Ravenscrag Slip: An excellent base for cone 6 glazes

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.

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

This was originally discovered as a 50:50 mix of G2917 Ravenscrag Floating Blue and G2908 Alberta Slip Floating Blue. The result is green. The recipe is a combination of the two with slight changes (0.5 cobalt instead of 0.75, 1.0 iron instead of 1.5, 2.5 rutile instead of 3.0).

For mixing instructions please see the master recipe, GR6-A.

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.

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.

Also known as G2936C.

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).

It is best to at least hold the temperature at the end of the firing to minimize firing defects. Even better, use the C6DHSC firing schedule.

As a transparent, this glaze is best suited for use on light-burning stonewares (e.g. Plainsman M340). On whiter porcelains and stonewares, it may not fire to a sufficiently transparent glass (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 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). For 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). Add colorants and variegators (with or without opacifier). If colorant additions affect melt fluidity, add or reduce the frit percentage to compensate.

If it cracks during drying, increase the roast clay at the expense of raw clay.

Glazes having a high percentage of Ravenscrag Slip are most often prepared using the traditional method of simply adding water until the preferred viscosity is achieved (the material has inherent properties that produce functional slurries for dipping). Control of drying shrinkage and slurry character is achieved by varying the proportion of roast and raw powder in the recipe. For us, a weight ratio of 80 water to 100 powder (2000 water for 2.5kg of powder) produces a 1.5-specific-gravity slurry that, although fairly runny, gives the right thickness on 1-2 second dip on 1850F bisque-ware (there is some dripping, but coverage is even and it is quick-drying). This recipe actually does not respond to flocculant additions that gel traditional mineral-blend glazes to a thixotropic state.

Roast or calcine your Ravenscrag Slip (or other clays) for much better results

Calcined or roasted clays are indispensable in making many types of glazes, they reduce drying shrinkage (and thus cracking and crawling) compared to those made using raw clay. In a glaze, you can fine-tune a mix of raw and roast clay to achieve a compromise between dry hardness and low shrinkage. This is Ravenscrag Slip, we roast it to 1000F (roasting is adequate to destroy plasticity and produces a smoother powder than calcining at higher temperatures). To make sure the heat penetrates for this size vessel I hold it for 2 hours at 1000F. Calcined koalin is getting harder to find, this same process can be used to make your own from a raw kaolin powder. One thing is worth noting: Weight lost on firing actually means that less of the roasted powder is needed to yield the same amount of material to the glaze melt, it can be anywhere from 5-12% less.

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.

Emulating a speckled reduction fired stoneware in oxidation

The body is Plainsman M340S. Cone 6. Left to right: G1214Z1 calcium matte base glaze with 6% titanium dioxide added. GR6-A Ravenscrag base with 10% zircopax (zircon). G2926B glossy transparent base with 10% zircon (this one produces the white "Kohler Toilet Bowl" appearance we are seeking to better). G2934Y silky magnesia matte base with 10% zircon.

Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe.

This recipe is also code numbered G2917.

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. 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.

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 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 dropped at 300F/hr to 1400F. This really helps to produce a dazzling defect-free surface in the GR6-M glaze. These are, of course, mix-your-own recipes and the pieces were dipped to get even coverage.

Ravencrag Slip vs Alberta Slip floating blues at cone 6 oxidation

Usable, reliable, non-crazing floating blue glazes are difficult to achieve at cone 6. Not these, they pass all the tests yet fire like the original classic G2826R floating blue from David Shaner. 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 (left) 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 (although drop-and-hold PLC6DS schedule usually fires more blue). The Alberta Slip version has zero cobalt so it is less expensive to make (but it does require the C6DHSC slow-cool firing schedule). It produces a deeper color over the L3954F black engobe on these pieces. Both of these produce a wide range of effects with different thicknesses, bodies and firing schedules.

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.

