8 Tips to Decrease Warping in Slip Cast Pieces

Have you ever had it, that moment of truth, when you wait so patiently for the kiln to cool, counting down the temperature to be able to crack that lid and take a peek inside? Sheer excitement, just like Christmas morning, you can’t wait to see the pots you spent so many hours, days, and weeks creating. 

As you open the lid and look down at the shelf of pots, it becomes obvious that many of your pieces are no longer nice and round, but horribly ovaled at the rim. Devastation hits you. So much effort has gone into those pieces, so many pieces that aren’t top quality products to be able to sell, and why is this happening? The questions come flooding in, the frustrations hit you, and you feel like closing the kiln and walking away, wishing those pots would just fix themselves.

Warping: we all hate it! It's something that doesn't show up until the end of the process, after so much time has already been invested into the piece and it can no longer be recycled back into clay.

"Why are my pieces warping?!"
"How can I reduce warping?!"

These are questions I get asked a lot, which is why I'm handing over my 8 tips to decrease warping and increase your success rate with your slip casting pieces.

Before I hand them over, remember that porcelain tends to warp more than other clay types because it contains more feldspar -- especially porcelain casting slips, which need to be made less plastic. Casting slips are also made to produce whiter and more translucent pieces which means there is less clay content in the slip, decreasing its plasticity.

All this means that porcelain clays warp more, and porcelain casting slip are even more susceptible to warping. Now...onto the tips!

8 Tips to Decrease Warping:

Remember, some forms and shapes are more susceptible to warping than others. Forms that are wider will tend to warp more than shapes that are narrow.

1. Cast your pieces longer: The thinner the cast, the more susceptible it is to warping. Try casting your pieces a bit longer to make them thicker. You’d be surprised how much this will help. 
2. Careful how you remove the piece from the mold: This will heavily affect warping. Take a small board or tile and lay it on top of your mold. Then holding the mold and tile together, turn the mold upside down onto your table top. Lift the mold straight up off the piece. Now you have two options: handle the piece as gently as possible and place it aside to dry. Or if you have a form that is highly susceptible to warping, take another tile and put it on your piece and use both tiles sandwiching it together, flip it over and place it to dry. You can even leave it on the tile for a couple days so you never touch it with your hands while it is still in the leather hard stage. 
3. Careful how you handle your pieces: As briefly mentioned above, you want to handle the pieces as little as possible. Extra handling can deform and warp your pieces. Don’t touch the pieces at their rims, touch the pieces closer to the bottom when handling. And touch them as infrequently as possible. 
4. Practice drying your pieces evenly: This is more for larger/wider pieces that are more susceptible to warping. Wrap your pieces in linens to slow down the drying process and decrease drafts. Keep the pieces under linens for about 2-4 days until the whole piece looks lighter in colour and feels drier. Then it can be fully exposed to finish drying completely. Linens are used instead of plastic as it allows pieces to dry faster, but yet slower than fully exposed. You can use old pieces of fabric, baby receiving blankets, or get an old flannel sheet, and cut it into sections to be used. 
5. Check your pieces and rotate them: Check on your pieces the next day and rotate the pots. This helps to have even drying which is important in reducing warping. You will likely be able to see that certain areas of the pieces are lighter in colour as they have dried more than other areas. 
6. Clean rims of your pieces when bone dry: To reduce handling and misshaping of your pieces, clean the rims when they are bone dry. This may seem really different than normal trimming and fixing timelines for regular plastic clay. But, it does make a difference, and works! 
7. Firing arrangements of your pieces: You’d be surprised that this plays a factor in the warping of your pieces. Obviously, you want every spot in your kiln to be full so that you maximize your space and minimize costs. So try and place the more susceptible pieces in the centre of your kiln shelves and the less susceptible pieces around the outside. This gives the pieces in the middle a more constant and even temperature and helps reduce warping. Do this for both bisque and glaze firing, but it does seem to be most beneficial in the glaze firing.
8. Kiln temperature and consistency: When temperatures are uneven and inconsistent in the kiln, and areas fire hotter than they should, warping increases. Try to have an even and consistent firing kiln. Test your firing temperatures by using pyrometric cones in multiple places in your kiln during firings. Evaluate if you have hotter areas and make adjustments or don’t fire your more warping susceptible pieces in that area. Keep your kiln parts up to date by regularly changing thermocouples and elements. Each kiln manufacturer has different average firing amounts for these parts, check with yours and make sure you are changing your parts on a regular basis. 

Implementing some of these tips will help to reduce warping, but if you really want to significantly decrease your warping rate, try all of these tips!

Hopefully the next time you open your kiln, you won't be devastated by your warped pieces, but instead enjoy the excitement of your beautiful creations!