Convergent Series

learning, using & teaching metal clay, and other aspects of life

Foil-Firing Base-Metal Clays

Posted by C Scheftic on 2011/07/06

One question that seems to keep recurring (everywhere from individual conversations to the big, international Yahoo group on Metal Clay) involves what people use for firing their various copper, bronze, and / or steel clays, so I decided to write a bit about what I’ve been doing.

At first (~2008) I used the stainless steel “steam table” pans that were the original suggestion for this. The clays fired OK (i.e., the metals sintered), but the pans flake black crud. (I’ve seen hints that the technical term is that they “spall,” though I know that word with a slightly different connotation, so I’m not sure…) Anyway, it’s not a crisis, but cleaning it up is just one more little thing to tend to, and I’m seeking to simplify this process as much as possible.

Later on, a number of ideas for other, alternative firing vessels started to circulate. Some people fire in used metal cans (i.e., reusing the kind food comes in, which would mean having the inner plastic linings burn off as you fire them; and, while cheaper than the steam table pans, they still flake). Others suggested building vessels out of fiber blanket. Neither of those held much appeal for me: I never tried either one.

Last winter and spring, Hadar Jacobson blogged about several other options, such as building a frame out of kiln posts, drilling an opening into firebrick, and building a box from ceramic cloth and T-pins. I tried the first and third of those. The posts are easier to find, the cloth is easier to use, but neither quite fit my “simplify” goal. Hadar also talks about working with several options in the instruction manual she provides. (Aside: Her manual is useful even if you’re using other copper or bronze clays! You may have to adjust specifics of the firing schedules to fit other products, but Hadar does a great job of explaining in a simple way what’s going on, what you want to have happen, and what might be going wrong if you encounter problems.) In the past year, also, several manufacturers came out with a range of fiber or ceramic firing boxes, but at least the ones I investigated appeared somewhat high-priced to me. (Or, perhaps a better way to say it is that the ones I checked seemed high for my budget for this, so I just stopped hunting. If you have found any well-priced ones, do let me know!)

Right after Hadar was here late last winter (when I gained motivation to do more with these non-precious metal clays) I decided to try something Hadar had not discussed, and to invest in some No-Flake Firing Foil. (I got mine from CoolTools.) That’s what I’ve been using quite happily now for the last few months. It does take a little fiddling-with before the first time you use it–you do have to fold it into the box shape–and then you should fire some test pieces to verify the temperatures to use–which you should do with any new firing vessel you try (or new kiln, or new carbon, etc.). But after that, this kind of box is both very easy and much more affordable than most of the other options.

The first photo with this post shows a newly-contructed firing pan before its first use. The foil comes with instructions on how to fold this particular box, and there’s a video available on the product-page. From my (somewhat basic) knowledge of origami boxes, this does seem to be a pattern that yields a relatively large-volume basic box from a given amount of material, so I did not try to second-guess the instructions there. But I will note two things about the instructions….

(1) They provide finished dimensions for various sizes of foil one might start with, but there’s no reason to limit yourself to just those. Make a box of whatever size will fit the foil you have and the size of your kiln. (Be sure to leave room for air to circulate all around the box!) Try a few paper models first, if you don’t already have experience folding boxes, so you see how it works and get a sense for the size. But, here’s the trick: the instructions list only sizes for rectangular boxes because that’s what you want to build. Not a square!

Any rectangular box will have a sort of “flap” of material that gets folded over the short edges and part-way around the long ones. (You should be able to see it on the photos with this post.) A square one won’t have that flap. You want the flap for two reasons:

  • Those “flaps” seem to increase the stability of the box, and
  • You can fold up the corners of the flaps on one long side of the box to mark the “front”–something that’s useful when you’re putting a carbon-filled box into your kiln (and especially important with front-loaders, because you don’t want to position any pieces along the un(der)heated front edge).

(2) The instructions and video use the traditional origami trick of making two folds at the very start that you just open back up again. They simply mark the center of the sheet. If you can find and mark the center-lines yourself, you can start with that rather than those two folds. That’s why my “new” box (above) has black ink lines, rather than folds, down the center. Why does that matter?

Well, some people report that these boxes only hold up for a couple of firings. Mine have held up much longer. The one shown here has been through several dozen two-phase firings, has not been treated with any particular care, and seems to be holding up just fine.

