search  current discussion  categories  glazes - chemistry 

si:al ratio

updated fri 31 may 96


Linda Arbuckle on sun 26 may 96

Thanks to everyone for the info and references. In response to Ron Roy's
comments below... any ideas what the boron does to skew the ratio? I'm
looking at clear glazes for about 03 that are just a bit less shiny than
most and fairly stable. Most have boron from Frit or gerstley. And of
course, glossy majolica glazes have boron from frit in many cases. I'll
check on the list of reference Bob Kavanah provided, too, and see if any
light bulbs go on. Just curious, as I've not encounted Si:Al ratios as a
factor. Very interesting that the surface quality can be maintained over
re-calculation for other cones if the Si: Al ratio remain consistent.
Computer glaze calc really makes this kind of looking at things more accessible!

>Hi Linda,
>The simple explanation as I understand it is: If the ratio of SiO2 to Al2O3
>is below 5 then you will probably have a matt glaze - unless you have B2O3
>as well. Between 5 and 10 you probably have different degrees of semi gloss
>and semi opake. Above 10 SiO2 to 1 Al2O3 the chances are you have a
>transparent gloss glaze. There are exceptions of course. The trick to
>keeping the character of a glaze while lowering or raising the maturing
>temperature is to keep the ratio the same. This works fairly well with
>balanced glazes but not as well with glazes outside recommended limits for
>durable glazes.
Linda Arbuckle E-mail:
Asst. Prof.
Univeristy of Florida
Department of Art 302 FAC
Gainesville, FL 32611 Ceramics: (352) 392-0228

Tony Hermsen on mon 27 may 96


Your question as to what boron does to skew the Si:Al ratio is intriguing. You
are really making me scratch my head here, trying to remember back to the old
glaze chemistry classes at U of C (Elizabeth Mould - great instructor, great
lady!). Notes taken from the 1973 edition of Ceramic Glazes by Cullen W.
Parmelee allude to this question but I don't know if they will help you in a
technical sense.

- Boron forms easily fusible silicate fusions
- lowers thermal expansion in amounts upto 12% but further addition reverses
this effect
- is an acid and usually forms a part of the silica network.

There is also the suggestion that B2O3 be put in the R2O3 column with alumina
when calculating a glaze formula because the sesquioxides (multiple oxides) may
function as either acids or bases. There is a further suggestion that the action
of B2O3 in a glaze depends upon the ratio of bases to silica existing in the
glaze before the addition of the B2O3. If this ratio is NOT greater than 1:2 the
glaze will be clear and will not craze. If the ratio is greater then opalescence
and crazing will occur when the B2O3 is added.

This doesn't really give you any hard numbers to plug into your glaze calc
software but it should give some insight into how to keep your glaze stable
within the temperature range and surface qualities that you want. Whew!

It's been a slice,

Ron Roy on mon 27 may 96

Hi Linda - again

The B2O3 is a glass maker - like silica - many matt glazes rely on
recrystalization on cooling and if there is too much glass maker (SiO2 and
B2O3) then the glaze cannot recrystalize during our cooling cycles. If we
would cool slower we could get more of our glazes to recrystalize.

Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849