search  current discussion  categories  kilns & firing - salt & soda 

: re: : loacalized salt glazing

updated mon 7 apr 03


iandol on sat 5 apr 03

Dear Friends,

John Britt contacted me direct and this is the reply I provided

"Dear John,
Yes, that is true. You can test it by putting Salt into a container =
which will withstand the heat and not be chemically reactive, ie Steel =
or Iron. Weigh out a fixed quantity, say 100 grams and heat it up to =
1250 for half an hour, then reweigh and find the difference. To get =
Sodium Chloride to evaporate strongly, that is Boil, at 1250 Celsius you =
have to drop the pressure inside the kiln from 101 kN/M^2 down to =

So your salt disappeared at about 850 Celsius. Firstly, were your dishes =
or tiles raw clay, bisque ceramic or vitrified ceramic? If raw clay, I =
would have expected some sort of reaction but it may just have soaked =
into the clay. Same for a bisque container. At Cone 10 it would melt and =
react with the clay, and as I said, it would evaporate at about the same =
rate as pure water held at 80 deg C.

All the evidence, both from my own experimentation and from reading, =
indicates that Salt, NaCl, reacts directly with the vitreous phase in a =
clay, once it has formed. This is the only way to explain how salt kiln =
effluent contains almost as much Potassium Chloride as Sodium Chloride, =
as well as small proportions of Iron Chloride, Aluminium Chloride and =
Silicon Chloride. I have said this in public lectures at
National Ceramic Conferences and it is on record. So far, no one has =
come up with a counter argument. Thermodynamic modelling shows that the =
popular chemistry is not possible at our working temperatures.=20
An interesting topic and one ripe for further research.
Best Regards,

Ivor Lewis. Redhill, South Australia