In the Studio: Refiring

Justin Rothshank’s planter, 6 1/2 in. (17 cm) in diameter, wheel-thrown earthenware, soda fired to cone 3, refired to cone 015 with various decals, refired again in a soda kiln to cone 3, refired again with various commercial decals to cone 015, 2019.

I occasionally choose to refire my work as an alternative finishing approach. There are several reasons why I decide to refire a piece:

  • To fix a poor glaze result including under-fired glazes, a lack of exciting soda surface effects, an ugly wadding mark, or a glaze defect such as pinholing.
  • To add decals, china paints, or lusters. A finished piece with beautiful glaze results may still benefit from additional layers
    of decoration.
  • To build up layers of decoration, firing to the hottest temperature for materials used  first and firing to the lowest temperature last. Background imagery should be fired first, with the foreground imagery fired last.

Prepping for Refiring

When preparing a piece to be fired again, the already fired, glazed surface can be difficult to reglaze because the surface is sealed and won’t absorb water, causing newly applied liquid glaze to drip off. Thickening your glaze—by air drying or adding products like Apt-II or CMC gum—can make this process easier. Preheating the pot will also aid in quickly drying the newly reglazed surface. I’ve also experienced success when I’ve sprayed a thin layer of glaze over a pot chosen for refiring, specifically highlighting and glazing certain areas that may benefit from a thicker application.

1 This pot was fired to cone 4 in a wood/soda environment. The glaze surface was underfired and rough.

2 I applied a thick yellow underglaze to the surface in a painterly approach.

When refiring pots that have a particularly dry surface after soda firing, I’ve applied new layers of underglaze to add color. Spraying, painting, dipping, or pouring the underglaze are all effective methods of application in this scenario, just like when adding underglaze to an unfired surface.

Cautions to Consider

Refiring ceramic wares at any time can cause them to dunt or crack. This can happen for several reasons. First, it’s important to be sure that there is no water in the pot—on its surface or absorbed into the piece. When this added water turns into steam, it can cause the piece to crack apart. This is relatively easy to control. Be sure your pot has dried thoroughly after reglazing it, and then program a slow ramp or temperature increase through water’s boiling point, 212°F (100°C).

The second reason dunting occurs is when too much cristobalite forms in the clay. Most of this formation happens above 1000°F (538°C) during quartz inversion. This cristobalite form of silica then inverts during the cooling process, mostly between 400°–500°F (204°–260°C).¹ Thus it’s important to cool the kiln slowly through these temperatures.

3, 4 I brushed Amaco’s low-fire matte glaze on other parts of the surface, then reapplied wads to prevent the piece from adhering to the kiln shelf and to enable consistent reheating throughout the refiring process. I then refired the cup.

Here are a few hints to help avoid cracking, dunting, or other refiring mishaps:

  • Be sure your piece is completely dry.
  • Fire slowly up to 500°F (260°C), and again through 1000°F (538°C). Cool slowly through those same temperatures. This prevents dunting caused by moving too quickly through quartz inversion and cristobalite inversion.
  • Be sure your piece fires evenly so that one side doesn’t get hotter than the other. Place it far from the kiln elements, elevate it above the kiln shelf for even heating, and place it in the most well-insulated part of your kiln to protect it from rapid changes in temperature that could cause thermal shock.
  • Consider using stilts, wadding, or soft brick to prevent the bottom of your piece from sticking to the kiln shelf. This will also enable consistent and even reheating and cooling during the refiring process.

Process

I recently fired a pot to cone 4 in a wood/soda atmosphere. The glaze surface was underfired and rough (1). To salvage this piece, I opted to refire it. I applied some thick yellow underglaze to the surface with a painterly approach (2). Next, I applied Amaco’s low-fire matte glaze (3). When refiring this piece, I applied wads again to prevent the piece from adhering to the kiln shelf. As the piece is reheated, residual soda on the surface of the cup will melt and cause the piece to stick. The new wads also enabled consistent reheating throughout the refiring process.

5, 6 The piece then went through additional firings to add decals.

Once the piece was refired with the underglaze and matte clear glaze (4), I applied various decals in a layered fashion with yellow roses first, then black decals applied on top (5). The piece was then refired again to cone 010. Finally, I applied red poppy decals and refired the pot to cone 015 in an electric kiln (6).

¹Tony Hansen, “Quartz Inversion,” DigitalFire, accessed December 10, 2019, https://digitalfire.com/4sight/glossary/glossary_quartz_inversion.html.

Excerpted from Low-Fire Soda by Justin Rothshank. Published by The American Ceramic Society, 2020. Available in the Ceramic Arts Network Shop at https://ceramicartsnetwork.org/shop.

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