Trouble with Coasters Warping

[Rachel Hawkins]

Hi all, Coming back to this forum after it helped me solve a looong battle with S-cracks several years ago! Here today with another technical problem... I've been making coasters in large quantities and struggling with warping that makes the coaster unusable/unsellable.  Here's my process -- I think I have an idea where my issues could be, but wondering if anyone else has tips or notices anything that might have to do with the struggle to keep the coasters flat.

1. Roll out slab using slab roller

2. Compress with metal rib

3. Cut circular coasters with cookie cutter

4. Here's where the problem maybe is -- usually the coaster sticks inside the cookie cutter and I gently press it free onto a piece of drywall, trying really hard to not do anything too drastic. But definitely occasionally picking up a wet slab or coaster. Maybe I should be waiting for the slab to be leather hard, then cutting the coasters, and peeling away excess as opposed to ever picking up the coaster before it is leather hard and thereby preventing the slab from "remembering" later on that once for 2 seconds I picked it up and it bent a little and now it wants to bend again? lol.

5. Stack drywall layers atop coasters, weigh them down with something, let them dry slowly over days

6. Once leather hard/nearing bone dry I do a little trim to neaten up the sides -- usually scrape over the coaster with a metal rib to flatten out, and use a damp sponge to smooth the sides at the end. Add underglaze decoration once bone dry. All coasters appear flat at this point

7. Slow bisque to Cone 04 - coasters still appear flat at this point, with maybe 1 casualty

8. Glaze medium speed to Cone 5 with hold - here is where, of maybe 25 coasters, 5-6 will be totally unusable (often a side curls up almost a quarter inch) and most of the rest are passable but just don't sit as flat as I'd like. 

Am I being unreasonable to want flat-flat coasters? Haven't seen a lot of potters selling coasters and wondering if this is why... 

Here's a picture to help show what's going on:

https://drive.google.com/file/d/1l3k1dYW3qxpCjqS3AcCmtHsT3jskLNbz/view?usp=sharing

Thank you so much for any tips/advice!!!

[akilpots]

i dont make coasters but i do make tiles sometimes so these tips should apply...

make slabs of stiff leather hard clay.

you can dust the inside of the cookie cutter with cornstarch to help each piece release.

make sure you are drying them out somewhere with no drafts. you want each one to dry evenly so like not having the left side dry faster than the right side.

the less drying they need to do when cut from the slab the better chance you'll have at them staying flat.

[Min]

If you use a claybody made for tiles it will be far less plastic than your typical claybody, less plasticity in a claybody helps tiles stay flat. (along with the practices you are already doing and @akilpots suggestions) Also, if you lay a piece of very thin plastic sheeting/film over the clay before pressing the cookie cutter down it will round over the edges so you don't need to smooth the topside eges of the coasters. 

[Kelly in AK]

I have to agree with what’s been said . A tile body, stiffer clay to begin with, that’s how tile makers do it . Non plastic clay in a hydraulic press  

Using plastic clay that shrinks a lot, I’ll share what I do to keep thinks flat. I’m a rolling pin guy, I always flip the slab and roll it in various directions (In fact, I beat the lump flat with a mallet before I start and strongly believe that makes a difference. It’s analogous to compressing with a rib.). With a slab roller it’s even more important, roll it it with a pin in the opposite direction of the slab roller. Compress both sides with the rib. Next is to flip the pieces over after they’ve dried a little, just a few hours. So much shrinkage happens in that first little while, it sets up the warp before you see it and everything you do after that is for naught.

A proof is to cut some 3 x 10 mm slabs and measure them after three hours, then again after 10 hours or overnight. The lion’s share of drying shrinkage happens early on, it needs to be even on both faces of your coaster. 

[Jeff Longtin]

I would abandon the cookie cutter process immediately. (Adding too much stress to the clay.) Maybe use a template and fettling knife instead.

A less plastic clay would also be a good start. 

Drying between sheetrock/drywall is good.

What kind of slab roller are you using? One with a single roller or two? If it has a single roller you might want to flip the slab between compressions. A double roller isn't a guarantee of no warp but it helps.

Beyond that welcome to the world of slabs. You would think it's easy but it can be way more complicated than throwing.

[Rae Reich]

I know your pain! Your Item 4 is where the problem is, I agree. Any bending will be remembered!!

Find or make out of clay a cookie-release shape that you can use to hold the coaster flat as you lift the cutter. I have dowels to push out clay stamped with smaller tubing, but it took a while to find something that fit perfectly within my large biscuit cutter.

Also, you can place one drywall board over your board full of coasters to keep them all flat as you flip them over (not too big of a board full of coasters, hold boards firmly together as you turn).

Ditto all the advice about drafts!

[dhPotter]

11 hours ago, Rae Reich said:

Find or make out of clay a cookie-release shape that you can use to hold the coaster flat as you lift the cutter. I have dowels to push out clay stamped with smaller tubing, but it took a while to find something that fit perfectly within my large biscuit cutter.

I do this with a bisqued piece because it shrinks just enough to fit inside the cookie cutter but loose enough to not get stuck in the cookie cutter. The bisqued circle gives you coverage over the whole shape, so you are not pushing to release in just a few spots. 

[Magnolia Mud Research]

I have been down a similar route long ago:  make a round object (a piece  of stiff flat tarpaper); place on the soft clay; use a sharp cutting tool (I used a small sharp pointed kitchen knife) to cut the circle of clay.  Start with a large flat sheet of clay on a piece of dry paper or cardboard, use the round object and the knife to cut the clay with the knife;  when all of the coasters are cut, then remove the clay between the coasters leaving the coasters on the cardboard.  The coasters are allowed to flat all the time.  smoothed the sharp edges of the coaster after the bisque. 

LT