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Hey guys,


Quick question I can't seam to find much information on.

I'm slipcasting a shape from a two piece mould and it leaves a seam down the side of my piece. I sand it as greenware then again as bisque before stoneware firing and wondering if seams are just inevitable. It could possibly be a combination of protruding just enough to be visible but also a discoloration effect.


I have experimented a little but just wondering if you guys find that seams are just unavoidable or are usually very easy to get rid of. I'm possibly not doing enough to remove them.

I'm not glazing the seamed area.



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Green ware: The majority of your clean up happens at this stage. In most cases all of your clean up is in this stage.

They make clean up tools for green ware...they are scrapers. I use an X-acto knife and a clean up tool to scrape the seams down.

You can dampen the seam with a sponge before you scrape.

Scrape your seam flush or pretty close to flush, then use a damp sponge to smooth it.

Your seam should be gone. If your mold has a bad seam you may have a little more trouble. You'll need to work with it a little more.

Don't fire it until it looks good.


In the bisque stage: Light touch up if needed

If it is fired low enough, it will remain soft enough that you can use a scraper or sand paper.


The exception to the above rules would be slip cast porcelain. Porcelain is often fired with the seams in tack, to a soft bisque for clean up. Then scraped and sanded. Slip cast porcelain is really delicate in the green ware stage.


Your seams should disappear with little indication that they were ever there.

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There's a post somewhere on here (can't find it at the mo) with a link to a document produced by Alfred Uni?.  


Anyway, it includes a diagram showing how the plates align during slipcasting, and recommends removing more of the seam than you would think, then putting a coil of clay back in to fill the "ditch".  


I have found that no matter how much you think you've removed the seam at greenware stage, it comes back to haunt you at bisque.  So you do need to "over-fettle" and remove more than your head says you should.


in the words of the more experienced on this forum:  TEST, TEST, TEST.  For me that means make six identical casts and fettle them in pairs, more and more aggressively, then fire one of each pair.  After firing you can compare before and after.


Good luck, nothing says "production made" like a poorly fettled seam.

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I find that fettling like you're scraping potatoes is the best way. [i assume everybody else does this,

but cannot be sure.] I also find taking 2 or 3 bites at the cherry gives better results, on a body that's

still a bit soft (the knife is pushing not cutting). A final wipe with a damp sponge helps.


One of those little snap-off craft knifes with an inch or two of blade extended works well. The remarkably

cheap plastic fettling tools work even better -- but you may need to remove the moulding seam from the

plastic tool first!


Regards, Peter


PS Experience mainly on porcelain.


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  • 2 years later...

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