Tuesday, December 4, 2012

More Angels and Cone 10 Glazes, substitutions and errors!

The glaze firing of the second two angels went well, much to my relief, as one of these was the commissioned angel that originally started me on this angelic mini series! The commissioned angel is shown in the first three photos, and I am pleased to report that it was well received.  At the end of this post I will say something more about the blue/green glaze on this angel's gown.

Next are photos of the other angel that was in the second glaze firing.

No wax resist or anything clever was used for applying the glaze to the striped gown, just 3 coats of glaze with a soft brush.  The red is Bailey's Red, well, almost... I substituted ball clay for china clay, I do that sort of thing at times.  I find that ball clay as a glaze ingredient does make the glaze easier to apply with a brush,  it also can help a glaze mature at a slightly lower temperature than if china clay was used.

Following this are angels you have seen before in my previous post, but I thought I would put them here with better lit photos and simple white backgrounds, so that you can see them better. 

I like the thought of angels that could appear to shepherds watching their flocks by night, as being good country characters, with big practical hands and sensible kind hearts.

Green/blue glaze.
The green/blue gown on the first angel in this post has a glaze that I developed recently.  It is a substitute for one that is in a Thames and Hudson glaze book (The Glaze Book A Visual Catalogue of Decorative Ceramic Glazes by Stephen Murfitt).  I love the look of the one in the book, it is a green that shades through to clear on rims and edges.  The glaze crazes, but it does so attractively.  There is a watery clarity to the glaze that reminds me of a cold copper stained pool high in the mountains.  The original glaze uses Cornish stone, and this is a very expensive glaze ingredient in this part of the world, so, much as I like it, I cannot afford to use it in anything other than small amounts.  I set about making an alternative.  Unfortunately you cannot simply substitute potash feldspar weight for weight with Cornish stone, because both are quite different materials.  It is necessary to attack on several fronts simultaneously!   I fired up the Insight glaze software that I purchased recently, and fiddled around until I had something that was chemically very similar to the original glaze, but used soda and potash feldspar in place of Cornish stone, and adjusted the other ingredients in the glaze, both the china clay and the silica had to be increased appreciably.

When I tested the glaze on a tile I found that, to my surprise, my substitute was much better than the original, giving superior coverage when applied with a brush (the extra china clay in my substitute would have helped with this).  The colour was almost identical, as was the crazing.  This is a glaze with character, and I suspect that it will do well as a copper red if fired in reduction, but I have not had the opportunity to test that yet.  At cone 10 the glaze will move, and it pools attractively in the bottom of bowls, giving a bluer colour where thick, green where medium and almost clear where thin.  It is not really suitable for the sort of potter where everything has to be the same, but it is a lovely addition to my glaze repertoire.

Here is the original glaze (listed as being for 1290 degrees C or 2354 degrees F with an hour soak)
Cornish Stone  28
Silica  20
Dolomite  18
Whiting  16
China Clay 12
Bone Ash  4
Tin Oxide 2
Copper Carbonate 1

My substitute (I fire to cone 10)
Potash feldspar  8
Soda feldspar  8
Silica  28.25
Dolomite  18
Whiting  17
China Clay  16
Bone Ash  4
Tin Oxide  2
Copper Carbonate 1

Whilst I think about it, do take care when following recipes from books.  Most glaze books that I have come across contain errors, unfortunately the Thames and Hudson has many really bad ones, some glazes are incomplete, and others have wrong amounts.  It always pays to read and to think about the glaze, ask your self, does the glaze sound sane and sensible to you?  How does it compare with similar ones elsewhere in the same book?  It annoys me that otherwise good and useful books can be let down in this way.  I could be wrong, but it does appear to me that the newer a book is, the more likely it is to contain errors..., we seem to be in a process of devolution rather than evolution, in a few more generations we will be conversing in grunts and dragging our knuckles on the ground!!  OK, and whilst I am on my soap box, I do wish that people publishing recipes used cone readings rather than temperatures.  There is a vast difference between a glaze that has been fired rapidly to 1290 degrees C, to one that has taken many hours to get there!  Cones give a truer record.


cookingwithgas said...

I know what you mean,grunt,grunt...icon,icon,.........kuckles? all we will need are thumbs to text with.
The angels are lovely.

Tracey Broome said...

I would love to have your glaze chemistry knowledge! I learn so much from your posts:)
The angels are just magical and the blue green is perfect!

Armelle Léon Bitterolf said...

Your angels look great, and the green/blue glaze is very nice, bravo !!!

Peter said...

Good Morning Meredith!
Text...., what's that? I still use clay tablets!

Hi Tracey,
Thank you for that, not sure that I know much really, but it is good to share discoveries and glazing tips! This very day I have put a tab at the top of my blog with a page of high fire glaze recipes. I'll keep on adding to this, and probably get round to putting tabs for earthenware and crystalline glazes too. It will be useful for me to have them there as well as on scattered test tiles and bits of paper, and I hope they might be of help to others too.

Bonjour Armelle,
Merci beaucoup! Le bleu-vert est comme la mer!

gz said...

Interesting substitution for Cornish Stone-I'm lucky I still have some of the original blue material!! Cornwall Stone is a close substitute by the supply companies, but doing 1:1 potash and soda feldspar is one I'll have to try next year

Julia said...

Those angels are amazing! What lovely work - and the glazes are beautiful.

angela walford said...

hey peter, bring on that tech talk and as always test test and test again, I don't know that any glaze works perfectly without adjusting to your local materials.

Peter said...

Hi Gwynneth,
The original blue material.. yum!

Regarding my Cornish stone substitute, The 1:1 potash and soda feldspar is only part of the story, you should note when comparing the recipe and my substitution that I also increased both the china clay and the silica by quite a lot, and the whiting by a small amount also, and the feldspars only add up to a small amount between them, they need the other the other materials adjusted too. Just doing a straight swap of feldspars alone for Cornish stone won't work unfortunately.

Hi Julia, nice to hear from you, thanks for the encouragement.

Hi Angela,
Good to hear from you, I've been meaning to visit your site for a while and catch up again. Testing... when it doesn't drive you crazy, it is almost fun finding out what works and what doesn't!