Earl Krueger on thu 9 sep 04
sulphATES, sulphITES, sulphIDES
They all contain the element sulphur.
What's the difference?
Sulphates and sulphites are generally readily, or
at least somewhat, soluble in water. Barium
sulphate is an exception to this rule as it is highly
INsoluble in water, which is why barium is sometimes
added to clay to bind up the sulphates which would
otherwise produce a scumming or blooming on the pot.
Sulphates are what one would use for obtaining
"watercolor" effects.
Sulphites are relatively unstable and in solution are
readily oxidized to sulphate. They are used to preserve
certain food items as they preferentially gobble up any
oxidizing substances which would cause the food to
spoil quickly. You probably won't run into them very
often, except in your bread and wine.
Sulphides, on the other hand, are generally insoluble
in water. Especially the metal sulphides such as
copper, iron, cobalt, etc. Metal sulphides are quite
common in nature as the water in the environment
doesn't dissolve them. Sulphides are more like our
normal glaze ingredients in that they don't dissolve in
the water in the glaze and sit on the surface as opposed
to being absorbed by the bisque.
The sulphate ion is pretty strongly bound together so
heating it to high temperatures does not break it down.
Instead it vaporizes and then will combine with water
vapor to form sulphuric acid which will corrode metals.
This is the most likely cause of metal hardware on your
kiln starting to rust. Of course, breathing the fumes into
your lungs will result in the same formation of sulphuric
acid. The lungs are not a good place for that so make
sure you vent your kiln well to the outside. Sulphates
are pretty ubiquitous and occur in significant quantities
in coal, oil and even your clay. The formation of sulphuric
acid from burning coal is the primary cause of "acid rain"
downwind from power plants and is a primary component
in smog.
Sulphides, when heated to high temperature in a reducing
atmosphere will form hydrogen sulphide gas, which has
already been pointed out as having the smell of rotten eggs
and being poisonous. (Yes, rotten eggs do give off hydrogen
sulphide.) In an oxidizing atmosphere the hydrogen sulphide
will be oxidized to the sulphate, in which case see above.
My concern with using any of the above in the pottery would
not be with the gaseous products produced by the kiln, as
long as they were vented to the outside and diluted with
sufficient quantities of fresh air. I would, however be cautious
in use of the products before they reach the kiln. Some are
much more dangerous than others and if you do use them
you should read and understand the Material Safety Data
Sheets for each individual chemical.
Hope this helps to clarify somewhat the role of sulphur in
what we do.
Earl K...
Bothell, WA, USA
BTW: An excellent method for making hydrogen sulphide
is to heat a mixture of paraffin and sulphur. The gas can
be bubbled through a solution of a heavy metal ion to
produce the insoluble sulphide of that metal. I AM NOT
RECOMMENDING THIS, I AM JUST RELATING A PROCESS
THAT HAS BEEN USED IN THE PAST IN CHEMISTRY LABS
FOR YOUR EDIFICATION.
Ivor and Olive Lewis on sun 12 sep 04
Dear Earl,
Where do you get this from <temperature in a reducing atmosphere will form hydrogen sulphide gas,
>>
Cu2S+2CO => ????? H2S ! ! !
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ron Roy on sun 12 sep 04
Hi Earl,
Just a little clarification in case anyone is wondering. In the paragraph
below when you say "which is why Barium is added to clay" you mean Barium
carbonate of course.
The implication here is that Barium Carbonate (which is slightly soluble)
will react with soluble salts containing sulphur in clay - turning them
into insoluble sulphates - and so prevent them from being carried to the
surface with evaporating water - to stop scumming.
Is that correct?
I have a question though - how come it is only Barium Carb that does the
job - as opposed to Strontium or Calcium carb?
RR
>Sulphates and sulphites are generally readily, or
>at least somewhat, soluble in water. Barium
>sulphate is an exception to this rule as it is highly
>INsoluble in water, which is why barium is sometimes
>added to clay to bind up the sulphates which would
>otherwise produce a scumming or blooming on the pot.
>Sulphates are what one would use for obtaining
>"watercolor" effects.
>
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Earl Krueger on mon 13 sep 04
On Sep 12, 2004, at 1:20 AM, Ivor and Olive Lewis wrote:
> Dear Earl,
> Where do you get this from <> temperature in a reducing atmosphere will form hydrogen sulphide gas,
>
> Cu2S+2CO => ????? H2S ! ! !
Ivor,
I am thinking more like:
CH4 + O2 => CO + H2O + 2H+
Cu2S + 2H+ => 2Cu + H2S
Earl K...
Bothell, WA, USA
Ivor and Olive Lewis on tue 14 sep 04
Dear Earl Kruger,
> CH4 + O2 => CO + H2O + 2H+
> Cu2S + 2H+ => 2Cu + H2S
You may well be correct if the oxidation of methane is a two step
process and Hydrogen Ions can exist long enough to diffuse into a
glaze.
Do the Thermodynamic numbers add up to give that proposal credence?
What does the Chemical Dictionary of Inorganic Compounds tell us?
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
.
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