What causes glazes to react with each other

[Brent Farler]

I have noticed that glazes often tend to either react when layered or are quite stable and do not mix.  If anyone has any insights into the chemical reactions I would like hear.  It seems like there are times you would like to encourage two glazes to interact with each other.

I have heard of some artists who multifire at successively lower temperatures to allow them to layer glazes much like a painter may add colors and details successively. I have found that refiring a piece with a second glaze over the original results in little to no mixing of the glazes - usefull to change a design but why is it that the glazes don't mix?

What if you wanted to encourage this?  Would the secret be in adding flux, adding something between the two glazes?  Would it be possible to change a fired glaze after the fact by adding a layer on top and refiring (for instance I have been able to reduce crazing by refiring with a pure silica wash, crazing is reduced although the surface becomes rough and matted).

About all I know about this is zinc in a glaze can change colors in some stains and some high iron glazes can produce some nice effects as the run down the side of a pot into other color glazes.

[Tom Anderson]

Brent:

Flux basics- calcium, magnesium and strontium are considered low flow fluxes: meaning they have a tendency not to run. Sodium, potassium, and lithium are high flow: meaning they have a tendency to run.

The base glaze should be high in calcium, magnesium and strontium; so it will be stable. The top layer should be sodium, potassium, or lithium so that it will run.

Want more run- increase the lithium.

Want even more run: add some frit- the molten lava river.

Want to freeze the glaze in place?  add lower percentages of alumina hydrate.

Some basic principles, without paragraphs of chemistry.

Tom

[Brent Farler]

rather than the flow/run what about the mixing of two glazes.  Would this imply if both are high in calcium, magnesium or strontium they won't mix and if high in sodium, potassium or lithium they would 'mix' i.e. tend towards more mixing of the colorants.

Another way to look at it is what makes one pair of glazes 'coat' one another versus what makes another pair of glazes mix and react with each other?

 

[George Lewter]

Glazes layered or in contact with each other can exhibit wild behavior due to eutectic melting. Material components that have higher melting points by themselves can have drastically reduced melting points when mixed together, or in contact with each other. Quite often two very stable glazes, individually, will run off a pot when layered together.

[Brent Farler]

sounds like, experiment and test, experiment and test, experiment and test, repeat.

I am guessing the amount of soak at the target cone can have an effect as well.  so fluidity + time at temperature

George Lewter said:

Glazes layered or in contact with each other can exhibit wild behavior due to eutectic melting. Material components that have higher melting points by themselves can have drastically reduced melting points when mixed together, or in contact with each other. Quite often two very stable glazes, individually, will run off a pot when layered together.