[Clayart] Sugar reduction of glazes

L TURNER magnolia.mud.list at gmail.com
Mon May 23 22:17:18 UTC 2022


The Hanford experiments were clear on the use of sugar as a reduction
ingredient in the containers where the experiments were conducted with
temperatures of between ~800 and ~1200 C  (cone 015 to 6).

My experience with cooking at home and over a campfire: overheating of
sugar during cooking produces first a dark syrup followed by a strong
"black stuff" stuck wherever it touched something; seldom did the sugar
burn clean in a skillet.

Properties of sucrose:
The overall data from two reliable databases indicates that sucrose
decomposes to water and carbon; the carbon is solid material.  Sucrose as a
"dust" (suspended in air) will, in adequate air, rapidly be converted to
carbon dioxide and water vapor plus a significant increase in volume (aka
explosion) as demonstrated in several sugar silo explosions; compacted as a
solid material the reaction will first decompose to water vapor and solid
carbon; however in a compacted material the temperature raise will be
slowed similiarly to the reactions of normal glaze reagents that produce
vapor as part of the conversion from solids to a viscous melt.

I have previously used sugar as an ingredient in a clear glaze based on
Portland cement with no problems; the glaze was used over half a semester
as the base for a clear, a cobalt blue, and iron glazes with the same base
glaze ; however the firing was in an gas fired cone 10 reduction kiln (was
a very good glaze by the way) so that data does not provide any information
on reduction (since the kiln was already in reduction); the experiment did
say that sugar does not screw up a glaze or make a mess.

While on the topic of reduction;  Hank Murrow, Mel, and others have made an
important observation that oxygen does not significantly migrates back into
the glaze; otherwise the reduced elements below the surface would have been
reoxidized.

Since not even "the Shadow knows" I will try to schedule a test some time
this fall. If you are interested, let me know.

LT


On Thu, May 19, 2022 at 6:08 AM Robert Harris <robertgharris at gmail.com>
wrote:

> When talking of local reduction, the most important part is the sealover
> temperature of the glaze. Any reduction needs to continue until this
> temperature, otherwise reoxidation will occur.
> A few points of consideration -
>
> In normal reduction firing carbon monoxide is the reducing agent. There is
> no reason to believe that this is not also the case with localised
> reduction from SiC. It is likely that thermal oxidation of SiC produces
> carbon monoxide which then reacts with copper and iron oxides etc.
>
> My best ability of recall says that under normal conditions (atmospheric
> pressure, low water content), thermal oxidation of SiC starts to occur
> around 1000C (1832F) and is slow - mostly because it is constrained by the
> crystal structure. Carbon oxidises first on the planes of the crystal. If
> you want to read more about thermal oxidation of SiC the most extensive
> research has been done under the auspices of the Nuclear Regulatory
> Commission (NRC) as SiC has been evaluated for use in cladding for nuclear
> reactors. This temperature profile takes place around or after sealover
> temperature of most stoneware glazes.
>
> Thermal oxidation of sugars to carbon monoxide probably starts around 400C
> (752F). In a normal electric kiln it will probably all have burned out
> before sealover occurs and therefore allows re-oxidation of copper and iron
> oxides.
>
> A final bit of personal evidence. I have tried to use powdered graphite. As
> far as I can tell it all burned out, but I have successfully used Silicon
> Carbide with the same recipe. Aluminium powder also works. Years ago I also
> bought some powdered silicon, I have never tried it, as soon after I bought
> it I had access to a fuel fired kiln.
>
> If you can find an organic compound that does not start to oxidise until
> 700 or 800C (just as a guess) then you might have some more success.
>
> Robert
>
> On Wed, 18 May 2022 at 19:36, L TURNER <magnolia.mud.list at gmail.com>
> wrote:
>
> > Has anyone added sugar (or molasses) to a glaze to get  "local reduction"
> > in an electric kiln?
> >
> > the question derives from reading the PNNL-14063 research report on
> Hanford
> > waste vitrification.  Sugar was added to the test batch of the materials
> to
> > reduce some of the oxides to either metals or lower oxidation states.
> > The report does not provide a lot of details, but enough to at least
> think
> > about what glazes would the "local reduction" be useful.  Creating
> reduced
> > copper or iron oxides in areas might have some artistic value. Yea,
> copper
> > reds with Silicon carbide does work (sometimes), but a spoon of molasses
> is
> > easier to get, etc., etc.
> > Think about it.  and let's talk about it.
> >
> > LT
> >
> > PS one can find some interesting insights on silicate melts (aka glazes)
> > outside of the usual box.
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