When pressed for more information, those reporting early failures say that their boxes seem to fail along the folds. Not necessarily the center ones in particular, but along folds somewhere. Now my theory is that they are not failing after, say, two or three or four firings: I think they are beginning to fail with their very first use, but the problem only gets big enough to see after several more. If you put a hole in the foil while folding it, it will get bigger with each firing, through the heating (expansion) and cooling (contraction). So the trick is to not put holes in the foil to begin with!

Thus, I chose to not make those first two folds, which must then be reopened. You’ve got to mark the center line accurately or the box won’t come out with everything lined up right, and doing it via that fold is an easy way to mark it. So either be careful folding and unfolding those lines or, if you can find both centers another way, do that.

Then, proceed with the rest of the instructions. Crease smoothly, but not so harshly that you rip little holes in the edge. Unfold smoothly too, also with care.

And, then, enjoy the treasures that emerge after being fired in such a box…

Please leave a comment if you’ve found anything useful in this post! It’s great to hear from readers. (I can see from my “blog stats” that you are out there! But comments offer even more motivation to keep on writing these notes….)

UPDATE: This box lasted for seven more months of regular use! Since this topic keeps coming up, and I keep pointing folks to this post, I’ve decided to add a link to my follow-up post.

11 Responses to “Foil-Firing Base-Metal Clays”

  1. Alice Walkowski said

    You must be spending lots of time in the studio. You’ve been doing so much Carol. Great looking piece and informative blog. I’ll have to try it


    • C Scheftic said

      I haven’t been spending as much time in studio as I’d like: to do that I simply need more hours in each day! But, off and on, I have spent a bit more time at computer, so at least I’m catching up on a few things I’ve been meaning to write about.

      If / when you try the foil, do let me know how it seems to work for you.


  2. Anthonia said

    Hello Carol,

    I found your blog by chance while trawling the net and I’m happy I did. Your pieces are very lovely and I particularly love the beads.

    I’m a metal clay artist by night and have only used the no-flake firing foil to make 3 boxes which I use to fire FF bronzclay. I must say I followed Mardel Rein’s instruction to fold and I’ve not had any problem whatsoever with the boxes. I completely agree that you can make any sized box subject to the foil as I made a bigger (taller) box than in the instructions.



    • C Scheftic said

      Thanks so much, Anthonia.

      I hope we’ll hear more from you here in the future. Maybe even see links to some of your pieces somewhere? I can’e help but be curious about what you’re firing in that even-taller box!



  3. I’ve used the foil box for FF Bronze, but have some of Hadar’s clay on hand as well. What firing schedule have you used in the foil box with Hadar’s clay. (I’ve never taken a class with her.)



    • C Scheftic said

      Hi, Becca:

      FYI, I worked quite happily with Hadar’s clays for a couple of years before I ever managed to take a workshop from her! I just started out using the firing schedule from the instruction manual you can find via a link on her blog:

      I don’t post the schedule here because Hadar does ongoing testing and refinement of it. She updates the manual regularly, and I just keep following her suggestions. The real key to success I’ve found is to do pretty much what she says [see note below] about testing your kiln, and then adjust a bit as needed. I have two kilns, and I modify the schedule details a little bit differently for each of them … based on the results I got from following her suggestions during testing.

      With the no-flake foil boxes, the difference between kilns seems to be a bigger factor than the box itself in adjusting the schedule. And I know nothing about your kiln….

      [Note:] Hadar makes quick little “throw away” test pieces. The only thing I do differently is that I spend an extra minute or two making a quick little “charm” element to test with. If it fails, I’ve not lost much time (especially since one shouldn’t have to test very often!). I it succeeds, instead of just tossing the test piece, I have a little piece to use as a gift, bonus, exchange, etc.

      Happy firing!


  4. Thank you for a nice and nerdly post about what sounds like a very sensible and affordable solution to the firing vessel.


  5. Stephanie said

    I just purchased the no-flake foil from CoolTools. Thanks for your and your “commenter” for the information on making a box from the foil. We want to be sure to not contaminate the kiln which is also used for fine silver enameling.

    Stephanie, San Diego Mineral & Gem Society


    • C Scheftic said

      You’re welcome, Stephanie.

      Although the no-flake foil avoids the problem of the steel containers that “spall” black bits into the firing chamber, it’s still possible you’ll end up with some dust, ash, and what-not in there. If you’re worried about contamination, I’m sure you know to vacuum it out occasionally anyway.

      Happy firing!


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