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

Cone 6 Ravenscrag Slip Glazes

This is not a glaze recipe page. Ravenscrag Slip is a glaze material, these are demonstration recipes of what can be done with it.

These are "not guaranteed-to-work glazes". This page focusses on the value of this material to make base glazes and provides some examples of variations. However circumstances differ, do testing and alter for your situation (for information on variegating, opacifying and coloring glazes click here).

Care should be taken not to fire glazes too fast or at too low a temperature. Witness cones are recommended to assure you have reached cone 6. These glazes have worked in our studio at Plainsman Clays but we recommend testing them in your circumstances and on your clay bodies for fit.

For functional glaze, consider testing for leaching, crazing and cutlery marking. For liner glazes for porcelains, please go here.

Courtesy of Digitalfire Reference Library

Ravenscrag Alberta Brilliant Cone 6 Celadon

Code: GR6-N

Plainsman Cone 6 Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip38.00
Alberta Slip 1000F Roasted38.00
Ferro Frit 313419.00
Ferro Frit 31245.00
Iron Oxide Red1.00
Cobalt Oxide0.50
Rutile2.50
Tin Oxide0.50
Copper Oxide Black0.50
105.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

This was originally discovered as a 50:50 mix of G2917 Ravenscrag Floating Blue and G2908 Alberta Slip Floating Blue. The result is green.

Alberta Ravenscrag Cone 6 Brilliant Celadon

The magic of this recipe is the 5% extra frit, that makes the melt more fluid and brilliant and gives the glaze more transparency where it is thinner on edges and contours. The extra iron in the Plainsman P380 (right) intensifies the green glaze color (vs. Polar Ice on the left). The specks are cobalt oxide agglomerates that were made by slurrying cobalt oxide and bentonite, then crushing it to sizes large enough to make the specks.

Cone 10R celadon at cone 6? Ravenscrag:Alberta GR6-N recipe.

Brilliantly glossy. The body is Plainsman Polar Ice porcelain. Firing is cone 6 oxidation. The reduction fired effect is particles (or agglomerates) from one of the raw metal oxides in the recipe (iron, cobalt, rutile; most likely the cobalt). If this glaze were ball milled the effect would be lost. Even though the glaze is so glassy, it is not running down off at the foot. The blue where it thickens on contours is because of the rutile, this can be removed for a truer Celadon effect (if it is not causing the specks).

A fluid melt glaze bleeds much more into adjoining ones

The outer green glaze on these cone 6 porcelain mugs has a high melt fluidity. The liner glaze on the lower one, G2926B, is high gloss but not highly melt fluid. Notice that it forms a fairly crisp boundary with the outer glaze at the lip of the mug. The upper liner is G3806C, a fluid melt high gloss clear. The outer and inner glazes bleed together completely forming a very fuzzy boundary.

 


Ravenscrag Cone 6 Clear Glossy Base

Code: GR6-A

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.

Ravenscrag Slip50.00
Ravenscrag Slip 1000F Roast30.00
Ferro Frit 313420.00
100.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

This is the base cone 6 Ravenscrag recipe, it fires as a transparent glossy. It has an addition of the most common North American borate frit, enough to produce a good glass at cone 6. Most other glazes based on Ravenscrag use this as a starting point (although they might substitute the frit for another).

The character of this is not the same as transparent glazes made from materials like feldspar, kaolin, silica, wollastonite and frit. This is a natural clay material having some iron content and particles you can feel (up to 42 mesh). So it forms a less sterile glaze, one having some variegation, fine speckle and slight blush or amber coloration. It is most glossy at a complete cone 6 (for a lower temperature you may need to increase the frit a little).

As a transparent, this glaze it is best suited for use on light-burning stonewares (e.g. Plainsman M340). On whiter porcelains it will not fire to a sufficiently transparent glass. For porcelains or white stonewares consider using G2926B or G3806C instead. On darker clay bodies this recipe may fire too cloudy. Is has a low enough thermal expansion to fit most bodies, but if it does craze consider substituting the Ferro Frit 3195.

As noted, this is a base recipe, well suited to additions of opacifiers, variegators and colors. Add zircopax to get a white (about 10%). The white produced will be somewhat variegated (non toilet-bowl-like). If you want variation with thickness to highlight edges of contours use a lower percentage of zircopax. For even better variegation, add some titanium dioxide (1-2% to a mix already containing 6-8 zircopax).

