Jump to content

Bob Coyle

Member Since 06 Jun 2013
Offline Last Active Feb 20 2017 02:13 PM

Posts I've Made

In Topic: New Studio Set Up In Montana

21 January 2017 - 11:52 AM

Looks like plenty of room.... for now anyway :)

In Topic: Silica In Glaze And Body To Ensure Good Fit?

25 December 2016 - 12:05 PM

Not much more needed from what John and glaze nerd said... except YES!  you really need to pre test every change in clay or glaze before doing a full batch.  Fire at as close as you can come to the conditions you will fire the batch. Firing cycle can also make a big difference in the final appearance of a glaze. many of us here have learned the hard way that even going from batch to batch of a commercial clay or glaze can produce problems, and this is especially true if you mix your own glazes and change raw materials.

In Topic: Silica In Glaze And Body To Ensure Good Fit?

22 December 2016 - 09:04 PM



No single component determines COE. it is a property of all of the bases present. If you know the COE of the clay body ( commercial bodies usually have this info) then all you have to do (theoretically) is match the COE of the glaze with the clay. There are many software programs that help you do that. From my experience they may not work as well as expected. They are good for a start, but they depend on a kind of a linear additive approach, which is not the way glaze works.  Adding or taking away SiO2 might get you to a better fit, but could alter the cone you need to fire to. The ramp you fire also effects  the final crystalline configuration of the glaze.  I have made glazes that calculated out with a close COE to the clay body, and well within major limits formulas and I still got some crazing.

In Topic: Silicone Caulking Cure Time?

01 December 2016 - 06:01 PM

It mostly loses solvent  in 24 hrs, but depending on the type, and how much air circulation it gets. it could take days to really cure out enough to not allow a bearing to slide off center.

In Topic: Help Diagnosing Glaze Or Kiln Problem?

20 November 2016 - 02:48 PM

My guess is that there is a reaction taking place between the two glazes. Looks like the bottom may be a copper based glaze and the reaction may be metallic copper being formed because of some reduction condition taking place... of copper oxide being formed because the top glaze is more basic than the bottom.


This is just a guess. Could be caused by just a contaminant in the glaze.  If you are firing electric, then your atmosphere is probably oxidizing.


Run a test tile with both of the glazes mixed together and see if the whole thing turns black.


PS... I hope those bowels are not being used for food.