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Pure Ravenscrag Slip

Code: GR10-A
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.

MaterialAmount
Calcined Ravenscrag Slip50.0
Ravenscrag Slip50.0
 100.00  

Firing Schedule

Rate (C)Temp (C)Hold (Min)Step
1012001
5055002
10098003
50130004
01300305
Freefall306

Notes

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.

Alberta and Ravenscrag Slips on two cone 10 reduction whiteware bodies

Alberta and Ravenscrag Slips on two cone 10 reduction whiteware bodies

Laguna B-Mix (left) and Plainsman H570 (right). They are fired to cone 10R with pure Ravensrag Slip on the inside and a 50:50 mix of Ravenscrag and Alberta Slips on the outside.

Plainsman H450 (left) vs. H550 celadon glazed mugs

Plainsman H450 (left) vs. H550 celadon glazed mugs

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.

Sterile white vs. pure Ravenscrag Slip as a liner glaze at cone 10R

Sterile white vs. pure Ravenscrag Slip as a liner glaze at cone 10R

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.

Calcining Ravenscrag Slip

Calcining Ravenscrag Slip

This is Ravenscrag Slip, I am going to calcine about 10 pounds of it in this bisque ware vessel to destroy the plasticity. I will fire to 1000F and hold it for 2 hours to make sure the heat penetrates. Why calcine? Because I have found that in some glazes having 70% or more Ravenscrag Slip, cracking on drying can occur if it is applied too thick. I love the working properties of these glazes and want to optimize them to avoid any problems. I am going to mix 75:25 raw:calcine on the next batch of glaze. However, Ravenscrag has an LOI of 9%, so I need to use 9% less of the calcine powder (just multiply the amount by 0.91). Suppose, I needed 1000 grams: I would use 750 raw and 250*.91=227.5.

G1947U transparent glaze (left) vs. Ravenscrag Slip at cone 10R

G1947U transparent glaze (left) vs. Ravenscrag Slip at cone 10R

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.

100% Ravenscrag slip on Plainsman H550 stoneware fired to cone 10R

100% Ravenscrag slip on Plainsman H550 stoneware fired to cone 10R

The surface is between silky and glossy.

CaO is a strong flux but it can cause crazing

CaO is a strong flux but it can cause crazing

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.

How to matte Ravenscrag Slip at cone 10 by adding talc

How to matte Ravenscrag Slip at cone 10 by adding talc

2,5,10,15% talc added to Ravenscrag Slip on a buff stoneware fired at cone 10R. Matting begins at 10%. By Kat Valenzuela.

2, 5, 10, 15% dolomite added to Ravenscrag Slip at cone 10R

2, 5, 10, 15% dolomite added to Ravenscrag Slip at cone 10R

This is a buff stoneware clay. Crystal development toward a dolomite matte begins at 15%. By Kat Valenzuela.

2, 5, 10 and 15% alumina hydrate added to Ravenscrag Slip

2, 5, 10 and 15% alumina hydrate added to Ravenscrag Slip

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.

2, 5, 10 and 15% calcined alumina added to Ravenscrag Slip

2, 5, 10 and 15% calcined alumina added to Ravenscrag Slip

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).

Ravenscrag Slip pure: Oxidation vs. Reduction

Ravenscrag Slip pure: Oxidation vs. Reduction

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.

Laguna B-Mix with Ravenscrag GR10-A, GR10-C glazes and 10% PES

Laguna B-Mix with Ravenscrag GR10-A, GR10-C glazes and 10% PES

Left two mugs are pure Ravenscrag Slip, right is RavenTalc silky matte. The speckled mugs have 10% of an A1:St. Rose Red mix added.

B-Mix with Ravenscrag Slip inside and Alberta Slip outside

B-Mix with Ravenscrag Slip inside and Alberta Slip outside

Fired cone 10R. The one on the right contains 10% of Plainsman A1:St Rose Red mix to add speckle.

Laguna B-Mix Cone 10R mugs with Alberta and Ravenscrag glazes

Laguna B-Mix Cone 10R mugs with Alberta and Ravenscrag glazes

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).

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By Tony Hansen

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