Ravenscrag logo

landscape near Ravenscrag

Plainsman clays logo

If we demand clay bodies with great working properties, why do we not demand the same of our glazes?

Cone 10 computer controlled gas kiln
Computer controlled gas kilns are less expensive than in the past. There is no better way to start serious production of high quality ware.

Glorious Cone 10R Using Ravenscrag Slip!

Are you serious about the production of high quality ware? Nothing beats cone 10R! It is good for quality and business.

  • Native materials: Glazes melt so much easier, so you can use raw and native glaze materials (gone are frits, borates and other low-melting and expensive materials). Ravenscrag and Alberta slip melt by themselves at cone 10.
  • Quality: Porcelainous and white stoneware bodies are so strong and the glazes have higher silica and alumina so they are harder.
  • Porcelain and whitewares fire blue-white instead of yellowy-white.
  • It is the home of the most magic oxide in ceramics: iron. Only at Cone 10 reduction is iron a powerful glaze flux, variegator and crystalizer, a colorant of many characters in bodies and glazes and a speckling agent like no other. And it is safe and cheap!
  • Cone 10 raw flux-sourcing materials are much less toxic.
  • Cone 10 bodies can have higher clay content, and therefore be more plastic (because less feldspar is needed). Glazes also need less flux so more clay in the recipe means better suspension and hardening.
  • The easiest mechanism to produce mattes lives at cone 10. Trying to crowbar magnesia or dolomite mattes lower using fluxes like boron glosses them, and the incredible soft silky surface is lost. Other lower fire matte mechanisms are fragile and trouble prone.
  • There is no aesthetic that can match cone 10R in porcelain or stoneware.
  • Firing consistency and cost: There are no kiln elements that deteriorate over time and fire slower, an old kiln fires the same schedule as a new one. Gas is cheaper than electricity and gas kilns can fire alot faster and this can more than compensate for the higher temperature (in industry fires are measured in minutes, not hours).
  • There is much less crazing at cone 10 because the glazes, by necessity, have much less high expansion fluxes and more low expansion silica and alumina.
  • There is little need to rely on complicated flux interactions to make things melt, the temperature does it all. There is a closer relationship between what the glaze chemistry suggests will happen in the kiln and what actually does happen.
  • .

Here is what you need:

  • A good automatic firing cone 10R kiln.
  • Make your glazes from Ravenscrag and Alberta Slip so you only need to stock a minimum of materials.
  • Plainsman clay bodies: No one on the planet makes better cone 10R native stonewares.

Glaze Recipes

Ravenscrag by itself fires to an excellent cone 10 glaze, especially as a silky matte base.

For functional glaze, consider testing for leaching, crazing and cutlery marking.

Courtesy of Digitalfire Reference Library

Alberta Slip:Ravenscrag Cone 10R Celadon

Code: GR10-E

A celadon recipe containing only two ingredients. It affords slurry and drying control because proportions of raw and calcined materials can be varied.

Alberta Slip47.50
Ravenscrag Slip47.50
Ferro Frit 31345.00
100.00

This is a celadon that you can tune to your needs. It works well on dark and light stonewares and porcelains. Adjust the proportions of Alberta Slip to Ravenscrag Slip to fine tune the color (more Ravenscrag for lighter color). Adjust the amount of frit to tune the amount of gloss and melt fluidity. Calcium carbonate also works, but may produce more bubbles floating in the matrix. If you fire to cone 11, then no flux may be needed.

You can fine tune the thermal expansion (e.g. frit 3134 for higher expansion, 3249 for lower, 3124 for nuetral).

Since this glaze is 90% clay, you will need to calcine half of the Alberta Slip complement to reduce the drying shrinkage if it will be applied to dry or bisque ware (if applied to leather hard ware it could be OK as is).