You can also add colorants and variegators (with or without opacifier). If colorant additions affect melt fluidity, add or reduce the frit content to compensate.

If you get cracking of the glaze during drying, increase the calcine clay at the expense of raw clay.

Roasting Ravenscrag Slip instead of calcining

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 transparent and Alberta Slip blue glazes by Tony Hansen

The mug is the buff stoneware Plainsman M340. Firing is cone 6. On the inside is the GR6-A Ravenscrag transparent base glaze. The outside glaze is GA6-C Alberta Slip rutile blue on the outside. The transparent, although slightly amber in color compared to a frit-based transparent, does look better on buff burning stoneware bodies this.

Cone 5R mug with GR6-A Ravenscrag glaze

Ravenscrag Slip at cone 5R and 10R

Cone 5 GR6-A glaze at cone 5R on Plainsman M340 (left) and pure Ravenscrag Slip at cone 10R on H550 (right).

 


Ravenscrag Cone 6 Floating Blue

Code: GR6-M

Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip40.00
Ravenscrag Slip 1000F Roast39.00
Ferro Frit 313420.00
Ferro Frit 312410.00
Add
Iron Oxide2.00
Cobalt Oxide1.00
Rutile4.00
116.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

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). Almost all users have thus have had a love-hate relationship with the recipe.

In this alternative base recipe we have transplanted the iron, cobalt and rutile into a the 80:20 Ravenscrag:Frit base (and added more frit to get more action in the 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 Alberta Slip Rutile Blue (because of the cobalt), this one produces the rich blue without needing the slow-cooling firing schedule.

This 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.

Adjust the specific gravity to about 1.44 (add some epsom salts to flocculate it if needed) to get a thixotropic slurry. It is important not to use this with too high of a specific gravity. Although it will be fluid and appear good at 1.5, it will apply to ware too thickly. To get 1.44 specific gravity use about 100 parts powder to 105 parts water.

How can you make Ravenscrag Floating Blue dance more?

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.

Ravenscrag Floating Blue on Polar Ice and M370 at Cone 6

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.

Ravenscrag Cone 6 Floating Blue on porcelain and a red stoneware

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).

Ravenscrag Floating Blue vs. Alberta Slip Rutile Blue

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).

A black engobe transforms the floating blue glaze over it

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.

Ravenscrag cone 6 floating blue thinner and thicker applications

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.

Close-up of Floating Blue on cone 6 dark/buff burning bodies

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.

Ravencrag rutile blue vs. Alberta Slip floating blue at cone 6

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.

Ravenscrag Cone 6 Floating Blue on buff stoneware

The clay is Plainsman M340. Unlike Alberta Slip floating blue, this version does contain a little cobalt to help guarantee the blue color.

A much better Cone 6 Floating Blue

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.

Better melting gives Ravenscrag Floating Blue more zip!

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.

 


Ravenscrag Cone 6 High Alumina Matte

Code: GR6-F

Plainsman Cone 6 Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip60.00
Ferro Frit 313420.00
Dolomite10.00
Calcined Alumina10.00
Cobalt Oxide2.00
Rutile Ceramic5.00
107.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

This is a high alumina matte glaze and is a favorite. It is not temperature sensitive and produces good results at cone 5, 6 and 7 (medium blue surface breaking an olive type green where thin). Most base glazes do not host alumina well and fire with pitting and uneven surface (alumina is very difficult to melt and dissolve). Be sure to use a smallest particle sized alumina you can find (it is available in very fine grades, much finer that the average ceramic material).

Ravenscrag glaze on Plainsman M340

 


Ravenscrag Cone 6 High Calcium Matte Blue

Code: g2851H

Plainsman Cone 6 Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip55.00
Whiting10.00
Dolomite15.00
Ferro Frit 313410.00
Silica10.00
Cobalt Carbonate2.00
Rutile Ceramic5.00
107.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

This glaze is temperature sensitive. It requires a slow cool down to allow crystals to grow. If the kiln cools too quickly you will get a glossy glaze. Has a nice soft smooth surface, with a variegated light blue color.

