Modification Date: 2016-10-01 18:06:25
Member of Group: RV10
Ravenscrag all by itself makes a great cone 10 reduction semi-gloss glaze. It also has great working properties.
|Calcined Ravenscrag Slip||50.0|
|Rate (C)||Temp (C)||Hold (Min)||Step|
Ravenscrag Slip is a revolutionary glaze material for stoneware. At cone 10R it can be used pure and produces a functional semi-gloss surface. The possibilities are endless on what you can do by adding things to this base material. You can make additions to gloss it or matte it more, color and variegate it and/or opacify it. Most will contain 90% Ravenscrag (you can make bamboo, tenmoku, celadon, iron crystal, white, matte, glossy, and much more).
Ravenscrag Slip is a clay, so it shrinks. We recommend starting a 50:50 raw:calcine mix when using it pure (use the calcining instructions at http://ravenscrag.com). Adjust the proportion to get the fastest drying possible while still maintaining hardness and good slurry suspension. When you have it right you can put multiple layers on the ware without lifting. Ravenscrag glazes do not settle in the bucket and go on even (when mixed correctly). For a slurry that performs even better, consider using a little more water and then gel the slurry with a little vinegar or Epsom salts (be very careful not to put too much in, test on a small quantity first). When gelled, this glaze suspends and applies even better.
The inside glaze is pure Ravenscrag Slip and the outside glaze is a 50:50 mix of Ravenscrag and Alberta Slips. Each of the glazes employs an appropriate mix of calcined and raw clay to achieve a balance of good slurry properties, hardening and minimal drying shrinkage. Ravenscrag needs less calcined since it is less plastic than Alberta Slip.
This picture does not fully convey how much better the Ravenscrag is as a liner (vs. G1947U). It has depth and looks much richer. It course, it could be opacified somewhat to be whiter and would still retain the surface quality (as long is it is not too opaque). The body is Plainsman H450. The outside glaze is pure Alberta Slip.
This is the Ravenscrag Slip I used to calcine at it 1850F (about 10lbs in a bisque vessel). I am now roasting it to 1000F instead, this produces a smoother powder, less gritty. I hold it for 2 hours at 1000F to make sure the heat penetrates. It is not actually calcining, since not all crystal water is expelled, so we call it "roasting". Why do this? Ravenscrag Slip is a clay, it shrinks. If the percentage is high enough the glaze can crack on drying (especially when applied thickly). The roast does not shrink. The idea is to tune a mix of raw and roast Ravenscrag to achieve a compromise between dry hardness and low shrinkage. Technically, Ravenscrag losses 3% of its weight on roasting so I should use 3% less. But I often swap them gram-for-gram.
Ravenscrag Slip is not ultra glossy but has a silky surface. It also contains some iron oxide and this colors the glaze somewhat. But the surface is much less sterile and pleasant to touch.
The surface is between silky and glossy.
2, 5, 10, 15% calcium carbonate added to Ravenscrag Slip on a buff stoneware fired at cone 10R. It gets progressively glossier toward 15%, crazing starts at 10% (test by Kat Valenzuela). Adding a flux only reduces the SiO2 and Al2O3, this pushes the thermal expansion upwards. 5% is actually sufficient. An alternative would be to use wollastonite, it supplies SiO2 also.
2,5,10,15% talc added to Ravenscrag Slip on a buff stoneware fired at cone 10R. Matting begins at 10%. By Kat Valenzuela.
This is a buff stoneware clay. Crystal development toward a dolomite matte begins at 15%. By Kat Valenzuela.
Pure Ravenscrag Slip is glaze-like by itself (thus tolerating the alumina addition while still melting as a glaze). It was applied on a buff stoneware which was then fired at cone 10R (by Kat Valenzuela). This same test was done using equal additions of calcined alumina. The results demonstrated that the hydrated version much more readily decomposes to yield its Al2O3, as an oxide, to the glaze melt. By 15% it is matting and producing a silky surface. However crazing also starts at 10%. The more Al2O3 added the lower the glaze expansion should be, so why is this happening? It appears that the disassociation is not complete, some of the raw material remains to impose its high expansion.
The Ravenscag:Alumina mix was applied to a buff stoneware fired at cone 10R (by Kat Valenzuela). Matting begins at only 5% producing a very dry surface by 15%. The matte is simply a product of the refractory nature of the alumina as a material, it does not disassociate in the melt to yield its Al2O3 as an oxide (as would a feldspar, frit or clay). The same test using alumina hydrate demonstrates that it disassociates better (although not completely).
Pure Ravenscrag Slip on a porcelain at cone 10 oxidation (left) and cone 10 reduction (right). The reduction fired sample is a very smooth pleasant semi-matte, the other is glossier but dimpled.
Left two mugs are pure Ravenscrag Slip, right is RavenTalc silky matte. The speckled mugs have 10% of Plainsman FiredRed (a concretion-containing red fireclay).
Fired cone 10R. The one on the right contains 10% of Plainsman A1:St Rose Red mix to add speckle.
B-Mix is a popular high-ball clay very plastic grey cone 10R stoneware in North America. The two mugs on the left have pure Ravenscrag Slip on the inside (the middle on the outside also), it fires almost transparent with a slightly silky surface. Pure Alberta Slip is employed on the outside of the left one and the inside of the right one. The outside of the right one is RavenTalc silky matte. In all cases the Ravenscrag and Alberta Slip are mixed half-and-half calcined and raw. B-Mix fires dark enough and with enough specks that a normal transparent glaze is not very interesting. But these Ravenscrag ones look much better (for liner glazes).
Out Bound Links
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By Tony Hansen