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jrm1

Terracolor transparent glaze over Spectrum underglaze

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I'm new to using underglazes and am having trouble with glazing on top of the brush on Spectrum underglazes. The Terracolor Transparent stoneware  glaze is patchy in thickness and milky here and there. The resulting image underneath was less defined than I had expected to see after firing. The underglazes were applied pre-bisque firing. I used disposable gloves and was very careful to keep the ware dust free and oil free. 

There is a small but noticeable indent or depression of the surface level on the glaze where it crosses from one colour to the next, as if there is chemical interaction between the glaze and certain colours. Three coats of underglaze were applied.

I'm after a flat and even finished surface with all the detail preserved in the design.

 

The firing schedule was 60 deg C / hr until 600, then 100 deg per hour to I think 1220. 

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Please post some pictures. It'll be much easier to diagnose the issue.

In general, if a clear glaze isn't clear, firing hotter will often help. The problem could also come from the glaze and underglazes being incompatible. Not all products play nice together. Application techniue could be the cause of the patchiness. You need to brush each coat of clear glaze in the opposite direction of the previous coat.

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5 hours ago, jrm1 said:

There is a small but noticeable indent or depression of the surface level on the glaze where it crosses from one colour to the next, as if there is chemical interaction between the glaze and certain colours. Three coats of underglaze were applied.

This can happen when one area of a pot is less absorbent to glaze than another. I know some of the spectrum underglazes start to have the fluxes glossing up the underglaze at cone 04 and subsequently makes it difficult to build up an even glaze layer. I bisque to 06 and dip into clear glaze and it solved the problem of too thin  glaze areas when using with some of the spectrum underglazes. Christmas red, jade, neon pink all are culprits for glossing up at 04.

Like Neil said pictures would really help.

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The red and yellow green shapes were painted onto the black background and they looked sharp before the glaze firing. When I dipped into the glaze, I could barely see the image underneath but I could see a kind of trace of it, like when light snow falls - you can see what lies underneath. That made me wonder at the time if the result would be flat. And it is uneven for sure. 

 

The clay was Scarva Earthstone stoneware ES5.

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On 12/20/2017 at 3:33 PM, Min said:

This can happen when one area of a pot is less absorbent to glaze than another. I know some of the spectrum underglazes start to have the fluxes glossing up the underglaze at cone 04 and subsequently makes it difficult to build up an even glaze layer. I bisque to 06 and dip into clear glaze and it solved the problem of too thin  glaze areas when using with some of the spectrum underglazes. Christmas red, jade, neon pink all are culprits for glossing up at 04.

Like Neil said pictures would really help.

I bisque at 1000 deg C - is that cone 06? Are you saying 06 is better than 04 for being able to get the glaze to attach to the underglaze evenly?

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1000C is approx cone 06, (depending on heating rate though). So it should be porous enough to take the glaze evenly. That milky cloudy look it has in places could just be from having it too thick there and/or underfired. I looked up the clay you are using and from herehttps://www.bathpotters.co.uk/earthstone-original-clay-es5/p1864 it's listed as having a 100C range in final firing temperature. That can be a problem for functional ware if you are not firing to the upper end. So, next question would be what temperature do you fire to and do you know the firing rate you use to get there?

There is a cone chart here, it explains the behaviour of cones and correlating temperatures.

edit: I just reread your first post where you said you are (possibly) firing to 1220C. That would be approx cone 6. Given that the clay you are using is rated to (approx) cone 9-10 (when firing at 60C/hr for the last 100C) then I would say the claybody is more of an issue than the glaze problem. Clay is going to be immature, which can be a problem for functional work. Pots will soak up water etc, they can leak, get very hot in the microwave, get mouldy on the bottoms, craze, not be as strong as they could be etc. There was a long discussion on this a few months ago and how difficult it is in GB to find a claybody that matures in the ^6 (midrange) area and how the majority of clays are listed as either earthenware or having a broad firing range which isn't brilliant for functional pots. I couldn't find the thread but maybe someone else recalls it?

 

Edited by Min
added last paragraph
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Hi Min, are you saying that the clay body needs to be fired even hotter to mature or get hard enough for tableware? The firing range of ES5 is listed as 1180-1280. Does immature mean it's not sufficiently fired?

 

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42 minutes ago, jrm1 said:

Hi Min, are you saying that the clay body needs to be fired even hotter to mature or get hard enough for tableware? The firing range of ES5 is listed as 1180-1280. Does immature mean it's not sufficiently fired?

 

Yes and yes. Clay and glazes should both mature in the same temperature range. Problem is it appears that you are using a glaze meant for midfire (cone 6ish i.e. your glaze fire going to 1220C) on what we on this side of the pond would call a high fire body. I found the thread I was referring to early, there are quite a few potters on that thread from England, perhaps a PM to them or start a new thread asking about claybodies available for cone 6 on your side of the pond. (they might not see the question in this thread) Other option would be to use high fire glazes with that clay, problem is firing to ^9 -10 is very hard on electric kilns, elements wear out far more quickly.

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Thanks both. 

I’ll contact Scarva tomorrow and see if they can reassure me about ES5 clay and its maturity in a glaze fire at 1220 deg C. I’ve had no problems with other glazes such as Lugano Blue by Terracolor, but I haven’t done a leak test. No milkiness and great gloss. The glaze goes on way too fast in dipping for my liking though , even when pretty runny. 

I’m still confused why a clay rated suitable for my glaze firing would sound to you like it’s not cooked enough in the glaze firing. I’ll carry on reading the thread you pasted. 

Back to the underglaze by Spectrum: should I try Amaco instead and glaze with their transparent glaze? I’d rather not have to  

 

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7 minutes ago, jrm1 said:

I’m still confused why a clay rated suitable for my glaze firing would sound to you like it’s not cooked enough in the glaze firing. I’ll carry on reading the thread you pasted. 

Back to the underglaze by Spectrum: should I try Amaco instead and glaze with their transparent glaze? I’d rather not have to  

 

If a clay is rated to go as high as cone 10, then it's under-fired at cone 6. It will work at cone 6, in terms of glazes not falling of and such, but it won't be sufficiently vitrified, so the body can soak up water when being washed, then sweat out that water onto a table afterward. Or weep water out the bottom of a vase through a crazed glaze.

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The glaze has a firing range of 1180 - 1250°C. Does that indicate a good match for the ES5 clay? If I went to 1250 I think it would run, at least that’s what I’ve been told. 

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So your glaze has, in theory, a range of 80C which is roughly cones 5 thru 8 and your pot went to approx cone 6 right? If it were my clay I would make some flat bottomed pots, just simple throwaway ones, don't glaze them at all, and scatter a few throughout the kiln and fire up to cone 8. Fill them with water and set on newspaper for a couple days. If the paper is wrinkled at all the pots are weeping and I wouldn't use that clay at that temperature for pots that need to hold water. If you want a more accurate assessment of the porosity and absorption of the clay there are instructions here on how to do that here and there is also a simple ink test there also. At the same time you are firing the unglazed test pots I would try your transparent glaze over all your underglaze colours, thin, medium and thick glaze applications and see what it does. Tall test tiles with room for the glaze to run and not mess up your shelves, or put throwaway thin slabs of clay under your test tiles to catch any possible runs. I would try those tests before worrying about changing underglazes and glazes, your uneven surface could just be the glaze not being melted enough, same for the hazy spots.

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Thanks, Min, for all the advice and apologies for my late acknowledgement! I'll follow up on that. To date I haven't had leaks but I can't be sure I've tested enough yet.

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