This base will also work well with other coloring oxides/stains. It is temperature sensitive and should be fired to cone 6.

Ravenscrag High Calcium Matte

Courtesy of Jonathan Kirkendall

Ravenscrag high calcium blue on M340

 


Ravenscrag Cone 6 Oatmeal Matte

Code: GR6-H

Plainsman Cone 6 Ravenscrag Slip glaze. See more at ravenscrag.com.

Ravenscrag Slip60.00
Calcined Alumina10.00
Dolomite10.00
Ferro Frit 313420.00
Add
Manganese Dioxide4.00
Rutile5.00
Illmenite Granular0.30
109.30

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

Note: For use as a straight glaze We are recommending GA6-F Alberta Slip oatmeal glaze instead, it looks better and has a better melt. Alberta Slip is already a stained material, so less colorants are needed to make darker colored glazes. Go the http://albertaslip.com.

However as a second layer over other more fluid melt glazes this can work well. Be sure not to put it on too thick.

Ravenscrag Cone 6 GR6-H Oatmeal over 5x20 cone 6 black

The black recipe was made using G1214M with 5% Mason 6666 stain. The oatmeal overlayer is 50% the thickness of the black. The more fluid under-black comes through leaving islands and vertical rivulets of the stiffer oatmeal. Good control of the glazing process is needed to get consistent results using this approach.

Ravenscrag oatmeal layered over black at cone 6

This is GR6-H Ravenscrag oatmeal over G1214M black on porcelain at cone 6 oxidation to create an oil-spot effect. Both were dipped quickly. You can find more detail at ravenscrag.com.

GR6-H Ravenscrag Oatmeal glaze cone 6 oxidation.

GR6-H glaze at cone 5R on Plainsman M370 and M350

 


Ravenscrag Cone 6 Raspberry Glossy

Code: GR6-E

A chrome-tin burgundy glaze using the Ravenscrag cone 6 base recipe.

Ravenscrag Slip70.00
Ferro Frit 313420.00
Whiting10.00
Add
Tin Oxide7.50
Chrome Oxide0.50
108.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

Chrome-tin (either from the raw materials or a stain) pink and red glazes can be difficult to achieve and keep consistent at cone 6. In ceramics, red is perhaps the most difficult and most expensive color in ceramics. The chemistry of the host glaze has to be sympathetic to the color development, the chrome and tin require high calcium, zero zinc and low boron (if the chemistry is wrong the color will likely be grey). Glazes that do work in this system are normally highly fritted, and as such are more expensive and difficult to work with. However this Ravenscrag base has beautiful working properties, you can do multiple layers and it has all the other benefits imparted by a high Ravenscrag Slip content. However, keep in mind that red is red, it is difficult; test this first on different kinds of clay to determine if is is suitable for you. Again, remember that tin is expensive, it may be better to use a pink or maroon chrome-tin based stain instead (although they are not much lower in price).

Medium to medium thick application is best. Breaks clear around edges to highlight irregularities in the surface. On darker bodies the red color is darker. Slow cooling can matte the surface.

Chrome tin pinks are easier using a stain than chrome and tin

The Ravenscrag Slip based burgundy glaze on the outside of these mugs is made by fluxing Ravenscrag with 20% Ferro Frit 3134 and adding 10% Mason 6006 burgundy stain (actually these have a little less stain, about 8%). This stain works better than using raw chrome and tin. This glaze functions very well on porcelains and breaks white on the edges to highlight contours.

Maroon and white mug before and after firing: What a difference!

The outer glaze is Ravenscrag GR6-E Raspberry, the bright maroon color is a product of the surprising interaction between the 0.5% chrome oxide and 7.5% tin oxide present. That small amount of chrome is only enough to give the raw powder a slight greenish hue, hardly different than the clear liner. While this color mechanism appears to be effective, it is delicate. A maroon stain is actually a better choice. It would fire more consistent would be less hazardous to use. And the raw glaze will be the same color as the fired one!

What happens if Ravenscrag Raspberry is applied too thick

These are Plainsman P300 mugs fired at cone 6. When the glaze, GR6-E, goes on too thick (as on the left) it is dark maroon and has a pebbly surface that does highlight contours. This went on too thick because the specific gravity of the slurry was too high, about 1.53 (even a one-second dip put to thick a layer on the pieces). When I thinned it down to about 1.45 and flocculated it using espom salts, it went on thinner, yet still evenly, and I got the result on the right.

