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Glaze Not Uniformly Glossy On Cone 04 Body

soft paste porcelain cone 04 porcelain gloss glaze problem

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#1 AnnaM

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Posted 07 April 2014 - 10:57 PM

Hi all.

Just in case you're not sick of my ongoing problems with these low temp bodies, I have another curly one :blink: .  The latest soft paste porcelain recipe is good (I put a picture and the recipe in the 'self glazing cone 04 porcelain' thread a couple of days ago). White, translucent, strong, but it seems to absorb the clear gloss glaze or something. In the photos you can see that some areas have gone glossy and some haven't. The other vitreous earthenware cups that were in the same firing, with the same glaze have come out beautifully as normal (the glaze is a cesco, not sure if you have that brand o/s, it is extremely glossy, and spreads out beautifully for a really smooth finish).

 

Can anyone tell me what the likely reason for this would be? These vessels had about 3 or 4 coats of glaze, which is usually plenty.

 

 

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#2 JBaymore

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Posted 08 April 2014 - 08:11 AM

Looks like uneven firing. Thermal lag in the kiln. Slow down the upper part of the firing a bit and see if that goes away. Same cone endpoint.... just slower to get there.

 

best,

 

..................john


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#3 AnnaM

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Posted 08 April 2014 - 08:42 AM

Thanks John. I'll try that.

 

Everything else in the kiln came out perfect though with the same glaze, even things that were right next to these. Could it be something in the body matting the glaze down?



#4 JBaymore

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Posted 08 April 2014 - 08:59 AM

To tell for sure if it is this you need to look at the PRECISE location they came from in the stacking in the kiln and also the exact firing curve.

 

Doubt it is something in the body.  But in ceramics "never say never".

 

best,

 

...................john


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#5 AnnaM

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Posted 08 April 2014 - 09:12 AM

They were on the top shelf at the back (my electric kiln is tiny - 33 x 33x 33cm - only 1cf or so, front loading) so they were close to the wall. It was 200C per hour to 850C then on full, but it takes my little kiln a long time for that last couple of hundred, it was four hours for the C04's to go over all the way.



#6 AnnaM

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Posted 08 April 2014 - 09:19 AM

Also the distribution of glossiness isn't confined to one side only, eg the sides that were facing the elements, its some top here, some bottom there. It looks like there were areas that didn't have glaze on them at all (despite having several thick applications).  Can glaze soak in to bisqueware?



#7 JBaymore

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Posted 08 April 2014 - 10:27 AM

Was the matt side facing the rear wall or facing into the stacking of ware (toward the center)?  Are there elements in the "back wall?  Are there elements near the vertical walls of the sides of the mugs where these were placed?  Are you sure the elements in the top area are working correctly?

 

Lots of info needed.

 

Hint... take digital pictures of a load before you unload it and AS you unload it.  That gives a lot of potential troubleshooting info to go on.  If you don't need it... just delete the images.

 

best,

 

...................john


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#8 neilestrick

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Posted 08 April 2014 - 11:58 AM

Did all of the pots of this clay body have this problem, or only some? If it was all of them, then I would be looking at the clay body rather than the firing. I have seen clay bodies that do absorb the glaze in odd ways. And you're dealing with a somewhat unusual body here, so it wouldn't be outside the realm of possibility. We had another thread here recently where someone was working with a body made from local clay, and it had a similar problem. As to why it happens or what exactly is happening, that is beyond my realm of knowledge.


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#9 Colby Charpentier

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Posted 08 April 2014 - 12:10 PM

I've seen similar things happen with commercial underglazes, where they appear to absorb glaze differently....



#10 Matt Oz

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Posted 08 April 2014 - 12:18 PM

I know some glazes have to be a certain thickness before they have a good melt, maybe it's a combination of some areas of the pot having thinner glaze coverage and the clay body.



#11 JBaymore

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Posted 08 April 2014 - 12:25 PM

The other vitreous earthenware cups that were in the same firing, with the same glaze have come out beautifully as normal

 

Hummmmmm.......... I have taken this comment (from the start) to mean that the "other vitreous earthenware cups" you are refering to there are made out of the SAME body as the one on the mugs with defects.  Is that the case?

 

If this is happening on all pieces of a different body.......... then I'd be looking to the body being the potential issue.

 

best,

 

.........................john


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#12 bciskepottery

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Posted 08 April 2014 - 12:29 PM

I'd be inclined to look at the clay body; as Neil pointed out, the clay body is somewhat unusual with a large amount of cullet in lieu of frit and silica. Especially with the glaze working okay on a commercial clay body in the same firing. The odd-man out is the cullet-based clay body. And I think John B. may have brought up the potential for inconsistency in cullet due to lack of consistency in the source glass used.

