Fresh out of the second (sinter) firing, this Gold Bronze Ultimate Premium clay acquired a really gorgeous patina. Too bad it had to be polished off…

Ros Bailey of Aussie Metal Clay asked me to test their newest base metal clays, Gold Bronze Ultimate Premium and Gold Bronze Origami. She sent a package of each all the way from Australia about a month ago, and asked me simply to play with clay and let her know how it went.

I’ve been a born product reviewer since I first read Consumer Reports at age 6. This isn’t news to anyone who’s followed my glass art posts over the years–I’ll bet you guys are rolling your eyes–but Ros couldn’t possibly know she was feeding a lifelong habit.

A few years back, when I made friends with a wheelchair (thanks to The Leg), I desperately needed something to DO. I’d moved into my mom’s house, away from my beloved glass kilns and sculpting studio because I needed one-level living. I needed something smaller-scale that could be set up (fairly) easily in Mom’s kitchen. I remembered my long-ago forays into metal clay…

comparing gold bronze metal clay

The three test clays: Aussie Ultimate Premium, Aussie Standard Premium, Aussie Origami, all in Gold Bronze.

Bingo. Bought myself some clay and a tiny kiln (an Evenheat Kingpin 88), and fell in love.

I’ve used quite a few metal clays since then, and made every newbie mistake. There are a surprisingly large number of metal clay manufacturers out there, from the original PMC (Mitsubishi), Hadar, Goldie, Prometheus, Aussie Metal Clay, Metal Arts, and CoolTools’ EZ line. Each has (a lot of) strengths; more than a few have some pretty glaring weaknesses, or at least eccentricities.

These clays come in gold, silver, iron, bronze, brass, copper, and probably every other metal on the planet if someone actually wanted it. They’re essentially finely powdered metal mixed with an organic binder and water, making what I’d call an almost-clay. Almost because, while they CAN be used like a potter’s earthen clay, they’re about as sculptable as silly putty when wet.

Dry, they’re another story. You form a rough shape when wet, then dry them to leatherhard. That’s when you add detail, sand, burnish, and refine until you have the shape you want, and fire. The binder and water burn away, leaving you with metal that “sinters,” or compacts and coalesces into a solid piece of metal.

Copper and red bronze clay pendants, ready for firing. I used the wrong charcoal on these…

…and wound up with little crumbled mounds of dirt. That’s what happens when you don’t follow the manufacturer’s directions (or make a test tile).

That’s how it works for silver clay, anyway.

Fine silver and sterling clays are remarkably forgiving and very much ideal for beginners. You simply make your piece, allowing for the inevitable shrinkage, and fire it into solid metal with a single, open firing.

I’ve only had ONE silver clay failure out of hundreds and hundreds of projects (I wish I could say that for glass). That time, my kiln ran amok, turning my nieces’ carefully crafted pieces into silver blobs covered in vermiculite. Naturally, it WOULD happen when I was firing someone else’s work…

This piece from a 3-5 card thick piece of 999 silver clay, was made with this cutter. Note how much the fired size has shrunk.

Silver clay is easy, but it’s also an expensive way to learn the medium, and it will lure you into a false sense of security. Base metal clays are much MUCH easier to screw up. They typically require two firings:

  • The first firing, usually to around 700F with a hold, burns out the binder and leaves you with a fragile-ish chunk of not-quite-sintered metal that closely resembles charcoal.
  • The second firing (usually at somewhere between 1450F and 1550F, a bit cooler than most silver clays) sinters the remaining powdered metal to form a solid piece that can be ground, bent, and polished.

The sinter firing is almost always done in a reduced oxygen atmosphere, created by embedding the pieces in carbon, ie., fine grains of charcoal created from cocoanut shells or similar.

That carbon is NOT your average aquarium store “activated charcoal,” either. It’s best to grit your teeth, follow the manufacturer’s instructions, and buy a supply of their recommended charcoal. At the moment, I have three different brands in my studio, each for different clays.