Cone 6 GR6-E Ravenscrag Raspberry glaze

Made using chrome and tin added to the cone 6 Ravenscrag clear base glaze recipe.

Ravenscrag Cone 6 GR6-C white and with 10% Mason 6006 stain

The body is Plainsman M340. This is a good alternative to trying to get a chrome-tin pink or maroon working.

 


Ravenscrag Cone 6 Transparent Burgundy

Code: GR6-L

A stain-based method to achieve this color using the Ravenscrag base recipe.

Ravenscrag Slip40.00
Calcined Ravenscrag Slip40.00
Ferro Frit 313420.00
Add
Mason 6006 Stain10.00
110.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

This is an excellent alternative to the GR6-E Raspberry because the recipe is simpler, just a stain addition to the standard transparent. It is easy to adjust the amount of frit to get the exact shade you want.

Because this is transparent (contains no opacifier), different thicknesses have different intensities of color. Because it employs a stain it will be very more consistent that one using chrome and tin. Add an opacifier to create a more pastel color with less depth.

No special firing curve is needed and the surface fires very clean and defect free (with a normal soak at cone 6).

A breaking glaze highlights incised decoration

This is the Ravenscrag slip cone 6 base (GR6-A which is 80 Ravenscrag, 20 Frit 3134) with 10% Mason 6006 stain. Notice how the color is white where it thins on contours, this is called "breaking". Thus we say that this glaze "breaks to white". The development of this color needs the right chemistry in the host glaze and it needs depth to work (on the edges the glaze is too thin so there is no color). The breaking phenomenon has many mechanisms, this is just one. Interestingly, this transparent base has more entrained micro-bubbles than a frit-based glaze, these enhance the color effect.

Add a Mason burgundy stain to Ravenscrag cone 6 Clear and wow!

This is GR6-L Ravenscrag Burgundy on porcelain at cone 6. The amount of stain is higher than usual (about 13% instead of 10%), thus the color is darker.

GR6-L Ravenscrag Burgundy fired at cone 6 on Plainsman M370

The stain has been cut to 10% giving the glaze more transparency and making it vary more in color with changing thickness.

Ravenscrag Cone 6 Raspberry glaze on dark and light burning bodies

The body on the left has 10% burnt umber adding (Plainsman M340) and fires chocolate brown (right is standard M340). The manganese (in the umber) is greatly affecting the appearance of the glaze (GR6-L).

 


Ravenscrag Cone 6 White Glossy

Code: GR6-C

Plainsman Cone 6 Ravenscrag Slip based white glossy glaze. It can be found among others at http://ravenscrag.com.

Ravenscrag Slip40.00
Ravenscrag Slip Roast40.00
Ferro Frit 313420.00
Add
Zircopax10.00
110.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

A white base glaze with all the advantages of Ravenscrag Slip base GR6-A. It is not drab-looking like a porcelain toilet, but is vibrant and interesting on stoneware bodies. It breaks to clear on the edges of contours and has variegation. It works well with added stains, the variegation of this base (adding a little titanium dioxide) makes the color much more interesting.

Adjust the amount of zircopax as needed to adjust the degree of whiteness.

Because it contains an opacifier, this can be a base for the addition of colorants to produce pastel colors (adding them to the transparent base will produce deep brilliant colors). We recommend stains rather than metal oxides.

How do you turn a transparent glaze into a white?

Right: Ravenscrag GR6-A transparent base glaze. Left: It has been opacified (turned opaque) by adding 10% Zircopax. This opacification mechanism can be transplanted into almost any transparent glaze. It can also be employed in colored transparents, it will convert their coloration to a pastel shade, lightening it. Zircon works well in oxidation and reduction. Tin oxide is another opacifier, it is much more expensive and only works in oxidation firing.

Ravenscrag white on Plainsman M340

Ravenscrag Cone 6 GR6-C white and with 10% Mason 6006 stain

The body is Plainsman M340. This is a good alternative to trying to get a chrome-tin pink or maroon working.

0.5% fine granular illmenite added to Ravenscrag cone 6 clear glossy white base glaze.

