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1230 C Semi-Clear Crystal Glaze Problem


PottaFella

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Hi guys, first post...

 

I am very new to ceramics, been throwing and making a year and have recently purchased a small-ish (2.6 cu ft, 75litre) electric kiln.  I know I have rushed in to doing a first firing, I have not used any cones, only the controller, so call me an idiot by all means. 

 

I used an un-grogged buff stoneware clay and mixed a semi-clear crystal glaze which has come out a really nice greeny colour but appears very burnt and rough texture on the inside of the pots which is no problem on a vase, but not much good on jugs and soup bowls. I have also got a bit of crazing.

 

I am posting some pics so you can see what has happened... from what I have read now, I think this type of glaze can be tricky to fire?  It would help hugely if anyone can tell me if they've seen this effect before and what I may have done wrong – for example, does it get much hotter inside the pots, so causing the burnt look? 

 

Here are the tech details...

 

The Glaze is number 207 from Emmanuel Cooper's "The Potter's Book of Glaze recipes"

 

It is listed at suitable for firing a wide range 1200 C - 1260 C (2192 F - 2300 F).

 

It is called semi-clear crystal glaze and in oxidation should be a "clear turquoise glaze with frothy green crystals" though the photo in book shows it as more bluey than turquoise.

 

Neph syenite 40

Dolomite 15

Whiting 8

Barium Carb 7

Flint 27

Bentonite 3

Copper oxide 1%

 

The insides of the bisque-fired pots were filled, then poured out pretty much straightaway, the outside were dipped in the glaze for about 5 seconds.

 

I fired at 150 degrees per hour all the way to 1230 C with a 30 minute soak, then  back down at about 150 per hour. The bottom vent was closed, the only other vent is a spy hole near the top of kiln and that was open all the time. 

 

Here's the pics and thanks for any help!

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post-61816-0-50085700-1387718612_thumb.jpg

post-61816-0-03403900-1387718616_thumb.jpg

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The inside may be due to glaze thickness, or in your case . . . thinness.  If you glaze the outside first, then the inside, the bisque was likely soaked from the outside dip and not able to absorb enough for the inside.  I've used a similar glaze and the dark color (which I like) tends to show in places where glaze is thin and where it breaks over edges (like the rim). 

 

As for crazing, try increasing the amount of silica/flint.  That will help improve the glaze/clay fit.  Adding more silica/flint may cause the glaze to become a bit more glossy.

 

What you may want to think about is using the green semi-matte on the outside and using a clear or white liner glaze on the inside.

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The inside may be due to glaze thickness, or in your case . . . thinness.  If you glaze the outside first, then the inside, the bisque was likely soaked from the outside dip and not able to absorb enough for the inside.  I've used a similar glaze and the dark color (which I like) tends to show in places where glaze is thin and where it breaks over edges (like the rim). 

 

I didn't write my post too well, because I actually filled and emptied the pot with glaze first, and then dipped it afterwards, sorry to have misled and I have corrected my post.

 

I do like your idea of using a different liner colour. I may be able to do this and re-fire the jugs.

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You should be cautious using a Barium glaze for a functional pot. test it wit a slice of lemon in it overnight. If there is a change in the coloration, the glaze is leaching. The best book for safe use of glazes is Mastering Cone 6 Glazes by John Hesselberth and Ron Roy.

 

Marcia

Good point thank you Marcia. Any ideas why it is so burnt inside but fine outside?

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I have three tips for you.

 

1.)  I think you'll find cooking analogies are misleading in ceramics. Clay and glazes don't get scorched or burnt. Ceramics which look "burnt" are usually merely under-melted.  Too much heat work give ceramics a nice fluid glassy look as they melt on your shelf.

 

2.) Nepheline Syenite is my least favorite raw material.  I've found glazes which rely exclusively on Nepheline Syenite as the melter become more reliable when adding in another melter like a man-made frit, gerstley borate, lithium carbonate, etc.  Although Nepheline Syenite melts at ^02 due to the high salt level, due to the high alumina content it doesn't flow well or mix well. I think you'll like Emmanuel Cooper's glaze far better after you've added 5% to 10% Ferro Frit 3134.  He fired primarily at ^10 and many of his recipes need something like a man-made frit to yield a similar look at ^6.

 

3.)  This is a tile of a very similar glaze, Philadelphia Green, where I've used Strontium Carbonate in place of Barium Carbonate. The top has three dips, middle two, and bottom one. As you can see, glazes which rely primary on Nepheline Syenite look very different depending on their thickness due to their lack of fluidity.

 

Philadelphia Green ^6

58.0% Nepheline Syenite

26.0% Strontium Carbonate

10.0% Ball Clay

  5.0% Silica

  1.0% Lithium Carbonate

  4.0% Rutile

  4.0% Copper Carbonate

 

If this were toast, I'd say the bottom of the tile got too burnt while the top is done just right - but this sends you down the wrong path.

 

The top of the tile has enough glaze that this very non-fluid glaze actually melts evenly. On the bottom, much of the sodium and potassium flux has melted into the clay, leaving insufficient flux to melt the glaze.

 

Thank you that is very helpful.

 

I had imagined it getting 'extra' hot inside the pots but it seems that is not the case.  I thought I had got similar amounts of glaze on to both interior and exterior (and so expected similar results), yet the contrast is extreme.

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The outside of the pot receives more radiant heat, so the outside is almost certainly hotter longer than the outside.

 

I'm beginning to understand. I wonder what will happen if I can get some more of this glaze (I've got plenty left) onto the inside of the jugs and give them longer soak or higher heat.

 

I guess my 1230 C and half hour soak was not enough?

 

I will certainly try ferro frit 3134 if I mix this type of glaze again, thank you!

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Thanks – I'm beginning to wish I'd paid more attention to chemistry lessons at school. 

 

I know what you mean, but it's a bit of a different ball game at over 1200°C:  my son has a BSc and an MSc in chemistry but he's no use at all if I ask him about glazing pots. :rolleyes:

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I agree with the above comment that it looks like too thin of an application on the inside.

Marcia

Thanks I am going to try adding some more glaze inside and re-fire at same temp.

 

I will try heating the pot and then pouring/swilling. If that doesn't work someone said try PVA glue on inside before glaze.

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