Ravenscrag Celadon and silky matte glazed mug by Tony Hansen

The outside glaze on this cone 10R mug (made of Plainsman H550) is simply an Alberta Slip:Ravenscrag Slip 50:50 mix with 5% added Ferro Frit 3134 (the Alberta Slip is calcined). This produces a stunning celadon with great working and application properties. Inside glaze: Ravenscrag Slip 90%, talc 10% (a matte having an extra ordinary silky texture). Learn more at ravenscrag.com.

Calcining Alberta Slip

Calcined Alberta Slip (right) and raw powder (left). These are just 5 inch cast bowls, I fire them to cone 020 and hold it for 30 minutes. Why calcine? Because for glazes having 50% or more Alberta Slip, cracking on drying can occur, especially if it is applied thick (Alberta Slip is a clay, it shrinks). I mix 50:50 raw:calcine for use in recipes. However, Alberta Slip 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 500 raw and 500*.91=455.

GR10-E 50:50 Alberta Slip:Ravenscrag Slip celadon at cone 10R

On a white stoneware and a porcelain. The glaze is transparent, it has depth and varies in shade according to thickness, breaking to a much lighter shade on the edges of contours.

Ravenscrag GR10-E celadon glaze

(50:50 Ravenscrag Slip:Alberta Slip) at cone 10R on porcelain (right) and stoneware (left).

GR10-E Ravenscrag:Alberta Slip with 10% calcium carbonate

At cone 10R this produces an overly melted glaze. It also crazes.

 


Pure Ravenscrag Slip

Code: GR10-A

Ravenscrag all by itself makes a great cone 10 reduction semi-gloss glaze. It also has great working properties.

Calcined Ravenscrag Slip50.00
Ravenscrag Slip50.00
100.00

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

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

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

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.

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

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

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.

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).

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

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

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

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

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

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

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.

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.

 


Ravenscrag Cone 10R Bamboo Matte

Code: GR10-J1

Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip51.60
Dolomite12.10
Talc2.60
Gerstley Borate4.00
Calcined Kaolin13.50
Custer Feldspar16.20
Add
Red Iron Oxide1.00
101.00

This is a variation of the GR10-J dolomite matte.

Bamboo glaze that is actually functional

The stunning cone 10R Ravenscrag bamboo glaze (GR10-J plus 0.5% iron oxide) on a Grolleg porcelain. Up close it can feel and look like a fine wood surface (when used on a porcelain). The cone 10 recipes page at Ravenscrag.com has more info.

How to turn a dolomite matte white glaze into a bamboo matte

Make cone 10R bamboo colors using the GR10-J Ravenscrag silky matte base recipe (right) and adding 1% iron (left), (0.5% centre). These samples are porcelain. This iron addition also works using the G2571A matte base recipe.

Ravenscrag GR10-J Cone 10R Bamboo glaze variation

Porcelain (left), buff stoneware (center), iron stoneware (right). Works well on all body types. On porcelain, interesting red tones and variations in tone appear.

 


Ravenscrag Cone 10R Dolomite Matte

Code: GR10-J

Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip51.60
Dolomite12.10
Talc2.60
Boraq 5 (Gerstley Borate sub)4.00
Calcined Kaolin13.50
Custer Feldspar16.20
100.00

This is the classic cone 10 reduction silky white matte, its feel, beauty and utility are very difficult to match in any other system. Unlike many feldspar saturated dolomite mattes in use, this one does not craze on stonewares and it does not cutlery mark or stain.

This glaze was developed from the popular G2571A, it has the same chemistry however it sources that chemistry from a completely different recipe based on Ravenscrag Slip (this is a good example of how Insight ceramic chemistry software can be used). The pleasant silky surface is almost identical (though slightly darker in color due to a little higher iron content). Add a little zircopax (e.g. 3-5%) to whiten the color if you desire or if you need to precisely match the G2571A. This glaze has the characteristic superb application and suspending properties of Ravenscrag Slip glazes.

Note that the calcined kaolin is important. If you use raw kaolin, the glaze will shrink too much during drying. For a slip glaze, raw kaolin would be OK. If you do not have the Boraq (Gerstley Borate substitute) then you can substitute real Gerstley Borate or another equivalent.

