Sunday, 19 June 2011

Strong can be too hard!

I used what I'd learnt from my recent strength test experiments to make sure my latest polymer clay jewellery was cured to make it good and strong. The pieces were cured at 160ºC for 30 mins. I was pleased when I handled them, they seemed good and strong. Then I tried to sand them. Yup they are strong, and hard and that makes them really hard to polish! I spent about 4 times longer than I normally would polishing this flower pendant and I am still not at all happy with the finish.

So my advice is, think about what you are making and what it's purpose is before you decide how to cure it! Make it strong enough, but not too strong!


  1. It never occurred to me that the stronger the clay the harder to sand. I'm pretty sure it's most clayers least favourite part of the whole process, I know it's definitely mine. Thanks for sharing with us. Gorgeous flower pendant.

  2. Cara, I'm confused. I bake my clay for an hour no matter what kind of clay I'm using. I bake it at 265 degrees. I use patty papers or deli papers to smooth things out if they are flat before I bake them and it makes it much easier to sand and buff.
    What kind of clay were you using and what did you use to sand it with? I gave up my sanding pads and went back to sand paper and I'm much happier with the results. Another thing you can do is put a little translucent in all your clays as you mix them. That makes them really shine when you sand and buff. Sometimes I add a little pearl just to give it highlights. Your pendant is beautiful. Might be the sand paper you are using. Don't forget to rinse out your sand paper a lot and change your water as you are sanding. The polymer gets stuck in the sand paper and clogs it up, same with the water.

  3. I agree with Bonnie. I use Kato clay which bakes really hard. I bake it for at least 50 minutes at 300. If it needs some rigorous sanding I start with 320 grit which will really do the job, then I go to 400, etc. If your sandpaper is worn out you will be sanding a lot harder and it will not do a good job.

  4. Muchas gracias por la información. Besos

  5. I use Kato, I used to bake at 150°C (300 F) and sand with micromesh and it was fine to sand, I got a beautifully shiny finish with little scratching as long as I took my time on the early stages. This pendant was baked at 160°C and the same micromesh I normally use didn't really 'cut' it so to speak. All these colours were mixed with some pearl. I even got out brand new sheets of micromesh as I was worried perhaps they were a little worn out. Normally when I sand I can see the clay particles in the water, well the colour of them anyway, not with this pendant. I also tried 120 grit sand paper, new sheet, same thing, not much coming off. I've done a lot of sanding and played about with different brands, grades etc so I would say that I can be confident that baking them hotter makes them harder and therefore more difficult to sand.

  6. Cara, I am so sorry but I am also going to have to disagree with part of your post here. In my opinion, the harder the clay is, the EASIER it is to sand and polish.

    When the clay is harder, the less it will clog up your sandpaper and the fewer scratches you will get at each stage. I also find, that baking at a higher temp for 30 minutes doesn't cure them as well long term, as baking at a lower temp for 60 minutes does, especially when trying to cure right into the center of your piece.

    Maybe at the high temp, its melting the outside layer or leaving some sort of residue that it is effecting your sandpaper's ability to cut into it? So in that sense, that hotter baking, could be making your piece harder to sand.

    You actually don't need to remove a lot of clay into your water to be doing a good job though. Starting with 120 grit is probably too low and is just causing you a lot of extra work. I also find that cold water rather than warm, will also help with speeding up the sanding process.
    Hopefully that makes sense and was helpful to you.

  7. Cindy please do disagree, it's these discussions which help us all learn!

    I agree that harder cured clay will be shiner when polished, but this piece was really difficult. I have another cured at the same time, I'm tempted to send it to you to see what you think.

    I don't normaly use 120 grit, I was just frustrated with the lack of sanding with the higher grades I was trying. I normally start with a 1500 micromesh cloth which is equivalent to 400 grit sandpaper unless I need to do some shape altering in which case I would usually start at 240 grit.

    Thanks for the cold water suggestion, I'll try that. I normally use warm cause it's more comfortable for me but if it makes sanding faster...

    I can see I need to do some more trials here. I did intend to try sanding all my baking trials to see how the curing affected their shine but have been putting it off.

    I also need to do some more baking time/temp trials I think. What I am taking from this experience at the moment is that 160°C is probably not the ideal curing temp unless you need super strength.I'll see what else I can learn though.

  8. Would it be worth half baking, sanding and then doing a long strength bake?

  9. possibly. I do find that a long strength bake will change the surface so you would need to resand but possibly only the finer grades. More research needs to be done here, hopefully I will find sometime before too long. Just off to have a look at your blog, looks interesting. Cara


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