So I use the right charcoal, and make test tiles for every new clay I use. I’ve learned from sad experience that if I don’t, I’m going to wind up with dirty crumbles instead of a solid metal sculpture. First step in testing Ros’ new clays: Make some sample tiles.

Making test samples

I roll out and texture sample tiles on the surface I’ll be drying them on overnight, to prevent distorting the soft clay.

My test process is pretty simple: Make sets of textured rectangular tiles of a specific size and thickness, let them dry naturally overnight, then fire the first set according to the manufacturer’s directions. I’ll vary time and temperature when firing the rest of the sample sets, helping me determine what works best for my kiln and methods.

After cutting and drying the metal clay will be slightly smaller than the original cutter. The “IB” stamped into the tile identifies the type of clay being used (in this case “Iron Bark”).

First, I roll out my clay to an even 5-card thickness, or about 1.25mm,* rolling out on the surface that will be used to dry the clay so that I won’t distort anything by moving it while still wet. I’ll use a wavy texture roller to re-roll the clay to 3 cards, or 0.75mm,* so that the thickness varies between 3 and 4 cards. My pieces are rarely smooth and even throughout, so this gives me an idea of how variable thicknesses will work with this clay.

Once I’ve rolled and textured, I use a cake cutter to make a precise rectangle, measuring exactly 16x26mm, for a total surface area of 4.16cm.

A group of test firings. The textured rectangles show color and shrinkage rates. The two-toned ovals compare compatibility of different clays (here, the base metal combined with silver). If the clays are compatible in a mixed-metal piece, the yin-yang shapes should stay attached. If they split apart, there may be an issue with different expansion/contraction rates that will make it difficult to use those metals together.

The clay will shrink a tiny bit (usually) in the overnight drying, but the real shrinkage will come during firing. Metal clay typically shrinks anywhere from about 6 percent to as much as 25 percent, depending on a LOT of factors.

Manufacturers usually post expected shrinkage rates in the 10-12% range, but my tests show a lot of variability in those rates–I go by what I’ve observed in my own studio.

I’ll make two identical tiles per clay type/firing schedule, fired in the same batch. That’s intended to catch any variability in either my process or the way the clay behaves in the kiln. If the two tiles have been created and processed in exactly the same way, they should end up the same size/shape/color. Sometimes, they don’t.

Metal clay shrinks a bit when drying and a lot more during firing. The wetter the clay, the more it seems to shrink. When your piece is too thin, the sides can shrink more than the corners, called “dogboning.”

Once firing is complete, I test the tiles for stability by bending them a bit and then pounding them flat on a steel block. A well-sintered piece should flatten without cracking or breaking. I look for flaws that might be caused by contamination or air bubbles, and check for dog-boning.

Dog-boning is where the straight sides of a piece contract more than the corners. It is usually caused by the clay being too thin for its footprint; the clay will shrinks into itself, with the shrinkage more pronounced on the sides than in the corners.

Lump or powder?

Thoroughly dried test tiles have had their burnout firing. Now for the sinter firing. Tip: Photographing your tests with notes makes it very easy to keep a record of your firing trials.

Both of my test clays arrived in lump form. Aussie is one of the few metal clay manufacturers that lets you purchase metal clay in either lump or powder form. Even if it wasn’t cheaper to begin with (which it is), powder is probably more economical in the long run. You only mix up exactly what you need and don’t have to worry about the clay drying out and needing reconstitution.

Mixing powdered clay, however, can give hit-or-miss results. A lot of factors go into making usable clay from powder, including the humidity when you make it, whether or not the powder has been sitting out, unsealed (in a humid environment that means the powder can absorb more water), and often, who’s doing the mixing.

It’s extremely easy to overhydrate the powder; if you mix in enough water to immediately get a clay-like consistency, it will wind up a goopy mess. Easy enough to fix–just let it sit, loosely covered, for a few days until it’s dried out a bit, then knead with your hands. Or, if you have more powder on hand, mix in more.

