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soda ash - na2co3 or na2co3.10h2o

updated mon 24 mar 03


Rod on fri 21 mar 03

Hello Folks,

Long time no post! I have an perplexing problem here. When I first laid my
eyes on a really nice Shino glaze a few years ago I fell love with it\them.
I did some studying tries many different glazes and then finally developed
my own to get the look I wanted.

Now the problem is that I had quite a whack of Soda Ash and I used that for
10 years. A ran out last year right before all the big shoes here in
Vancouver. I went out and purchased a new bag from our local supplier. When
I mixed my recipe with this Soda Ash\Sodium Carbonate all things suddenly
went to hell and I mean hell in a extremely hellish way.

My beautiful shino turned into pots covered in a hideous green charred black
looking glaze. Obviously there was way WAY to much flux. So I promptly gave
my assistant hell for not paying close attention during mixing and asked
them to mix another batch. The next batch came out the same. What the
^%!^@*#* is going on.

I never have had the Soda Ash I had tested yet. I thought I would try and
save some money by bending the ear of some of the guru's here.

I'm assuming that the new Sodium Carbonate is Anhydrous Soda Ash or simply
Na2CO3. Whereas I have strong suspicion that my old Soda Ash is Na2CO3.10H2O
or Hydrous Soda Ash.

The Hydrous Soda ash goes into solution allot faster than the Anhydrous
which explains why I never had any problems with mix and glaze scenarios.
Whereas with the Na2CO3 on one of the incidents opening the kiln last year
(after 2 remixes) I looked in and went Ahhh this is okay now. This was a
load that was glazed right after the glaze was mixed. Some of you probably
know what happened 5 days later when I glazed another batch. You are right
when you day there was a steady stream of &%@#%$# and other vitriolic
comments as well as allot of "what the .... is going on here". The Green
Glass was back just like the damn cat. So obviously the Na2CO3.10H2O goes
into solution instantly whereas the Na2CO3 really needs to sit for a few
days before you glaze to ensure that the soda has gone into solution. Or sp
it seems to me. I'm not a chemist but this has perplexed me enough to enroll
into an inorganic chemistry course ;)

Anyways to make a long story even longer....

NA2CO3.10H20 has a molecular weight of 286
NA2CO3 " " 106

Based on this it would seem that the old Soda Ash was 63% water.

So if the recipe once called for 12% it would now call for 4.44% of the
new soda ash to get the same results? Does this make sense? It there a
way of looking at it? Or am I just addled and tired ?

One last thing are there other combinations of Soda Ash our there besides
2 mentioned above? Will Soda Ash pick moisture up and or weaken over time?

Thanks in advance,

Roger Graham on sun 23 mar 03

Rod: Recipes involving soda ash confuse me too. There are three stable forms
of the stuff. Na2CO3 (anhydrous sodium carbonate... it's a white powder),
then Na2CO3.H2O (sodium carbonate monohydrate... it's a white powder too).
And finally Na2CO3.10H2O (sodium carbonate decahydrate... usually clear
glassy crystals, often in big transparent lumps).

Here's the relevant paragraph from Sherwood Taylor's ancient textbook "
Inorganic and Theoretical Chemistry":

"Anhydrous sodium carbonate is a white solid, which melts at a red heat (850
degrees C). It combines with water, becoming hot and forming the
monohydrate. When crystallized from water in the ordinary way, below 32
degrees C, the decahydrate Na2CO3.10H2O (soda crystals, washing soda) is
formed in large transparent crystals. This salt is efflorescent, gradually
forming the monohydrate when exposed to air. Long exposure to air produces
some bicarbonate. When heated it melts at 30 degrees C, and on further
heating deposits the monohydrate."

So here are some small tests to try:

1. Add a few drops of water to a spoonful of the white powdery stuff. Does
it get warm, or hot? If so, it's likely to be the anhydrous form, giving off
heat as it combines with the water. Does it just stay cool? If so it's
likely to be the monhydrate.

2. Heat a spoonful of the white powdery stuff over a low flame, or over a
stove hotplate. Does it melt before your eyes into a clear syrupy liquid? In
this unlikely event, it was the decahydrate. If it just sits there and
doesn't seem to change, it's either the anhydrous form or the monohydrate.

3. If you have reason to think it's either the anhydrous form or the
monohydrate, weigh out a small amount (say 100 grams) into a jam tin or some
such container. Then heat it over a flame, close to red hot if you can. Or
sit it on a stove hotplate and let it get as hot as possible, for say 5
minutes. When it's cool, weigh it again. If there's negligible change in
weight, you had the anhydrous stuff. A loss of about 14 percent in weight is
what you'd expect for the monhydrate, driving off the single molecule of
water from Na2CO3.H2O Some weight loss, less than 14 percent, would suggest
the stuff is a mixture of the anhydrous form and the monohydrate, in which
case you would now have made it all into the anhydrous form anyway.

Roger Graham, near Gerringong, Australia