A black engobe transforms the floating blue glaze over it

This is M340 stoneware glaze fired to cone 6 using the C6DHSC schedule. The L3954B engobe fires deep black (it has 10% Mason 6600 black stain). The engobe was applied by pouring and dipping at leather hard stage (inside and partway down the outside). After bisque firing the piece was glazed inside using the base GA6-B Alberta Slip amber base. The outside glaze is Alberta Slip Rutile Blue GA6-C (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers).

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 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 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.

Firing one cone lower improves the glaze result

The mug on the left was fired to cone 6 using the PLC6DS drop-and-hold schedule. The one on the right was fired using the drop-and-hold slow-cool C5DHSC schedule. This is the same body, Plainsman M390. The most visible difference is the outside GR6-M glaze, it is turning out a little different at cone 5, firing a lighter less variegated blue. That being said, it sometimes turns out that way at cone 6 also, so there may be other factors. However, a more important difference is the inside glaze, L3500G. Notice how much more glassy and perfect it is. How is it possible to be so much better at cone 5 than cone 6? The slow-cool is the answer, that firing drops at 150F/hr from 2100F to 1400F.

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.

We have had a report of this leaching. This is credible because of the 10% calcined alumina in this recipe, only a little of that goes into solution in the melt as Al2O3 because it is high refractory (it is added to achieve matteness here). The significant colorant addition is also a concern. We are working to use the titanium/stain mechanism to create a similar appearance (e.g. the way we do with red stain in G3939A).

For mixing instructions please see the master recipe, GR6-A.

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.

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. Red is perhaps the most difficult and 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 (that is why this recipe has 10% added calcium carbonate).

Remember that red is red, it is difficult to acheive in glazes; test this first on different kinds of clay to determine if is is suitable for you. It may be better for you to use a pink or maroon stain instead instead of raw chrome and tin (e.g. the GR6-L recipe).

Try different thicknesses to see which works best for you. This breaks clear around edges to highlight irregularities in the surface. On darker bodies the color of the glaze will be darker. Consider reducing the percentage of colorant to get a lighter color if needed. Slow cooling can matte the surface.

For mixing and firing instructions please see the master recipe, GR6-A.

Ravenscrag Cone 6 white glaze with 10% Mason chrome tin stain

The body is Plainsman M340 and these two glazes are based on the GR6-A recipe (Ravenscrag Slip + 20% frit). The GR6-C creamy white glaze adds 10% Zircopax to opacify it. The pink version, our code number GR6-L, adds Mason 6006 stain instead. The GR6-A base is zinc-free and just hits the 10% minimum CaO recommended to get color development with a chrome tin stain. This recipe also couples a low MgO level (MgO can kill the color in chrome tin stains).

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 glaze. 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!

GR6-E and GR6-L Ravenscrag Pink glazes side-by-side

The GR6-L (the two on the left) employs raw chrome and tin, the GR6-E uses a stain (Mason 6006). The L does not melt quite as well (because of the 10% whiting in the recipe to make sure it develops the chrome-tin color well). The GR6-E is does not need the whiting. The E is more flexible because one can choose different stains to get different colors. Of course the intensity of the color can be adjusted by varying the percentage of colorant. And the L could be made to melt better by increasing the percentage of frit.

This is an excellent alternative to the GR6-E Raspberry recipe. It is simpler, just a stain addition to the standard transparent. It is more reliable. It is easy to adjust the amount of stain to get the exact shade you want. This base recipe is unusual in that it supports the development of chrome tin stains, these are available in many shades from pink to red, consider trying a different another if this color is not right for you.

Because this is transparent (contains no opacifier), different thicknesses have different intensities of color. Because it employs a stain it will be more consistent than one using chrome and tin. Add an opacifier, like titanium, to create a more pastel color with variegation (e.g. 2-4%).

No special firing curve is needed and the surface fires very clean and defect free (with a normal soak at cone 6, or even better, the C6DHSC schedule).

For mixing instructions please see the master recipe, GR6-A.

