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Celia UK

Member Since 01 Jul 2012
Offline Last Active Yesterday, 03:56 PM
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Topics I've Started

Glaze Testing

15 May 2015 - 07:37 AM

Delving into the realms of mixing my own glazes I found a Greg Daly recipe that covered a range of firing temperatures and was the sort of turquoise I love. On a white stoneware test tile (bisqued to 1000oC), glazed to 1074oC it was a lovely bright matt turquoise. I decided to use this for the interior of a set of 7 bowls a friend asked me to make as gifts for old college friends, at a reunion.
I made up a 2 litre batch to the same recipe but with 1% bentonite added. (After reading various posts on CAD!) The specific gravity was 1.64. Not sure of this significance of this and I left it as it was, as it seemed ok for pouring to me.
The only other differences between the test tile and the bowls were that the bowls (v thinly rolled - about 1/10" final thickness, and hump moulded) were bisqued to 1100oC and glazed fired to 1120oC. The outsides had a very dilute manganese wash and I poured the insides for 10 seconds.

This is the recipe
Neph Sy 60; Barium Carb 30; Lithium Carb 5; Kaolin 5
+ Bentonite 1; Copper Carb 4
The test tile and finished bowl photos are attached. Not sure how clearly the colour difference will show. The bowls were much darker, less bright than the test tile and were slightly more towards satin than matt which is very finely crazed (barely perceptible). I do actually think the colour became somewhat brighter as the pieces cooled, if this makes sense to anyone?

So, from my explanation above, can anyone suggest the most likely reason for the difference in the final colour?
Glaze thickness? Firing temperature? Bentonite? Manganese seeping through from outside?

Also, if I want to experiment around this basic recipe, what approach would be worth taking? Which of the ingredients would it be logical to adjust. I have books explaining line blends and triaxials, but some pointers as to how to do that with an existing recipe would be much appreciated.
Now I've fulfilled my friend's brief - (she was delighted) I have time to spend on glaze testing.Attached File  image.jpg   5.1KB   1 downloads[attachment=5326:image.jpg]

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.