 


Ravenscrag Plum Red Cone 6

Code: G2896

Plainsman Cone 6 Ravenscrag Slip based iron-red glaze. It can be found among others at http://ravenscrag.com.

Silica12.30
Ravenscrag Slip34.90
Ferro Frit 313414.10
Talc13.50
Ulexite10.20
Red Iron Oxide15.00
100.00

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

Iron red glazes are common in the cone 6 range. The red color is a product of iron silicate crystals forming during the cooling cycle in the kiln; it can be difficult to develop a process that gives repeatable results. While it would seem logical that these glazes should have a very fluid melt and a slow cooling cycle during firing to give the red iron crystals time to grow, in actual practice we have not been able to confirm either assumption. However it does appear that a thick application is needed to encourage the crystallization (and thus there is a danger that too much thickness will result in it running down off the ware). Thus experience is needed to achieve a workable thickness to be able to manage vertical surfaces. Just try to apply the glaze just thick enough that you can tolerate the amount of running.

Since this recipe employs Ravenscrag Slip, its application properties are superior to many others.

G2896 Ravenscrag Plum Red iron red cone 6 glaze

Original development of this recipe was done to match the chemistry of Randy's Red (a popular recipe). At the time we did not do any special firing schedule to encourage the growth of the red crystals.

Iron Red glazes look a little different in a flow tester

A GLFL test for melt flow comparing two cone 6 iron red glazes fired to and cooled quickly from cone 6. Iron reds have very fluid melts and depend on this to develop the iron red crystals that impart the color. Needless to say, they also have high LOI that generates bubbles during melting, these disrupt the flow here.

 


Ravenscrag Silky Matte for Cone 6

Code: G2928C

Plainsman Cone 6 Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.

Ferro Frit 313411.00
Talc8.00
Ravenscrag Slip60.00
Nepheline Syenite13.00
Wollastonite1.50
Calcined Kaolin12.00
Add
Tin Oxide5.00
110.50

The firing schedule may be important to developing this glaze effect.
Cone 6 Standard Firing Schedule
Cone 6 Slow Cool Firing

This works well on Plainsman M340, but especially on a whiteware like M370. Produces an ivory white with some fleck. The surface is very silky, reminiscent of a cone 10 dolomite matte. Its matteness is adjustable by varying the amount of calcined kaolin (or simply blending in a glossy recipe to shine it up a little). The mechanism of the matteness is high MgO in a boron base of low Si:Al ratio.

Do not use regular kaolin, the glaze will shrink too much during drying.
Use enough water in the slurry so it flows well, it will apply very evenly without drips. If there is not enough water and the slurry is too creamy, it will crack during drying and crawl during firing. Calcine part of the Ravenscrag to reduce the shrinkage if needed.

The tin oxide is included to whiten and opacify the glaze, if you remove it the color will be quite a bit darker, especially on darker clay bodies (Ravenscrag contains some iron). The 1.5 wollastonite is a remnant of how this glaze was created; it started as Moores Matte, a well known Gerstley Borate based matte recipe. We first reformulated it to substitute the GB for a frit (while maintaining the same chemistry) and then incorporated Ravenscrag Slip to supply as much of the rest of the chemistry as possible. A further adjustment was made to make the surface more silky. The silky matte G2934 followed this, it being a cleaner, whiter variant having no Ravenscrag but the same fired surface.

Since this has some iron, colouring it with stains could produce more muddied colours than you might want. Consider using the G2934 base instead if needed.

Ravenscrag Cone 6 silky matte does not work well on dark burning bodies

Like Plainsman M390 on the right. It is good on M340 (a buff stoneware on the left), but it is even better on a porcelain.

Compare two glazes having different mechanisms for their matteness

These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio (you can find more about it by googling 1214Z). It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived a 300F-to-ice water IWCT test). This looks like it could be a great liner glaze.

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.

A matte and a glossy liner glaze

Left: Ravenscrag G2928C matte on inside of mug. Right: A clear glossy. The matte needs to be soaked in the kiln long enough to make sure it develops a functional surface, especially on the bottom. Mattes are not always the best choice for food surfaces, but you can do it if you blend in enough glossy glaze to make it smooth enough not to cutlery mark.

 

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