A breaking glaze highlights incised decoration by its variation in thickness

This is the Ravenscrag slip cone 6 base (GR6-A which is 80 Ravenscrag, 20 Frit 3134) with 10% Mason 6006 stain (our code GR6-L). 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, the GR6-A transparent base has more entrained micro-bubbles than a frit-based glaze, however these enhance the color effect in this case.

Ravenscrag Cone 6 Raspberry glaze on light and dark burning bodies

This is GR6-L Ravenscrag Raspberry glaze fired at cone 6 using the C6DHSC schedule. The body on the left is Plainsman M340. On the right is Coffee Clay. The black burning color of the Coffee Clay is due to a 10% raw umber addition, the manganese in the umber interacts on edges where it is thinner.

Ravenscrag Cone 6 white glaze with 10% Mason chrome tin stain

GR6-E and GR6-L Ravenscrag Pink glazes side-by-side

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

Bad and good glaze application: The difference was the rheology.

This is GR6-L, is the standard GR6-A Ravenscrag Slip cone 6 base recipe + 10% chrome tin stain (the body is Midstone, the inside glaze is G2926B, the firing schedule is C6DHSC). Chrome tin stains are picky about their host glaze, if it does not have a compatible chemistry they fire grey. Obviously, there is a love affair going on here! But the mug on the left has an issue. The glaze on the left has gone on in varying thicknesses and these are producing crystallizations and runs and the incising is not being highlighted. The one on the right is under control. What is the difference? The rheology of the slurry for the bad mug was wrong - the specific gravity was too high (the water content was too low). Even on a quick dip it was building thickness unevenly and way too fast. And there were drips that were so big they had to be shaved off with a knife! After the addition of a lot of water, to take the specific gravity from 1.55 to 1.45 it was watery enough to accept some Epsom salts to make it thixotropic. The difference was amazing, it went on totally smooth without a single drip, producing the result on the right.

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

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. Recently we have begun using a mix of tin and zircopax (3% and 5%) to minimize crawling.

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 deeper more brilliant colors). We recommend stains rather than metal oxides.

For mixing instructions please see the master recipe, GR6-A.

Consider using G2926B clear with added Zircopax if this recipe does not produce the degree of whiteness and homogeneity you need.

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 Cone 6 white glaze with 10% Mason chrome tin stain

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

Iron red glazes are common, but tricky, 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. Only a few people have discovered the proper combination of recipe, iron oxide percentage and firing curve (especially cooling).

Warning: Iron Red recipes will confront you will troublesome or expensive materials (e.g. bone ash, tricalcium phosphate, lithium carbonate, gerstley borate, ulexite). Getting and using these will put your dedication to the test, especially when testing produces initial failures.

Warning: High iron content causes glazes to gel, making application difficult. More water is needed to get fluidity, which causes higher drying shrinkage which leads to cracking during drying. Don't take this warning lightly, working with iron reds is working with "buckets of jelly" for glaze.

Are you sure you want to do this? If yes, then this might be a good starting point.

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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 (thus there is a danger that too much thickness will result in it running down off the ware). 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.

Ravenscrag Plum Red recalculated to use frit instead of ulexite

The original recipe, from which Ravenscrag Plum Red was derived, employed Gerstley Borate to source the boron (the melter). That was a problem because it gels glaze slurries (there is already 15% iron oxide present and that gels also). Thus ulexite was instead to source the boron. That was many year ago but now, ulexite is almost impossible for potters to get. We reformulated again, this time sourcing the boron from a frit (actually two frits). Frit 3249 was used to source some of the MgO needed (this glaze has high MgO levels) since it melts so well and also sources lots of boron. There is now a little less Ravenscrag Slip to suspend the slurry, but that is not a problem, the iron will gel it.

G2896 Ravenscrag Plum Red iron red cone 6 glaze

Original development of the G2896 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.

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

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.

Comparing 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. 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 liner glaze 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.

Cone 6 Ravenscrag Slip Glazes