Open Shelf Firing Base Metal Clays by Martha Biggar


I recall so clearly when I first heard about metal clay, back in the late 1990s, in the Rio Grande catalog. I thought I might like to work with it, and took my first class in 2000 (a two-week stint at Arrowmont in Tennessee, with Linda Kaye-Moses). I also recall, equally clearly, hearing about and then using Metal Adventures’ (Original) BRONZclay, when it came on the market in 2008. Such an interesting and different take on metal clay!

And look at us now: we have several versions of silver clay, plus a multitude of base metal clays, with more coming. What Bill Struve started experimenting with in 2006 has grown into an international community of inventors and users, with clays coming from many parts of the world. While I am personally far from a scientist, I do have an inquisitive nature that wants to know many things about the materials we use.

Most of us are aware of some differences in the base metal clays, like color and shrinkage rate. I’ve taught many classes that help others get the feel of different clays, but never went beyond the basics where firing is concerned.   This is the subject of my latest set of experiments, and this article deals specifically with open-shelf firing of base-metal clays; torch firing is the subject for my next major experiment and another article.

Let me explain exactly what I mean by open-shelf firing. This is not the stage one/open-shelf-to-burn-off-the-binder type of open shelf that is preparatory to carbon firing, this firing takes the metal to its peak temperature, holds for a period of time, and then quenches in water. photo 1This photo shows the stainless steel grid I use for this process.

Note: two edges are bent down to create “legs” which allows an enameling fork to be used to place and remove the steel during firing. This eliminates any need for stilts or other kiln furniture and adds stability to the shelf.

Why all the interest in this method? It’s another way to fire that, while requiring a kiln, does free up time in the kiln, and for someone who’s working with just a piece or two at a time, it is a perfect way to utilize the almost “instant gratification” that this technique offers. Also, unlike torch firing, which does take practice, open shelf allows even beginners to succeed with firing.

Also, since I see so many similarities between base metal clays, I wondered if (since Art Clay Copper and Prometheus both utilize this process) other metal clays can be fired in a similar manner. I tested, and I found I was right. The only difference in firing many types of clay is the top temperature, as in firing with carbon.

All my test pieces were made in the following fashion.  All clay was rolled and I used a set of rolling slats to ensure that each piece will be the same thickness, I rolled each type of clay into a slab 1.2mm thick, then removed .2mm and rolled in a texture using the same photopolymer plate each time. My final thickness is 1mm, which is similar to 4 playing cards in thickness. I cut each slab into a circular shape with a cutter, and let each piece dry until I could flip them and scribe the brand of clay onto the back side of each. I made these pieces over a period of time; each was allowed to dry naturally. I didn’t spend a lot of time sanding each circle but did knock off the rough edges.

photo 7
Although I could have made these circles perfectly plain, I decided to check one other variable in this test, that of the design flaking off and disappearing in the fire scale. I was pleasantly surprised to find that this issue is a non-starter: the thickness that pops off during quenching is negligible.

Materials used for testing: dried circle(s), stainless steel screen, heat-resistant gloves, enameling fork, kiln (preheated to the top temperature), water in steel container for quenching, long tweezers, and a heatproof surface on which to lay the shelf. Since some of the clays will fire at the same temperature, they can be fired at the same time (refer to table below for firing temperatures).

This photo shows the process of placing a hot shelf.
This photo shows the process of placing a hot shelf.
photo 4
Quenching the hot piece in water
photo 6
Testing sintering with bending pliers.

I programmed my Paragon SC-2 kiln to reach each temperature needed, then added a hold time (see table). Since the kiln temperature will drop as the door is opened when you first place the shelf, the kiln won’t start timing until temperature is reached. You must stay close, however, both to place the shelf inside when the kiln reaches top temperature, and to retrieve the shelf and quench the piece when the firing time is finished. With the firing complete, carefully remove the steel shelf with the fork, place it on a fireproof surface, and quickly quench the piece(s) in water. You will be able to touch them almost immediately upon quenching. A lot of firescale will pop off during this action; you may want to pickle the pieces after quenching if firescale is still present. Or simply brush with a brass brush.

photo 5a
Properly sintered, circles. I’ve fired multiple times and tested each type of clay to determine sintering with both bending pliers, (note that some base metals, because of their components, do not bend) and with the water drop test (this involves mixing a drop of dish soap in ½ cup water, then placing a drop on the surface of your metal, if it is completely sintered the drop will not be absorbed; the dish soap acts as a surfactant and lets the water wet the metal, otherwise it will bead up even if the metal is porous or not sintered).

And now let me offer a couple of caveats: at first I was reluctant to experiment with open-shelf firing because of the safety factor, basically opening and working out of a hot kiln. I soon realized that this is what enamellists do all the time, and that there are appropriate tools to make this a safe enterprise. Secondly, don’t quench directly into pickle, as it is an acid and can pop up and burn you. Most important of all, remember that this works in my SC2 kiln; if you try this make test pieces, experiment with the temperatures if necessary, make it work for you. So, let your common sense prevail…and enjoy the experiment.

Where will I go from here with this new knowledge? I won’t say that I will never use the carbon method again; I was partially successful in firing the leaves (in the first photo), however I’ll use carbon firings for production pieces where time is a factor, and I will have to experiment with complex forms like rings or lentil beads before I go completely carbonless. I will certainly show students how this works; for some this may become their favorite method of firing! And of course there are those new clays and ones that aren’t even out yet, just asking for an experiment. That’s for another trial…

kiln firing table Martha Biggar wwwcre8tivefirecom

To see the table larger: click on the image.

Please note that I’m still testing! New grid with updates as time permits. Enjoy and experiment, make test strips until your kiln fires these pieces accurately; these top temperatures work in my Paragon SC-2 kiln, test to make sure your kiln is firing accurately, or, add and subtract degrees as needed. You may want to increase the hold time for up to 15 more minutes but no more. ~Martha Biggar

Suggestion: wear green-tinted safety glasses made especially for those who lampwork or enamel if you open your kiln while it is red hot. 

About the Author:
marthaBetween the two, Ed and Martha Biggar have over fifty years of experience with glass, and twenty-five years of experience with metal clay. They consider themselves a team as they teach, work, farm, and live as an artist-couple in southwest Virginia.   Ed and Martha are as passionate about teaching and sharing their knowledge as they are creating new and exciting works.  Both experiment and write prolifically; both teach regularly at the Bead and Button Show in Milwaukee and BeadFest in Philadelphia, as well as private classes around the country and near their home studio.  The Biggars regularly exhibit their work, including as members of the Southern Highland Craft Guild.  Besides making time for their own artwork, the couple maintains several horses, mules, and donkeys on their farm in Draper, Virginia, along with several types of specialized produce, including figs and asparagus.
Contact Ed and Martha through their facebook site,, and watch for the unveiling of their new website, around the first of February.

3 Responses to “Open Shelf Firing Base Metal Clays by Martha Biggar”

  1. Helene Schoutissen

    I have a Paragon topload kiln and no door that opens to remove the item after firing. Is the open shelf firing a good Idea? I am a bit afraid of burning myself. Very greatful for your answer. Greetings from Sweden

  2. Sue Henry

    I also open fire Prometheus copper clay but have a terrible time removing the black oxide or firescale. Quenching immediately from the kiln is key, but even that isn’t fool proof. And pickle doesn’t seem to remove it. Same story with the Prometheus bronze. Suggestions?

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