Tip #12  Decorating with Oxides

You’ve probably heard of oxides, but how are they used?  This is by no means a comprehensive list, but some ideas to help you understand oxides better and a basis for experimentation. 

First, how does oxide compare to stain?  An oxide is a raw material.  A stain is a commercial blend of raw materials (including oxides) to achieve an accurate, consistent color.  Glazes and slips made with stain will generally be more uniform, whereas those made with oxide will be more splotchy, speckly, and variable (in other words, beautiful!).  There are 4 main colorants used.  Sometimes you will see them as oxides, sometimes as carbonates.  For example, cobalt oxide and cobalt carbonate.  The oxides are generally stronger in color and coarser grained (more speckly.)  The carbonates are generally finer grained. 

Oxides can be added to clay (mix with water then knead in), but this is expensive for deep coloring of large pieces.  They can be added to slip or glazes.  At stoneware temperatures, they can also or used on their own.  Mix with water (mix often as it settles out fast), brush on bisque, fire.  This is a way to get colored but still “natural” looking surfaces.

Iron Oxide.  This comes in many varieties and can provide a wide variety of colors under different firing conditions.  Types of iron oxide are red iron oxide, black iron oxide, and yellow ochre.  At earthenware temperatures, up to 4 percent oxide will produce amber and honey glazes.  At stoneware temperatures it can be applied directly to stain the clay surface.  It is often used in this way to highlight textured surfaces.  Can also be added to glazes.

Cobalt Carbonate and Cobalt Oxide.  This is the most powerful color and produces various shades of blue.  It can be harsh used by itself, so is sometimes mixed with iron, manganese, magnesium or copper to create more subtle colors.  Like iron oxide, it can be added to glaze, or can be applied to the clay surface and fired to stoneware temperatures.  By itself it will tend to create a dark slate metallic finish.  It can be mixed with manganese and iron to product rich black slips.

Copper Carbonate and Copper Oxide.  (Note: Copper should not be used in soluble glazes for food and drink containers; in addition to not being safe, it will leach into tea for example, making your tea have a coppery taste.)  Not very effective used on it’s own.  Better to add to or cover with glaze.  In an electric kiln it will create a variety of green shades.  In alkaline glazes it will create turquoises.  It may achieve red colors in a reduction kiln.

Manganese Dioxide and Manganese Carbonate.  In glazes creates colors such as mauve, purple and brown depending on the other ingredients present.  By itself it produces an attractive brown with tiny metallic specks at stoneware temperatures (dioxide is specklier than carbonate).  It is often mixed with other oxides such as cobalt to create purples and iron to produce rich browns.

There are others, such as chromium and nickel, but they are not as predictable throughout all temperature ranges, and should be handled carefully as they are toxic. 

If you make your own glazes, you already have seen many oxides used in glaze recipes.  It is probably best to stick with recipes (at least as a starting point) for colored glazes.  But there are many other ways to use oxides.  So…

Here are 10 ways to experiment with oxides.

1. Brush oxides on greenware, bisque and/or glaze. 

2. Make some slip (see recipes in previous tip of the week) and add some oxides to create colors.  To get more uniform colors mix well.  To get more random, blotchy colors, don’t mix too well.

3. Brush oxide wash over an unfired glaze, then fire.  Be very careful when handling as you may smudge the oxide.

4.. Brush oxides on, then apply glaze.   Best to dip or spray to avoid brushing the oxide off (and if dipping, best to put some glaze aside so oxide doesn’t contaminate your whole batch of glaze.)  Generally the stronger the oxide wash, the more it will bleed through the glaze.
* Both 2 and 3 works well with Opulence glazes, and many of the other glazes.  Where specified by the manufacturer this is explained on the specific glaze page.

5. Mix ball clay with your oxide/water.  Is reported to gives a better consistency and tones down the color.

6. Brush a couple different oxides on, overlapping in areas.

7. Sgraffito.  Brush on oxide.  When dry, scratch with a sharp tool through the oxide to show the clay underneath.  Cover with transparent or translucent glaze.  Or do the same thing with oxide over unfired glaze.

8. Spatter wax on the surface, and paint the oxide wash over that.

9. Use other masking techniques, such as torn, wet newspaper, and paint an oxide wash over it. 

10. Sprinkle a variety of different oxides on a newspaper.  Place leather hard objects onto the oxide mixture (for example, a piece of tile).  Or use a piece of Styrofoam or a sponge to pick up the oxide and transfer it to your piece.  Keep the pattern as it lands, or smear it around.  In this case if you’re once firing you can add glazes.  Otherwise bisque and apply glazes; the oxides will still interact with the glazes when fired together.

Keep in mind, Oxides are strong colorants, so a little bit goes a long way.  In a solution you will probably  only want about 2-8% or you will end up with black. 

Use a respirator or mask when handling the dry oxides. 

Finally, remember that using oxides like this will provide unpredictable, but sometimes beautiful results.  Test!  And Take Notes!  (I had to throw that in because I am lousy at taking notes, but getting better.  It definitely helps.)

Note: Much of this information was obtained from the book Creative Pottery by Peter Cosentino.  This book is out of print, but he has several other great books such as Potter’s Project Book, and Encyclopedia of Pottery Techniques.  We have a few Encyclopedia of Pottery Techniques in stock, and for the others keep an eye out on e-bay or used books sales.