Drying and Firing Cristina Córdova

Appears in the Jul/Aug 2023 issue of Pottery Making Illustrated.

Shrink Slabs 

When working large, I build on top of a shrink slab, which is loaded in the kiln with the piece and fired underneath it. A shrink slab is a perforated clay waster slab that will buffer the friction between the bottom of a heavy piece and the kiln shelf to avert cracking and warping (1). It is made of the same clay body as the sculpture and will shrink at the same rate during the firing. To make one, I roll a slab about ½ inch (1.3 cm) thick and slightly larger than the footprint of my sculpture. Then, I pierce or drill holes all over the slab spaced every 1 to 2 inches (2.5 to 5 cm), and let it stiffen some before building on top of it. I also put a layer of sand or grog between the shrink slab and the kiln shelf. This layer of sand is very helpful for sliding a sculpture off of a cart or work board and onto the kiln shelf, and it also helps the shrink slab contract easily during the firing. Silica sand, sold as “play sand” at hardware stores, is good for this purpose. 

Firing Supports 

There are some sculptures that require the help of external supports during firing to prevent sagging or cracking. Primarily, these are pieces that have elements projecting off of the main form, such as a figure with an outstretched arm, or pieces that are carefully balanced with a lot of mass supported on a relatively small base, such as a standing figure. Even a standing figure that is fired laying down may require support, if the head, arm, or leg does not make contact with the kiln shelf. In these situations, the ideal supports are those made from the same clay body as the sculpture, because they will shrink at the exact same rate during the firing (2). You can use clay slabs to make standing cylinders, perforate them with holes for quick drying, and place them under cantilevered portions of the sculpture. For taller supports, another option is to stack soft kiln bricks into a tower and top it with a smaller clay support that makes contact with the sculpture. If a figure sculpture is laying down in the kiln and needs just a short support under an appendage, it is often possible to put just a small piece of kiln brick or a pile of sand under it. 

Once a sculpture is glazed, it becomes harder to support during firing, since the glaze will stick to anything it touches. In this case, you can use the same kind of supports, but top them with commercially available firing stilts so that only their small metal prongs make contact with the glaze surface. If the piece has already been bisque fired at or above the glaze temperature, it won’t continue to shrink, so any kiln furniture can be used for support under the stilts. If the entire surface of a sculpture is glazed, the whole thing can be fired with stilts between it and the kiln shelf. Just remember that if the piece is heavy, you may need many stilts spread out to support its weight. 

Drying 

Most of my sculptures are loaded into the kiln when they are leather hard, before they are completely dry. At this stage, the clay is strong and not yet brittle, so it is an ideal time to move the piece. Drying can continue after the sculpture is safely in the kiln (3). If you have time, you can allow the sculpture to air-dry completely before starting the kiln by leaving the lid open with a fan blowing across the top to help circulate air. You can also start the kiln immediately on a drying cycle if you need to speed up this process. Even if the clay is totally dry, it is important to use a kiln-drying cycle before the true firing begins. The key is to raise the temperature gently and hold it as long as necessary at 180°F to 190°F (82°C to 88°C). This is safely below the 212°F (100°C) boiling point of water, at which point moisture in the clay can turn to steam and cause a crack or explosion as it escapes. How long you hold your sculpture at this temperature depends on how large, thick, and dry it is when you start the kiln. For my largest sculptures, if fairly wet to start, I may hold this preheat for up to 60 hours. For small, dry pieces it might be as little as 2 hours. Tip: A helpful way to gauge dryness is to hold a mirror or piece of glass to one of the kiln’s top peepholes and see if it fogs up with condensation. If it does, there is still moisture in the kiln, and you should probably continue the drying cycle. It never hurts to dry a sculpture too long, but rushing this stage can be disastrous! It pays to err on the side of caution after you have invested so much time in making your artwork. 

Firing 

After the drying cycle, start the real firing. If using a digitally controlled electric kiln, the initial drying at 180°F to 190°F (82°C to 88°C) is usually programmed along with the rest of the firing, so the kiln will continue straight into the firing after that period. For smaller sculptures, it can be okay to use a preset slow program, but for larger pieces, you will need to program a slower, gentler firing to keep your sculpture from cracking during the firing or cooling. The paper clay I use for large-scale sculptures can be fired all the way to cone 10, but I choose to fire only to cone 2, which gives it sufficient strength for a sculpture. Firing below the point of full vitrification (when clay particles fuse to become glass-like) puts less stress on the piece, keeps it from shrinking as much, and reduces the likelihood of cracks and warping. Above are my recommended firing schedules. 

See more of Cristina Córdova’s work online at www.cristinacordova.com or on Instagram @cristinacordovastudio. 

Excerpted from Mastering Sculpture: The Figure in Clay by Cristina Córdova and published by Quarry Books, an imprint of The Quarto Group. To purchase, visit https://mycan.ceramicartsnetwork.org/s/ product-details?id=a1B3u00000B2lCXEAZ.