The two most common variations are bamboo and robin's egg blue (see photos here).

Mar 2015: We have found a tendency for this glaze to craze on some stonewares after decal firings. See the linked picture.

Ravenscrag dolomite matte

GR10-J Ravenscrag dolomite matte base glaze at cone 10R on Plainsman H443 iron speckled clay. This recipe was created by starting with the popular G2571 base recipe (googleable) and calculating a mix of materials having the maximum possible Ravenscrag Slip percentage. The resultant glaze has the same excellent surface properties (resistance to staining and cutlery marking) but has even better application and working properties. It is a little more tan in color because of the iron content of Ravenscrag Slip (see ravenscrag.com).

Why did this piece came out of a decal firing crazed?

This Cone 10 matte mug has been refired to attach decals. During the refire the Quartz-containing body passed up through quartz and cristobalite inversions while the glaze did not (all of its quartz was converted to silicates during the previous glaze firing). The sudden expansion in these two zones stretched the glaze and cracked it. Had that glaze been better fitted (under some compression) it would have been able to survive.

Two great dolomite matte cone 10R recipes on iron stoneware

GR10-J Ravenscrag silky matte (right) and G2571A matte (left) on a dark burning iron speckled stoneware at cone 10R. Surfaces have identical feel (the chemistries are very close). The former fires a little darker color because of the iron contributed by the Ravenscrag Slip.

Bamboo glaze that is actually functional

The stunning cone 10R Ravenscrag bamboo glaze (GR10-J plus 0.5% iron oxide) on a Grolleg porcelain. Up close it can feel and look like a fine wood surface (when used on a porcelain). The cone 10 recipes page at Ravenscrag.com has more info.

A refined-material cone 10R dolomite matte (left) vs. one made using Ravenscrag Slip

GR10-J Ravenscrag silky matte (right) and G2571A matte (left) on a buff stoneware at cone 10R. Surfaces feel identical, the slightly darker color is due to iron content in the Ravenscrag. The former was formulated to mimic the latter using as much Ravenscrag Slip as possible yet still maintain the same chemistry.

How to turn a dolomite matte white glaze into a bamboo matte

Make cone 10R bamboo colors using the GR10-J Ravenscrag silky matte base recipe (right) and adding 1% iron (left), (0.5% centre). These samples are porcelain. This iron addition also works using the G2571A matte base recipe.

Robin's egg blue at Cone 10R: add 1% cobalt oxide and 0.2% chrome oxide to GR10-J Ravenscrag silky matte. Does not work well on porcelains (left), very well on buff stonewares (right). Inside of center mug is GR10-J.

Ravenscrag GR10-J Cone 10R Bamboo glaze variation

Porcelain (left), buff stoneware (center), iron stoneware (right). Works well on all body types. On porcelain, interesting red tones and variations in tone appear.

 


Ravenscrag Cone 10R Silky Talc Matte

Code: GR10-C

Plainsman Cone 10R RavenTalc matte glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip90.00
Talc10.00
100.00

This glaze is more silky (less matte) than the GR10-J glaze, it is much more beautiful visually and to the touch than a photo can convey. It works well all all clays, but especially buff and dark stonewares.

Ravenscrag based silky MgO matte at cone 6

Cone 6 Ravenscrag Silky Matte on Plainsman M340 (left) and M370 (right). The inside of the M370 mug is a transparent glossy. This recipe produces a silky ivory-coloured surface of very good quality. Go to Ravenscrag.com for more info.

H435 with Ravenscrag 10% talc inside and Ravenscrag Silky Matte outside.

Fired at cone 10R

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).

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.

Difference between oxidation and reduction! GR10-C matte on Plainsman H443

Same body, same glaze. Left is cone 10 oxidation, right is cone 10 reduction. What a difference! This is a Ravenscrag Slip based glaze on a high-fire iron stoneware. In reduction, the iron oxide in the body and glaze darkens (especially the body) and melts much more. The behavior of the tin oxide opacifier is also much different (having very little opacifying effect in reduction).

