Alumina Hydrate

Alumina Hydrate

I make a lot of PMC pieces that have various thicknesses. For example, this Raven.

13-802a 3

As you may know, firing a piece with various thicknesses can warp or change shape as they are fired in the kiln. The metal clay shrinks to the area of highest mass or density, which means it does NOT shrink evenly or consistently. And, it can be a challenge after the firing process to flatten a bent-out-of-shape, multi-layered piece.

I have discovered if I sink my pieces in alumina hydrate that they remain pretty much their original shape. 

The trick to alumina hydrate is you need to bury the entire piece. If you don’t you get twice the warpage. Bury flat pieces flat and rings or rounds things evenly north and south or east and west, doesn't matter. When you open the kiln door you'll see little chimneys where the burnt off binder and steam came out.

Al(OH)3 or hydrated alumina is a fine granular white powder that is typically added to glazes as a source of alumina. I’ve read about a kiln wash using alumina hydrate with water. Then I’ve read that one should NOT use alumina hydrate for washing electric kiln shelves because it radically shortens the life of the kiln’s elements. But, here I’m talking about using it dry and not as a wash.

RioGrande sells “PMC Alumina Hydrate” with the following description: "Alumina hydrate is a material ideal for supporting delicate pieces during firing--especially wet pieces with curved surfaces. Package contains 1 lb. of silica.” Wet pieces? What?

Alumina Hydrate Rio

Photo from RioGrande’s website

Firing 1

I use alumina hydrate in these three ways. 

The part of the piece buried will be heated longer because the alumina hydrate maintains the heat longer than air; therefore, that part may be smaller that the other parts. If you want your pieces to fire and sinter evenly and are burying them (see below), place them north and south or east and west or in the needed orientation so shrinkage is uniform to the most important direction. Or bury them deep in the alumina hydrate. See the sunken charms above.

Slight Curvature
An example of using alumina hydrate for curvature is this: I have earrings with a lot of small detail on them. I put the detailed front of the earring up and place them in the alumina hydrate. As above, the part of the earring touching the alumina hydrate will hold heat longer, shrink more as the metal sinters, and this results in a nice curve to my earrings. Remember, metal clay shrinks in the direction of the highest mass, when you are thinking about curvature.

Maintain Shape 
For pieces that I do not want to warp, I bury them in it (see below). I stir the alumina hydrate with a toothpick. I dig a hole or flat area and place the piece in that area. I might gently flick alumina hydrate on the piece. More often I tap the container so the surrounding alumina hydrate lightly packs around my pieces. I have made some humongous pendants of varying thicknesses and they have all come out in the same shape as they were when I put them in the kiln.

Alumina Hydrate

My container holds a lot of alumina hydrate.

Alumina Hydrate

I set the container on kiln stands and fire away.

I have two buried Wolf Pendants in this container. This is what the container looked like after being fired.
You can see two little chimneys and some cracks where the burned off water and binder escaped.

To find my buried pieces, I take a tool and search for them.

Alumina Hydrate

Here is a Raven pendant similar to the Wolf pendants above that just came out of the kiln.
I’m holding it so you can see the different thicknesses and how straight it remained.

Alumina Hydrate

Just another view.

I will caution you on this: wear an N95 mask when you brass brush the silver, especially if you use a brass brush on a flexible shaft or Dremel. I do not think it wise to breathe the fine dust of alumina hydrate. Also wear eye protection.


In view of comments, here is additional information.

If one wants to prevent dry-clay pieces from joining or fusing in the kiln, most metal clay artists simply keep them a distance apart. The truth is, even if they were touching and stick together at first they will break apart in the finishing process or with a little pressure. If you want to make something like a metal-clay chain, you could make all the links and fire the chain. In this case, it would be a good idea to sink this chain into alumina hydrate, which would further prevent any links from sticking together. 

How long does alumina hydrate last? I think it is a pretty inert material. I have used my batch for over ten years. And I would guess I could use it another ten years.

Here’s a biggy. I make these road-trip items, as shown below. Recently, instead of firing with the roads up, I put the road side down and pressed these into alumina hydrate. Big mistake. I think what happened was the tiny pieces of the stripe in the road were either pushed to the side or pushed off the earring. This happened despite my close attention to the process of adding wet clay to dry clay. See free course at I Love Silver how I repaired these. 

The lesson here is if you have delicate syringe work, do not be pressing the syringe or delicate parts into alumina hydrate. You can bury pieces gently (see above), allowing the alumina hydrate to go around the piece by gravity as you gently tap the firing container.


Missing road stripes

Here is the Materials Safey Date Sheet (MSDS) on the substance, in case you are tuned into MSDSs and want to know everything about alumina hydrate.

 For online courses in metal clay, go to I Love Silver, where you learn how to design and create your own silver creations.

© Kris A Kramer 2018