Monday, June 4, 2012

More Bronze Metal Clay Firing Experiments and Kiln Testings

Second test batch still over-fired. (But I just found something online to possibly explain this… argh!)

At first I thought my copper hadn’t sintered (which REALLY confused me… if one can over-fire and one can under-fire at the same time???), but then I realized my impromptu sintering test was dumb.

As you can see in the “before” photo (the rectangle piece that is copper outside, bronze inside), I decided to apply pressure to that ripped corner (after firing, I mean) and sure enough I managed to break it off. I assumed this meant the copper hadn’t sintered, but then I read back on some of Hadar’s blogs and see that:
1) It is VERY possible to break fully-sintered metal clay pieces by hand or with pliers (jewelry doesn’t need to be indestructible… just healthy enough to serve its purpose as jewelry)
2) Sometimes the inside of fully-sintered pieces still looks powdery

Besides, I took that copper piece and used my grinding tool on it and it was copper all the way through without any patches of powder. So in my book, fully-sintered.

The set-up…
Media: bronze and copper separately and together, all pieces 6 cards thick
Kiln: top loading brick
Firing Container: round fiber firing container, no lid
Charcoal: coconut shell carbon, acid washed
Phase 1 Firing: full ramp to 1000 external pyrometer reading (kiln setting 870), hold for 2 hours (then opened the kiln and removed the bowl to cool)
Phase 2 Firing: full ramp to 1450 external pyrometer reading (kiln setting 1340) (although this is debatable), hold for 2 hours (did not open the kiln directly after firing)

I did notice, however, that my charcoal seems to be retaining its heat for a long time after the kiln is off. I didn’t hear the kiln beep (signaling the end of phase 2), but I went out about 2 hours after it was probably finished and when I lifted the lid saw that the charcoal was still bright (bright!) orange with extreme heat.

I’m suspecting that the over-firing has something to do with switching from a steel container to a fiber container.

And sure enough, I just found this on a website: “The manufacturer of the Fiber Container recommends an alternative firing method of firing without the lid and 25° F cooler than specified for the clay type. Container should be removed from the kiln immediately upon completion of the firing cycle. “

Here are the before firing, after firing and steel brushing photos.

It turns out I can sand the blisters and pitting down on the bronze (which also revealed that it was fully sintered), but that’s not an ideal situation as not all shapes of jewelry lend themselves to vigorous sanding.

I’m also spending a huge amount of time going back over Hadar’s blog… like practically from the beginning, because I have questions that I don’t know the answers to.

When Hadar introduced her Quick Fire bronze and copper clays, it was only a one-phase firing. But in the latest instructions, I see nothing that has a one-phase firing. I’ll continue reading the blog (in chronological order) to see if things changed along the way.

Round 3…

The set-up…
Media: bronze and copper separately and together, all pieces 6 cards thick
Kiln: top loading brick
Firing Container: round fiber firing container, no lid
Charcoal: coconut shell carbon, acid washed
Phase 1 Firing: full ramp to 950 external pyrometer reading (kiln setting 875)*, hold for 2 hours (then opened the kiln and removed the bowl to cool)
Phase 2 Firing: full ramp to 1380 external pyrometer reading (kiln setting 1300), hold for 2 hours (did open the kiln directly after firing)

(* Yesterday, when I fired at 870, I had an external pyrometer reading of 1000. Today, firing at 875 gives me an external pyrometer reading of 950 degrees. I wonder if it’s because the ambient temperature of the day is about 30 degrees cooler.)

I also raised the box about a half inch and put the pieces back to horizontal layout per Hadar’s online instructions.

Still mega pitting (and shrinkage). Still looks like over-firing, right? And yet phase one was at 950 (even accounting for the fiber box would only suggest I fire at 970, so still well below the worry zone). And phase two was 1380 which is way below 1470 (1440 adjusting for the fiber container).

This next photo shows you how I know my pieces ARE being sintered (which is one relief, at least). I’ve sanded down (you can tell how far down because in the bronze circle, I’ve sanded past the pitting!) and everything is still intact. All signs point to proper sintering.

The circle with bronze in the middle and copper around it looks different because I added Baldwin’s patina so you can see the difference more strongly between the copper and the bronze.

I think I’m going to try Hadar’s stove-top method for phase 1. Then I won’t have as many kiln factors to contend with. Kiln will only be for phase 2.

So I picked up a camp stove burner for $9 at K-Mart. May try this later today.

So my latest questions are:

1) How fast (or slowly) do I cool my items after phase 2? Do I open the kiln lid as soon as the holding time is up? I feel like I should because these fiber containers seem to stay hot for so long. But I don’t want to risk the integrity of the pieces.

2) To fire phase 1 on a stove-top, I’m supposed to put a hole in the lid of my fiber box. I can’t see where you can buy replacement lids for these so I’m hesitant to do that. The box is $26. But now I can’t remember what else I’d be using it for, so if I’ll always only be using it for base metal clay and I need a hole in the lid… I guess I’ll put a hole in the lid.


  1. I share your pain. I too am having problems with pitting and am able to snap the pieces even though they appear to be fully sintered. I have tried the camp stove, changing the temps, ramping more slowly and nothing seems to help. The white bronze and copper are giving me the same problems. i am wondering if it might be something I'm doing in the mixing. This is super frustrating. Any insight would be appreciated.

  2. I have been working with Hadar's clays for a very short time, but just as you have, I am having terrible problems with sintering and i'm not sure where in the process i'm off. Is anyone here still trying to figure this product out and if so, would you be willing to share your info with me???

    Thanks, Tracy

  3. Tracy, for me, it's not that I'm getting bad results. It's just that I want to make sure I'm getting the best results I can be getting. Also, by doing my own experimentation, I'm learning more about the process which helps me when it comes time to troubleshoot.

    Here, I've started running the tests again, just to fine tune the process in conjunction with my own kiln (each kiln has its own particular quirks).

  4. This is very cool! Thanks for the information!