RavenTalc silky matte for cone 10 reduction

GR10-C Ravenscrag cone 10R silky matte glaze (90% Ravenscrag Slip, 10% talc) produces stunning surfaces and has excellent slurry and application properties.

GR10-C Ravenscrag glaze on iron stoneware

GR10-C Ravenscrag glaze on grey stoneware.

 


Ravenscrag Cone 10R Tenmoku

Code: GR10-K1

Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip95.00
Calcium Carbonate5.00
Add
Iron Oxide10.00
110.00

Like a typical tenmoku, this fires to a brilliant deep gloss that will run if applied too thickly. It appears best on porcelain on white stoneware. Tiny iron silicate crystals grow on the surface (depending on the speed of cooling).
Adjust the calcium carbonate to fine tune the degree of melting.

Compare Ravenscrag and Alberta Slip tenmokus at cone 10R

GR10-K1 Ravenscrag tenmoku (left) compared to Alberta Slip tenmoku GA10-B (center) and pure Alberta Slip (right).

Add 5% caclium carbonate to a tenmoku. What happens?

In the glaze on the left (90% Ravenscrag Slip and 10% iron oxide) the iron is saturating the melt crystallizing out during cooling. GR10-K1, on the right, is the same glaze but with 5% added calcium carbonate. This addition is enough to keep most of the iron in solution through cooling, so it contributes to the super-gloss deep tenmoku effect instead of precipitating out.

Tenmokus made from Alberta Slip and Ravenscrag Slip

GR10-K1 Cone 10R Ravenscrag Tenmoku (right) compared to Tenmoku made from Alberta Slip (left, it is 91% Alberta Slip with 5% added calcium carbonate and 2% iron oxide). Left is Plainsman P700 porcelain, right is H570. Tenmokus are popular for the way they break to a crystalline light brown on the edges of contours.

 


Ravenscrag Iron Crystal

Code: GR10-L

Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip100.00
Add
Iron Oxide10.00
110.00

Without any added flux Ravenscrag will produce an iron crystal surface with 10% iron oxide.

An awesome iron crystal glaze recipe for cone 10R

This iron crystal glaze is Ravenscrag slip plus 10% iron oxide fired to cone 10R on a buff stoneware (Plainsman H550). Since Ravenscrag slip is a glaze-by-itself at cone 10, it is an ideal base from which to make a wide range of glazes. It has its own website at http://ravenscrag.com. It was originally formulated using Digitalfire Insight software. The project built on the merits of a specific silty clay that was noted to couple very good suspension and drying properties with a low firing temperature. The process involved calculating what minerals needed to be added to it to produce the chemistry of a middle-of-the road silky cone 10 glaze; the product was Ravenscrag Slip.

Fixing a crawling problem with Ravenscrag Tenmoku

Crawling of a cone 10R Ravenscrag iron crystal glaze. The added iron oxide flocculates the slurry raising the water content, increasing the drying shrinkage. To solve this problem you can calcine part of the Ravenscrag Slip, that reduces the shrinkage. Ravenscrag.com has information on how to do this.

A cone 10R iron crystal glaze using only Ravenscrag Slip and Iron

Ravenscrag Slip iron crystal glaze fired at cone 10R on porcelains. The recipe is simply 90% Ravenscrag Slip and 10% iron oxide.

Ravenscrag Iron Crystal

Ravenscrag plus 10% iron oxide on the outside, Ravenscrag Slip plus 10% talc on the inside (GR10-J). Fired at cone 10 reduction.

 

Oxidation

Courtesy of Digitalfire Reference Library

Ravenscrag Cone 10 Oxidation Variegated White

Code: GR10-G

Ravenscrag Slip100.00
Add
Talc10.00
Tin Oxide4.00
114.00

Ravenscrag Slip can make an interesting variegated ivory at cone 10

GR10-G Ravenscrag white glaze at cone 10 oxidation

 

Ravenscrag Slip is a product of Plainsman Clays and near Ravenscrag, Saskatchewan, Canada.