a fragile mechanism and affected even by the small color difference in these bodies. The small amount of extra iron in the M370 affects the glaze character more than expected.
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 coverage.
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
G2587 - Floating Blue Cone 5-6 Original Glaze Recipe
Floating Blue is a classic cone 6 pottery glaze recipe from David Shaner. Because of the high Gerstley Borate content it is troublesome, difficult. But there are alternatives.
GA6-C - Alberta Slip Rutile Blue Cone 6
Plainsman Cone 6 Alberta Slip based glaze the fires bright blue but with zero cobalt.
GR6-A - Ravenscrag Cone 6 Clear Glossy Base
This Plainsman Cone 6 Ravenscrag Slip base is just the pure material with 20% added frit to make it melt to a glossy natural clear.
Floating Blue page at gerstleyborate.com
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