#13 AnnaM

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Posted 08 April 2014 - 05:04 PM

Yes, the kiln has elements in three walls but not the door. 

 

No, all the other cups (that turned out perfectly with the same glaze) are a commercial earthenware clay fritted with Ferro 4131 though.  (I do this because none of the commercial earthenware clays in Melbourne are capable of holding water unglazed and they are extremely weak, too weak and seepy for functional ware). So different body for those.

 

There is no matt 'side' as such, there is uneven mattness & gloss around the whole vessels (which is why I was wondering if it could have soaked in)

 

The problem is confined to those vessels. Also these two problematic vessels had two more coats of glaze on them (4 coats in total) than the ones that turned out perfect which had two.

 

I have made this recipe using Ferro 4110 (in USA 3110 I think?) and didn't have the same problem (except where a pot had underglaze on it, but the naked area loved this same glaze). I'm wondering if adding in the extra ingredients that would have been introduced by the frit would have any effect? I think the 4110 is a high boron frit? The cullet obviously misses the boron and whatever else is in the frit. From previous threads though, my understanding is that adding soluble boron into a clay body might not be feasible, is that right?



#14 JBaymore

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Posted 08 April 2014 - 05:55 PM

You NEED tha analysis of the cullet.  Lacking that....... there is NO way to know what is going on... other than to say it is likely the body you are using causing the issue.  But you can;t track that down without knowing the chemical analysis of the cullet.

 

Personally.... since the LABOR is the high coast of producing ceramics..... if the frit body is working well... I'd stop fussing with the variable cullet and just use the one that works well.

 

best,

 

.....................john


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#15 neilestrick

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Posted 08 April 2014 - 07:37 PM

Personally.... since the LABOR is the high coast of producing ceramics..... if the frit body is working well... I'd stop fussing with the variable cullet and just use the one that works well.

 

Bingo. Even expensive raw materials are cheap in the big picture. Break it down to a per pot cost and you'll probably see that you're not saving much by using the cullet. Add in the time and materials wasted every time the cullet formula varies and messes things up, and you'll likely find that it's not worth the time.


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#16 AnnaM

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Posted 10 April 2014 - 09:47 AM

I'm not offended, but I don't understand the question Babs! What do you mean embezzlement?

 

I wish I could just use frits, but in Australia they are really expensive, between $54 & $60 per 5kg

 

I had them send the breakdown of the cullet (which is below in case anyone is interested), is there any point in comparing recipes in insight and then adding whats missing with the absence of the particular frits?

 

 

 

Chemical Approx. Limits mol%
SiO2                      62 – 84%
Na2O                     6 – 16%
CaO                       5 – 14%
MgO                       0 – 6%
Al2O3                     0 – 2%
K2O                        0 – 2%
LiO2                       0 – 2%
FeO/Fe2O3            0 – 0.6%



#17 JBaymore

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Posted 10 April 2014 - 03:32 PM

The "ranges" they are showing you there (if the formatting is what I think it is) say it is pretty much a useless material to depend upon for any kind of consistency. The analysis is all over the place batch to batch.

 

For example the sodium can vary from 6% to 16% mol wt.......... that is a night and day difference. 

 

This stuff might best be used for concrete aggragate. 

 

What is it mainly being sold for?

 

best,

 

.................john


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#18 Babs

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Posted 10 April 2014 - 07:35 PM

I was referring to a hacker post which has subsequently been deleted.
Books, I included a couple, were posted yesterday so should arrive soon
Babs

#19 Nancy S.

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Posted 10 April 2014 - 08:15 PM

I've had that problem when I've brushed on glazes and inadvertently made them uneven. Usually I get matte areas where the glaze was thinner, but this sounds like maybe you are having issues where the glaze is thicker?

 

If your 2-coated pieces are fine, 4 coats might just be too much.



#20 AnnaM

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Posted 10 April 2014 - 09:49 PM

Could be Nancy!

 

Babs, you are a legend thankyou!

 

John,  its used in glass manufacture, containers, electrical glass, construction and automotive glass apparently, stuff like that. I think she said its the stuff they mixed with paint for road marking to make it reflective as well. The powder is actually tiny glass beads, it's typical soda-lime glass.

 

When I spoke to her she said even though the proportions given are a range, that the actual proportions generally don't change - but she couldn't give me a specific composition for the batch I have.

 

I'm going to see if I can get one done I think.






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