Me, I mist distilled water over the clay powder with a cheap spray bottle. I mist just enough to coat the top surface, let it sit a minute, and then try pushing the powder into crumbles.

For one of my Ultimate Premium samples, I covered a tiny 2-inch crucible with a thin layer of clay, then built up texture and added a foot to make a bowl.

If it doesn’t happen, I mist a few more times. Then I roll it up in plastic wrap, compress it into as neat a ball as possible, and seal the package. I leave it that way overnight, which allows the water to evenly penetrate throughout the clay. In the morning, I have a lovely, workable clay.

Think “making pie crust” and you have exactly the right technique for powdered metal clay.

Now, a lot of folks CAN mix and use powdered clay immediately, if they’re very careful with the water. I’m not one of them; every time I’ve tried it I’ve ended up with something that closely resembles the gloppy slime toy I had as a kid.

The wetter the clay becomes, the more shrinkage and texture issues it can have as well. Even with a correct firing, overwet clay can leave the surface of your piece looking like a dried-out lakebed.

Lump form is premixed; you simply pull it out of the package and use it. I find it’s not only ready to go, it’s also more consistent in texture and shrinkage from package to package. So my strong preference is for lump form.

Ultimate Premium

Both clays arrived in lump form, which was great. I received 50 grams of the Ultimate clay, enough to make two good-sized pieces and a fair amount of slip (clay with enough water added to resemble thickish cake batter–you use it to glue pieces together, repair small cracks, etc.)

I tested both clays against my own 100-gram package of regular Gold Bronze clay, the older type from Aussie.

The Ultimate Premium Gold Bronze was, ultimately (heh-heh) my favorite of the three Gold Bronzes I tried. It had the nicest consistency, and worked pretty easily. The lump form seemed a little dry–rolling and flattening a 1-inch ball of freshly-opened clay tended to open some cracks–but it was the least sticky of the three, and the easiest to work.

I formed the bowl over a little crucible that I’d dusted with cornstarch. Getting it off was still a bit of a hassle, so here I’m repairing scratches with a bit of slip.

I used it to form a 2-inch bowl with a smooth interior and a patchwork of textured paisley shapes. The pieces stuck together easily, and I was able to roll the clay down to 2 cards in thickness (about .5mm) that was still workable.

I let it dry in air for about 10 minutes before trying to pick up it up; that little time was enough to dry the clay surface and set it a bit, so it held up well.

bamboo pendant in Aussie Metal Gold Bronze clays

Bamboo pendant, about 3 inches tall, unfired. Stems and frame are made of Ultimate, leaves and decoration of Origami. Note the rough texture.

Working the wet clay is only half the battle, though; most metal clay artists form reasonably rough shapes in the wet clay, let it dry, and do most of their work by carving, sanding, and adding more clay or slip to the dried piece.

Ultimate retains just enough flexibility to be carveable, but is fairly soft and grainy. It didn’t take detail quite as well as a good earthen clay at the same “leatherhard” moisture level, but it certainly supported the detail I needed to add.

Leatherhard earthen clay is a dream to carve and very much where I’m comfortable working, and the Ultimate did well there. It’s not quite as carveable as EZ960, but is the closest I’ve seen, and extremely workable.

I tried combining both Ultimate and Origami clays in a second piece, a series of bamboo poles and leaves set up as a pendant. I made the stems and mountings from Ultimate, again, they tended to look a little grainy and rough when just built, but a few swipes with sandpaper resolved that well.

The Ultimate bowl showed a couple of small cracks in the lining (interior) clay near the rims. This likely was because the clay was a little too dry when I joined the darts I took to get the bowl shape in the initial sheet, so that they opened back up during firing. As a further test, I mixed up some ClayStay/Ultimate slip, painted into the cracks until it stood proud of the surface, then went through the entire firing process again.

Working with Origami Flex

Forming a small vessel on the back of a spoon

Origami Flex; a small bowl formed on the back of a stainless steel scooper. Although the wet clay is sticky, after an hour in the dehydrator the sloped shape allowed the piece to literally fall off the spoon. Note the “sandy” appearance of the wet clay–most of that can be sanded smooth when it dries.

