Perspectives I As Far As I Know CLAYTIMES · COM n 20TH ANNIVERSARY • AUTUMN / WINTER 2015
Developing Rich Electric Kiln Surfaces ( continued from previous page )
strong role in the fired quality of the glaze . A thicker application may be better , or even a lot better , than a thinner one . Some glazes can be quite magical when they are applied very thickly . How thick is too thick ? In all truth , you ’ ll never know until you try it . I like to make tea bowls for this kind of testing , since they can be quickly thrown and can fit into a blender for dipping the test glaze . Leave about two inches of bare clay at the bottom of each piece and dip the teabowl upside down so it is thickest at the top . Fire the pieces on a ruined shelf or on a piece of a ruined shelf , in case the glaze runs or drips .
It ’ s not unusual for a glaze to crack ( like a mud puddle ) when it is applied in a very thick coating , and that cracking can sometimes cause crawling . In that case , begin by asking yourself if the crawling looks good — sometimes glaze “ flaws ” can be pretty nice . If not , then try applying your glaze in layers , allowing it to partially dry between layers . A trick I use is to force dry the piece in a microwave oven that I have in my studio : a couple of minutes on high ( followed by a brief cooling ) will often get the piece dry enough to apply another layer ( warning : don ’ t do this if you ’ ve applied wax as it may trap the steam and cause the piece to blow up . I don ’ t use wax on pots that I ’ m going to microwave and I ’ ve never had one blow up ).
Another trick for layering is to add a very small amount ( perhaps a quarter of a percent ) of CMC ( sodium carboxymethyl cellulose ) to the glaze . CMC is an organic gum that acts like glue for the applied glaze . With CMC , a second layer of glaze can be added to the pot without it lifting the first layer of glaze off the bisque . The secret to CMC is to use no more than you absolutely must . Too much CMC will seal the surface of the bisque ( as the glaze is applied ), resulting in a dipped glaze that remains wet on the surface of the piece , rather than quickly drying as the water is drawn from the glaze and into the bisque ware .
Besides applying the glaze more thickly , you can adjust the cooling profile of the firing to help make the glaze more visually varied and interesting . This is definitely one of the best ways for getting richer glazes from an electric kiln . The glazes in fuel-fired kilns are often richer because the kiln has thicker walls and heavier refractories , so cooling occurs much more slowly than in a small , thin-walled electric kiln . However , it ’ s easy to fire down an electric kiln , either manually ( in a kiln with switches ) or automatically in a computer-operated kiln .
Most of the characteristics that we describe as “ richness ” or “ complexity ” in a fired glaze are actually formed in the still-liquid glaze during the cooling cycle . If the glaze cools too quickly then there isn ’ t sufficient time for these changes to occur and the glaze can appear bland . During the cooling , glazes can form a wide range of microscopic crystals that result in matte surfaces , visual textures , and changes in color . There are even some beautiful affects that are the result of the separation of two different liquid glasses ( imagine the separation of oil and vinegar in a salad dressing ). These changes occur during the first few hundred degrees of cooling , while the glaze is still soft enough for its constituents to reform into new structures . In a low-fire kiln ( cone 04 ), the changes tend to occur between about 1940 ° F ( the approximate temperature of cone 04 ) and 1400 ° F . In a medium range ( cone 6 ) firing , most of the changes occur between 2000 ° F and 1600 ° F .
I first started doing this to my lowfire pots in the early ’ 90s , long before
I owned a computer-operated kiln . Once the kiln setter tripped I lifted the tab and pressed the button to restart the kiln . I then turned the three kiln switches to low and let it fire an additional two hours . A low setting wasn ’ t enough power to keep the kiln from cooling — it simply slowed it down . That change alone made the surfaces far richer .
In a computer-operated kiln you can simply program an additional cooling ramp , or you can choose a temperature within these ranges and program the kiln to hold that temperature for a period of time . Choosing a different temperature for that hold can result in very different glaze qualities .
It ’ s not at all unusual to hold a glaze for one to three hours . Like thickness , you never know what too much is until you ’ ve seen it . Not all glazes are improved by slow cooling or a long hold , so you ’ ll always need to test before instituting a wholesale change in your practices . A very long hold — or a hold at too high a temperature — may cause a matt glaze to become too rough or abrasive . On the other hand , a hold at too low a temperature may do little or nothing . Like glaze thickness , the best results will come from testing and experience .
In the next issue I ’ ll continue this subject and provide you with additional opportunities for achieving rich and beautiful surfaces from an electric kiln . As always , you can reach me via e-mail at ppinnell1 @ unl . edu . [
Peter Pinnell is Hixson-Lied Professor of Art at the University of Nebraska at Lincoln . E-mail him at ppinnell1 @ unl . edu or contact him via his Facebook page at www . facebook . com .
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