: JBaymore - Viewing Profile

oldlady, on 19 May 2013 - 02:09 PM, said:

JBaymore, on 19 May 2013 - 12:03 PM, said:

That (in red) is the key phrase in there..... any of the recipes that don't have those materials listed are the ones I am talking about.

Does that help?

best,

...................john

Min, on 19 May 2013 - 12:38 PM, said:

The iron in the reduced FeO state is an active flux on silica (SiO2), for sure, and it begins its action at quite low temperatures. So yes, it will add some fluxing of the available silica and keep that silica from potentialy converting to the chrstobolite phase form. Unfortunately, iron fluxed silica glass is very brittle... so the body's glassy phase in that case is not a very optimal one to use to "glue things together".

Plus the iron/silica melt has a low melting temperature and also a low volitilization temperature... possibly accounting for some additional bloating issues.

A particulary bad combo is a somewhat high iron body with inadequate sourcing of other fluxes. You end up with a brittle body that is getting "loaded" upon uneven cooling (or reheating in food service) by excessive COE factor from the cristobolite... and BANG..... dunting.

best,

.......................john

Body formulations with nothing supplying additional sources of fluxing materials to cause more of the SiO₂ in the clay components (both as trace admixtures and from the kaolin/metakolin silica ejection) to convert to a glassy phase. Depending on the firing range, if you don't see an addition of feldspar, neph. sy., frit, sometimes spodumene, or the like in the formulation..... there there is the possibility of the issues I mentioned.

best,


....................john

Be careful taking bodies with little devloped glassy phase (becasue of no spar, frit, or neph. sy. content .... yeah...spodumene counts here.....) up to higher temperatures above the 1100 C / 2012 F range........ particularily with longer firing cycles. They are prone to having the non-melted silica on the body converted to the cristabolite form of SiO₂ ..... which has a nasty huge impact on the COE of the body. Can cause dunting issues on kiln cooling...and also cracking in the domestic oven temperature range. The amount of cristobolite developed is linerally related to the time spent above 1100 / 2012.. ..... twice asl long... twice the conversion level.

Some of these above recipes would tend to have that issue, I think. Cone 6 is typically reached above that temperature for most typical rates of climb.

best,

.......................john

Welcome Anne. Glad you found us.

best,

.......................john