My Origami sample also arrived in lump form, although I only had 25 grams of this one so the projects had to be smaller. It proved both stickier yet faster to dry out than Ultimate Premium, but still handled very well.

It’s advertised as a clay that allows you to roll it into extremely thin sheets; I was able to get usable pieces only 1 card thick, so that’s definitely true. That thickness will probably dogbone significantly if you try to fire it by itself, but thin, stable sheets of clay are a godsend when you’re trying to create layers and depth without creating a thick monster.

I made a bowl in Origami, an oval, rather primitive coiled pot mounted on a big stainless scooping spoon.  It was far simpler than the Ultimate bowl, mostly because I had much less clay to work with.

I smoothed an oval base onto the back of the spoon, smoothed it down, and then built it up with coiled snakes of clay.

The snakes were made by putting the Origami through the extruder, and it worked beautiful. This is a sticky clay, though, so I found it helped to start with a cylinder of Origami lightly rolled in some cornstarch to make the extruder move the clay more easily.

comparison of fired aussie metal gold bronze test tiles

Test tile firings showed differences between the clays in shrinkage, color, and texture.

Such thin snakes dried out fairly quickly, but still remained workable long after I thought I’d get cracking. Brushing the coils with a little distilled water helped. Like most metal clays, the line between “keep it moist” and “slime” is very thin and easy to cross; too much water, and you’ll find yourself trying to sculpt a puddle.

Like the Ultimate, this clay can look very rough and grainy–almost sandy–when just formed, and it tends to break off in smaller crumbs when carving. Very fine detail (such as the eyes and lips on a centimeter-wide face) can be a little frustrating to carve–I developed a technique of carving a bit, using a tiny brush with slip to refine lines, and then lightly carving again to get the precise detail I wanted.

Workability-wise, either of these new clays beat out the 100gm package I already had for sculpting. They’re stronger and more forgiving of the carve-dampen-fill-carve process I employ. Although Origami would be the obvious choice for sculpting, though, I much preferred Ultimate for that task.

Firing results

Firing was entirely without incident (yay). I don’t torch-fire because a lot of my work is large enough to make torch-firing an exercise in frustration. Instead, I fired these in my PMC kiln.

The fired test tiles showed acceptable metal (in my kiln) with a final firing at 1490F for 2.5 hours, but maximum strength for grinding, forming, and polishing by holding at that temp for 4 hours. With either method, I was able to create sturdy metal tiles that could be bent and hammered back to flat without cracking. I brought them up to a 600-grit polishing wheel, and they buffed up nicely.

polishing a fired metal clay vessel

Polishing the Ultimate Premium clay on the JoolTool. It was a little heartbreaking to polish off that lovely patina.

More surprising, though, were their textures and colors. All three clays were “Gold Bronze,” but each had a distinctly different color. It was most pronounced before polishing, but there was still a noticeable color difference in the finished tiles.

The Origami tiles exhibited a warmer color with a smooth, very polishable texture. The Premium color was a bit cooler, but it shrank the least of the three clays and had the finest final finish.

bottom of the Aussie Gold Bronze paisley bowl

Good detail shot of the textures in the paisley bowl: Aussie Metal Clay, Gold Bronze Ultimate Premium

The color of my older Premium Superflex clay was right in the middle, but it had other issues: The tile developed lumps or bubbles, so that the surface (see photo) looked lumpy. I’m not sure what happened there–the bumps are clearly arranged in a line across the center of the tile, and to the best of my knowledge there wasn’t anything under/over the tile that would have caused it.

The older clay also exhibited the most shrinkage, about 35% of total surface area. It also exhibited some dog-boning, which tells me that this clay wants to be thicker than my standard 3-5 cards.

The Premium shrunk the least, about 31% of total surface area. Neither the Premium or Origami exhibited dog-boning.

fired bamboo pendant

Post-firing, it looked a bit burned