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Increasing strength of small pieces


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#1 Puzzlebox Art Studio

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Posted 22 May 2013 - 05:02 AM

My studio produces some small, easily-droppable pieces like pendants for necklaces and keychains. I believe that since I started working here, the kiln manager made some improvements (perhaps just increasing temperature) but I'm really not eager to test too many pieces that we spent a lot of time painting by throwing them around. This concern popped into my head today because I DID drop one from the most recent firing but it didn't break, but then another employee dropped a different one from a firing a few months ago, which did break.

We're in small-town Thailand. The clay we are using is simply called "white clay" but is not porcelain. (looks brown when wet). The pendants are cut from slabs (rolled w/ pins). Don't remember the temp for bisque fire, but underglaze fire is at 1150 C and glaze fire is at 1240 C. Each pendant is 1/4" thick at maximum, but sometimes become thinner after sanding. The glaze doesn't run, so we glaze down the sides of the pendants, only cleaning off the very bottom. Looks like this when finished.

Does anyone have general thoughts on what makes some small ceramics sturdier than others? Is there anything we should be doing? Since our customers also live in Thailand, the loss of an $8 pendant due to breakage is a bigger deal here. We don't want people thinking this is a foolish purchase! Plus, as the painting technique on the pendants grows steadily more detailed, we'd like to bump that up to $15, but don't want that to be risky for customers.

#2 Benzine

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Posted 22 May 2013 - 07:49 AM

My studio produces some small, easily-droppable pieces like pendants for necklaces and keychains. I believe that since I started working here, the kiln manager made some improvements (perhaps just increasing temperature) but I'm really not eager to test too many pieces that we spent a lot of time painting by throwing them around. This concern popped into my head today because I DID drop one from the most recent firing but it didn't break, but then another employee dropped a different one from a firing a few months ago, which did break.

We're in small-town Thailand. The clay we are using is simply called "white clay" but is not porcelain. (looks brown when wet). The pendants are cut from slabs (rolled w/ pins). Don't remember the temp for bisque fire, but underglaze fire is at 1150 C and glaze fire is at 1240 C. Each pendant is 1/4" thick at maximum, but sometimes become thinner after sanding. The glaze doesn't run, so we glaze down the sides of the pendants, only cleaning off the very bottom. Looks like this when finished.

Does anyone have general thoughts on what makes some small ceramics sturdier than others? Is there anything we should be doing? Since our customers also live in Thailand, the loss of an $8 pendant due to breakage is a bigger deal here. We don't want people thinking this is a foolish purchase! Plus, as the painting technique on the pendants grows steadily more detailed, we'd like to bump that up to $15, but don't want that to be risky for customers.


I would say the clay you are using, sounds like the low fire white clay, that I use in my classroom, but the glazing temperature, would rule that out.

I terms of what makes some pieces stronger than others, there are numerous factors, such as how they are dried, glaze thickness from one piece to another, etc. Some of them could have small cracks, from drying quickly, or even mishandling, that don't fully break until later.

I'm sure there will be others here, better able to give you some more insight.
"Anything worth believing, is worth questioning"

#3 trina

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Posted 22 May 2013 - 08:53 AM

You could try mixing your clay with paper. (If you are actually using a low fire clay, the glazing temps sound too high) There are quite a few recipes on this site. It would make your pendants stronger, easier to sand and lighter. You might have to fire a bit cooler but easy to test. T

#4 Puzzlebox Art Studio

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Posted 22 May 2013 - 12:07 PM

Interesting. We are constantly making paper pulp because we also make some paper mache products. I had some mugs I was making as test pieces, that formed a few small cracks when they dried. I tried mixing some of that pulp with clay and a little glue to fill the crack. It got damn hard and I guess I am not sure what will happen after firing...does the paper not burn out?

I will look for recipes. Does lowering the temperature affect how the glazes work, though?

#5 Benzine

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Posted 22 May 2013 - 12:28 PM

The paper will indeed just burn out.

As for the glaze, you want to fire it, to the recommended temperature, or it won't fully mature. Under fired glazes, harden, but may not change to the correct color, or become glossy, if it is supposed to become glossy that is.
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#6 oldlady

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Posted 22 May 2013 - 09:14 PM

i admire your work and your ambition. keep going. maybe you are the new Josiah Wedgewood of your country. your artists display amazing skill at painting. maybe you could develop a decal transfer for that art to be applied to simple pots. someone here can point you in the right direction.

as for your original question, sometimes things just land differently and some break but others do not. i know that i have shown fish in baskets with 100 fish at a time and had people approach them so carefully that i have pulled out one of them and banged it on the table. no breakage.

did it all one saturday. on sunday i broke the first one i tried. i have dropped pieces on my concrete floor without damage. they still ring so i know the clay is intact. your glaze firing temperature looks like about cone 7 according to my book. are you firing these three times, once to bisque once to set the underglaze and again for glazing? if i read your post correcly. from the many photos on your website it seems you are only firing twice.

it is not necessary to fire so many times. underglaze goes on greenware and once it is dry you could glaze and fire it only once. this takes a little experimentation but it would save time and fuel. i am assuming that you are putting a clear glaze on the pendants. it appears from all the photos that the finish glaze is being applied by brushing. single firing works best if the final glaze is sprayed. sorry, that may not be possible for you.
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#7 Puzzlebox Art Studio

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Posted 23 May 2013 - 03:01 AM

Three times. I thought the original ceramics teacher/kiln manager told me that firing fewer times might result in weaker pieces...but we have lots of miscommunications due to language barrier! I've never tried underglaze on greenware. Clear glaze is done by dipping. We don't have a sprayer, but wish we did!

Prior to coming here, I had very rarely used underglaze and didn't know much about it. I would worry about painting on greenware because I imagine mistakes can't be fixed as easily as on bisque, although for pendants that's less important than a piece that took more time to build.

So my confusion with the paper clay is...if the paper burns out, would this not leave tons of tiny holes and weaken the clay after firing? Honestly, I don't know how it works but am just trying to imagine it. I did a little searching on this and am seeing that it's stronger to build with than plain clay, but in my mind the lighter result after firing would be even more delicate than before.

#8 weeble

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Posted 23 May 2013 - 06:16 AM

My opinion is that the paper clay I've worked with isn't as stong once fired as the plain clay. But I also know there are others here with far more experience with it than me! They're much stronger in the green stage, but that advantage evaporates in the first firing. Or maybe I should say it burns up :)

A thought I had on your problem is how reliable is your clay source? If its something local the quality control on the clay body itself may be less than we're used to seeing here in the states. Perhapse the supplier made a change in the formula that has resulted in this change? It could also be-as someone said- just luck of the draw that the one broke but not the other!
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#9 oldlady

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Posted 23 May 2013 - 09:00 AM

actually, i find it easier to remove underglaze from a greenware piece by just scraping it off with the edge of a knife or similar tool. a knife fits into small areas without disturbing what is already there next to the bad part.

do you cut each shape with a knife? if so, think about using metal strap bent to shape for each one. i would be happy to donate some "cookie cutters" to your organization that you can bend to fit what you want to do. if you use them over thin plastic sheet, your edges are automatically rounded on the top of the piece. flip the cut side and rub the edge with a finger and both sides are done. it becomes a much more finished work surface. learning how takes practice just like everything else to do with pottery.

your group is doing a fine job of helping the local youth and your economy. i am sure there are other potters here willing to help you out.

clay is strong. making pieces will get easier. you will eventually accept that some breakage is just part of the business and though you will be sad to see the beautiful painting damaged, it will get easier.
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