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Author Topic: To pre-bake or not to pre-bake?  (Read 416 times)

nannyL

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To pre-bake or not to pre-bake?
« on: April 09, 2017, 06:46:27 PM »
I'm in a bit of a muddle just now. I've made 4 fun frog sculptures in the last 2 days using Premo. The plan was to use Pearl-Ex and mica powders on one and then bake, while the other I would bake and then paint. The Pearl-Ex one looked fantastic as it was put in the oven with the unpainted one, and I was soooo excited!

Then disaster struck! Part way through baking, the unpainted one toppled over, knocking the other one down as well. The unpainted one was a little off balance due to positioning of the frog, so I know it was my own fault. The Pearl-Ex one fell to bits (the unpainted one got a dent but was otherwise ok). I'd used wire as a core for the bases and the frogs, but now realise the base wasn't wide enough to keep them upright. The unpainted one is now painted using acrylics, and that is another story! (It looks like a 3 year old has gone mad with the felt tip pens! Not sure if I'm just not an acrylic painter or if the paints are very poor quality. Anyhoo, I'm liking the Pearl-Ex results better, so probably gonna stick with that). The Pearl-Ex one wasn't salvageable so sadly got put in the bin.

Round 2: I decided to do them in stages and pre-bake in between each stage. Now, I know some people don't see the point in pre-baking, especially for something like these frogs, which aren't particularly detailed or complex. But I thought perhaps doing the pre-baking stages might help keep each section in the right shape/position. It has  :D except that the new unpainted one has a crack down the belly (the thickest part, though the base doesn't have a crack, which is about as thick). The new Pearl-Ex one doesn't (that I can see). I have done pre-baking on other projects and not had this problem. From doing a search on here, it seems that the temp was a little high. I do follow the guide of 130* for Premo, but wonder if I ought to drop it to 120*? Or do you think the pre-baking part is the issue, seeing as the centre of that thicker part won't have baked really during the pre-bake, and will then start expanding properly during the final bake? I really don't want any more disasters :( so any advice would be great :)

Beadbug

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Re: To pre-bake or not to pre-bake?
« Reply #1 on: April 09, 2017, 10:36:13 PM »
I don't know if this will help but I've watched other polymer clay artists on YouTube and they begin their projects in a cold oven, maybe this reduces the stress on the clay. Perhaps worth a try?

Carrie

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Re: To pre-bake or not to pre-bake?
« Reply #2 on: April 11, 2017, 05:23:44 PM »
I don't think myself that baking too high would cause the crack, especially as the base is ok.  It won't cure at 120C anyway, and a bit over 130C is not a problem, especially if you cover the things to prevent scorching.  It is a mystery, but the pre-baking might have caused it if it was only partially cured at that time - did you cure it long enough to fully cure the first time?  I generally cure most things for 45 mins to an hour these days....seems to be what is generally recommended! My pieces aren't thick and solid.

Is there any possibility that you could rest them in cornflour/cornstarch?   I imagine they would have to lie sideways: would that be a problem?  If the base wasn't level enough afterwards sanding would cure that.

A further thought...exactly how thick are these pieces?  Could you model them round something like a cotton/paper ball (the type often used for hollow beads) or a papier mâché ball or a solid lump of crumpled kitchen foil?

It is so frustrating for you after all the effort...Hope somebody has further ideas.

ejralph

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Re: To pre-bake or not to pre-bake?
« Reply #3 on: April 15, 2017, 11:01:16 AM »
Respectfully, I would probably disagree and say the crack could be caused by heat.

I've seen it a lot with the heat causing thicker sections to crack. Very typically I see this with Fimo's newer formulation and it is the reason I suspect they dropped the suggested baking temp to 110 degrees.

I think the reason is this - today's clay formulations are not really as flexible as they used to be (different plasticisers)

I think what happens with thicker lumps of clay is the outside fully cures before the inside has really got to temperature. So as the inside clay starts to reach temp, it expands - but the outside crust, for want of a better word, is already cured and doesn't have the flexibility to absorb that expansion so it cracks.

With thinner sections, this isn't an issue because the inside of the clay is reach temp pretty much the same time as the outer layer.

With Fimo certainly, I did a s***load - erm, a LOT of testing on this. I mean I really went to town on it all. And I came to the conclusion over and over that 110 degrees was the highest temp I could bake solid lumps of clay without cracking occuring. Flat sheets, fine - i could get them baked at 130 no issues.

So I would wonder if Premo has gone the same way and might benefit from a lower but longer bake?

Emma
Emma from Sunny Sussex, UK
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Carrie

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Re: To pre-bake or not to pre-bake?
« Reply #4 on: April 15, 2017, 12:48:58 PM »
Also, whether or ot a thick piece cracks seems to be in the lap of the gods...a pretty random occurrence ???


ejralph

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Re: To pre-bake or not to pre-bake?
« Reply #5 on: April 24, 2017, 07:13:39 AM »
For me, Kato clay would pretty much always crack if I mixed it with any other clay. And Fimo always if thicker pieces and baked over 110

I literally haven't once been able to bake a round solid ball (15mm diameter upwards) of Fimo above 110 and NOT have it crack. I tried many different smaller ovens, I used multiple thermometers in different areas of the main kitchen oven and even had the engineer in to calibrate it so I knew the temps were absolutely accurate. I tried baking from cold. I tried putting clay into the hot oven - no difference. I tried baking at 115, 120, 125 - all the same, solid balls always would crack. Only once I stuck to 110 degrees would there be no cracking.

If it were not for the fact that Fimo themselves changed the recommended baking temp to that, I'd have thought I was going bonkers! It is one of the main reasons I moved over to Cernit after being pretty loyal to fimo since the 80s because the oven I use just isn't so reliable holding 110 as it is holding a slightly hotter temp, so I needed a clay that could handle that ok.

But as I always point out - I seem to be in the minority of people reporting this. I've come across a handful of people now who have had the same results as me but many others still claim to be baking solid Fimo clay pieces at hotter temps. I have no idea what they are doing differently - and I've done a fair bit of trouble shooting with others to try to work it out.

I post about it just in case other people are having issues with Fimo - it is a fantastic clay - just if you are having cracking issues, try baking lower and longer.

Emma
Emma from Sunny Sussex, UK
www.ejrbeads.co.uk Projects, articles, bead galleries & more
www.ejrbeads.co.uk/shop Art beads, clay & jewellery supplies

Beadbug

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Re: To pre-bake or not to pre-bake?
« Reply #6 on: May 01, 2017, 12:04:09 AM »
I don't seem to have a problem with cracking and I bake some pretty big beads. I don't think I'm doing anything different, I condition the clay then bake it. I use a combi oven to cure in and even though I had to change it recently and cover my work with baking parchment it still works well. I Use fimo pro as my basic do all clay so I really don't know why there's a problem. The only thing I can think of is that I soften it a little with diluent as fimo pro does seem more dense.