silicon carbide in red reduction glazes for an oxidation environment?

Jon Britt talks about adding small amounts of silicon carbide 600 mesh or smaller to any red reduction glazes to make them turn out red in oxidation.

Does anyone have any experience doing this?

What red reduction glazes are a good candidate?

How much silicon carbide do you add?

Any additional advice on this procedure would be greatly appreciated.

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addendum to the above post.

I do know what putting too much silicon carbide causes. About six months ago I had some leftover red reduction cone 6 glazes, just a bit of this and a bit of that. Just as a shot in the dark experiment, I added a couple of pinches of silicon carbide 600  and hosed them on some test tiles. Obviously a pinch or two was far too much. In cone 6 oxidation, I got the ugliest, most pitted glazes I've ever seen, with not a hint of red in them.

Silica carbide, SiC + 2 O2 oxidizes in the kiln to silica SiO2 and carbon dioxide CO2.  Both the silicon and the carbon create a localized reduction, but the carbon dioxide creates a lot of glaze foaming as it off-gasses.

Each gram of Silica carbide becomes 1.5 grams of silica when fired.  So you want to replace silica with silicon carbide at that ratio.

Atomic Weight

40.096 Silica Carbide
28.085 Silicon  70%
12.011 Carbon 30%
28.085 Silicon
16.000 Oxygen
16.000 Oxygen
60.085 Silica
60.085 / 40.096 = 1.499

I would assume that you would not want to replace very much silica with silicon carbide. Tom Turner (article attached) talks about 1/3 to 1/2% silicon carbide but he's firing at ^9. I guess you'd have to add a bit more frit to his ^9 oxblood to fire it at ^6.

Do you think there need be any other changes necessary to move this glaze to cone 6.


Exactly correct. 

Unassisted by an early melting frit, nepheline syenite, lead or boron material like gerstley borate the fluxes in this Cone 9 glaze do not melt and become fully active until Cone 8.  These fluxes are Custer Feldspar, Calcium Carbonate and Barium Carbonate.  Tom Turner uses 5% Ferro Frit 3110 at Cone 9, but at Cone 6 we need to replace much more of the Custer Feldspar with something which melts at a much lower temperature.  Barium still won't be a very active flux, but like zinc it is usually important to create color change in glaze so may be required in his recipe.

The question you need to answer is whether added boron will adversely affect the outcome of this chemically reduced copper red glaze.  Boron frits are compatible with atmospheric reduction copper reds, so that's hopeful. The  Cone 06 Chun Turquoise recipe below is a baby blue in oxidation, but a copper red in a reduction atmosphere.

When entering Tom Turner's recipe I increased the level of Tin Oxide from 1% to 1.5% to maintain the 3:1 ratio of Tin to Copper required for a copper red.  At temperatures higher than Cone 6 there is a loss of tin and copper due to evaporation, which is made worse by his one hour hold at Cone 9. His 2:1 ratio of tin to copper suggests that he is losing copper faster than his tin.

Tom Turner refires his copper red pieces to Cone 019 (1,252 F / 678 C), a Strike Firing, to allow the molecules to reorient themselves enhancing the red color.  This will be more successful than a slow-cool in this range during the initial firing as there will be better temperature control.

Notice that Magnesium Carbonate is named Light Magnesium Carbonate in Digitalfire to differentiate it from another form of magnesium carbonate more common in the ceramic industry - Magnesite. 

Magnesite is easier to deal with being a denser material which is a coarser material held together with an organic binder.

So I did the strike on my bowl that is in the other post(also a different recipe) & there was no change.  I have seen some copper red recipes with RIO added to them & Tom Coleman even suggests putting RIO over the top of the glaze.  I think if you struck that the iron would definitely help with the red color.  Tom Turner suggests there is a certain temp for tuning the clear back to red.  He says they do it in glass blowing where they will have the clear glass & when they heat it to a certain point it becomes red.  Anybody do glass blowing that can figure that one out?  jhp

actually, I do have an o blood copper red cone 6 glaze formula. I even some already mixed from earlier efforts with my gas oxidation kiln.

Oxblood, Cone 6 Reduction   Gloss  semi-opaque glaze.

Talc           3.3

Whiting   14.3

Frit 3134 13.3

Kona F-4 46.2

EPK          6.4

Flint        16.5

Tin Oxide 2.2

Zinc Oxide 4.3

Copper Oxide 1

V-gum T     1

maybe I'll give this stuff a shot.

Simply lower the flint from 16.5 to 15.75, add 0.5 silicon carbide and see what happens.

Tom Turner suggests a thick layer of glaze protects the red color from re-oxidation better than a thin layer does.

Following Tom Turner's thinking, I'll  put a layer of clear over the top of some of the test tiles.

On my next test, I am going to bisque some copper on , then glaze with the normal glaze and then spray some RIO over the top.  I am half tempted to also put some RIO in the base glaze.  I have noticed some other Copper Red recipes with RIO in them already.  Test, test, test.......   jhp

Did you ever try?

Jeff Poulter said:

On my next test, I am going to bisque some copper on , then glaze with the normal glaze and then spray some RIO over the top.  I am half tempted to also put some RIO in the base glaze.  I have noticed some other Copper Red recipes with RIO in them already.  Test, test, test.......   jhp

There's another possible way to help trap the copper

Make glaze "A"(goes on first) with the copper and silicon carbide then Glaze "B"(same base glaze) except this glaze has the tin and covers the "A" and traps the copper as it's reduced by the Silicon Carbide and migrates into the Tin coated  glaze.

This method has been used by several potters firing regular cone 10 reduction copper reds and might work well for this.


So here is the pot that I bisqued some copper/SC onto.  I was worried that when it came out of the bisque, that it would have turned pink or red, but it was still green when it came out, so I sprayed the copper red glaze w/SC added & fired to ^5 and came out with this.  Not very pretty...  There were some issues with spraying the glaze with the previous glaze bisqued on & it was really bumpy when it came out of the bisque, so it was hard to tell how thick I was spraying the second coat on. I haven't had much time to do pottery these last few weeks.  In between weeding & daylilies & harvesting cherries, my time has been elsewhere.  Besides, it's just too @%#* hot to fire a kiln anyways!   jhp

Brent Farler said:

Did you ever try?

Jeff Poulter said:

On my next test, I am going to bisque some copper on , then glaze with the normal glaze and then spray some RIO over the top.  I am half tempted to also put some RIO in the base glaze.  I have noticed some other Copper Red recipes with RIO in them already.  Test, test, test.......   jhp

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