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Celia UK

Member Since 01 Jul 2012
Offline Last Active May 02 2015 11:46 AM
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Topics I've Started

Bubbles In Glaze

27 April 2015 - 01:22 PM

I dipped my 'sky' plate (white stoneware - Valentine's ES5), in a Potterycrafts, opaque, leaded, mid-fire glaze (that I see now has a recommended firing range to 1100oC) then sprayed areas using an airbrush, with a dilute mixture of copper carbonate and cobalt carbonate. Fired it to 1200oC (approx cone 5) - oops! With a 10 minute soak. This is the first time I've fired as high as this in my small electric kiln.
The areas with the oxide over are almost ok, but the white areas have tiny bubbles - almost granular in appearance. Photos attached.

So....I've realised the White opaque was fired 100oC above its recommended range, but is that likely to be the problem or is it something else? And why is this only on the areas that weren't sprayed over with the oxide wash? Any ideas?

Attached File  image.jpg   6.3KB   1 downloadsAttached File  image.jpg   5.38KB   1 downloads

Glaze Settling Out - What To Add?

25 April 2015 - 06:04 AM

So, my next problem - in need of your help.
I have tried to get my head round the issue of flocculation and Deflocculation several times but still can't quite get it! Right now, the theory isn't exactly what I need, it's urgent, practical advice.

I've just mixed up 2 litres of a bought, lead free, transparent stoneware glaze (fires 1180oC - 1250oC).
1 kg : 800mls water. It immediately settles out and will need to be stirred every few minutes to keep everything mixed. What is it I need to add to keep the ingredients in suspension? I have Epson Salts, Dispex, CMC gum and some Bentonite. So, firstly, what to add to this batch and then, if mixing another batch, should I add something at the dry materials stage?

Thanks in anticipation!

Do I Need To Re-Bisque?

23 April 2015 - 06:18 AM

I did a bisque fire last night in my 40litre electric kiln with sitter and digital controller. Digital controller set to finish at 1000 oC. 03 minibar in kiln sitter. I grabbed a cone pack from my shelf (don't often use these!) and placed it in the Middle shelf.

This morning, kiln off and cooling. On opening the lid, the bowls all look fine - 2 different types of stoneware, but none of the cones shows any sign of bending.

When I checked, I had used 03 04 and 05 cones which I'd prepared for an earthenware glaze firing. So my dilemma now - Is the work bisqued enough? Do I need to re-bisque to a higher temperature on the controller? Why didn't at least the 05 cone bend a little if the kiln reached 1000 oC as programmed? As I've never use cones in a bisque firing before and only a couple of times in glaze firings, I can't compare this to any other firing, perhaps all my previous bisque firings have been the same without me realising???

I'd welcome anyone's thoughts on this.

Glaze Stoneware Or Earthenware?

21 July 2014 - 03:49 AM

In amongst a batch of ceramic materials & equipment I bought from a potter who was going travelling are two buckets of mixed, liquid glaze. One is a 'matt turquoise blue' the other 'black'. This was a hurried purchase as the young lady was leaving the country the following day and I'd already taken up an hour of her time sorting through her garage! She told me that the black was very runny, and to be careful, and it is labelled as such. I don't know if these are earthenware or stoneware glazes, they were her own mixes. A couple of questions. ( I do plan to test, test, test. ) I currently use a smooth white earthenware, fire in a small electric kiln., glazes usually to 1060/1080oC

1 - what will happen if they are stoneware glazes and I use them on white earthenware firing to earthenware temperatures.

2 - as I already know the black is runny, other than only using it on a rim and expecting it to run, is there anything I can add (without having any idea of the original recipe) that will make it less runny?

3 - what would happen if Earthenware is fired above the recommended temperature? E.g. If I bisque an earthenware piece to 1000oC then glaze to a stoneware temperature?

Many thanks, in anticipation.

Treating Rust On Kiln Lid

18 May 2014 - 03:43 PM

I have a small amount of rust on the outer edge of the vent hole in my kiln lid. Some tiny flakes fell into the kiln & on to a white pot on the top shelf in the last firing - spoiling the pot. Does anyone know if Hammerite would be ok for treating this? Not sure if available outside the UK, but it can be painted directly on to rust once flakiness is brushed off. I've used it on the outer casing of my small electric kiln, but not sure if it can withstand the heat coming up through the vent hole. Does anyone know how hot this part is likely to get during firing? Is there a specialist product for this?