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vapourisation of salt

updated sun 29 jun 03


iandol on sat 28 jun 03

Dear John Britt,=20

As I understand things, all substances, solid or liquid, at all =
temperatures exert what is called "Vapour Pressure". Everything has a =
tendency to allow molecules or atoms to break away from the material =
surface. This allows the to exert pressure which increases as =
temperature rises. Change from solid to fluid and it increases even =
more. As temperature rises so does the rate of evaporation. There is a =
temperature at which the Vapour pressure is equal to or greater than the =
pressure outside the liquid, in common terms, atmospheric pressure. When =
the temperature pushes the vapour pressure to the atmospheric pressure =
the substance boils.

How does this relate to Salt, Sodium Chloride? While it is solid there =
is almost no vapour coming from the surface. You cannot smell it. But =
throw some on a fire and you will soon smell the vapour. Sodium Chloride =
melts at 801 deg C and has a boiling point of 1413 deg C at atmospheric =
pressure. So at your temperature of 1500 deg F, just above the melting =
point, the vapour pressure is very low. At about 900 deg C the vapour =
pressure is two tenths of one percent of the atmospheric pressure, =
meaning that if melted in a good vacuum it would start to boil away. I =
rate that as being about as fast as water evaporating on a frosty =

So what happens when we throw coarse salt into a kiln at 815 deg Celsius =
or higher. We cause instant thermal shock. Lots of crackling, spitting =
and stuff flying everywhere. This is called "Decrepitation", causes a =
dense white or grey cloud, a particulate fog. Small fragments and =
droplets settle on the pot, melt and start to react chemically with some =
of the surfaces they touch.=20

I say and you may quote me by saying "Under the influence of Heat Salt =
decrepitates, melts if the temperature is high enough and reacts with =
some of the compounds in clay which it touches"=20

Enjoy the weekend.

Best regards,

Ivor Lewis. Redhill, South Australia