Non-Traditional Black on Black Firing – Jo Ann Weldon

For the 2014 SWKC held at Tijeras, NM, I chose to experiment with a modern technique for obtaining black-on-black pottery. This year’s firing was to be done without the use of hot coals under the firing base. It would rely only on “top wood,” wood placed around the firing chamber and lit after the pottery was in place.

2014 Results, Discussion, and Hints

The results of this test showed that a black on black reduction firing with top wood providing the only source of heat could be done successfully. My pot fired deep black, and the shine on the pot was maintained. Other pots in this firing showed the same black and shine; however, several of them spalled. Spalls usually happen when there is an air bubble inside the wall of the pot. The air expands as the pot heats up and the pressure causes a chunk of the surface to blow off. The only way to avoid this is to wedge the clay thoroughly to be sure you have all of the air bubbles out of the clay before you build. It is also very important that the pots are dry. If a pot is even slightly wet, it will probably not survive this black on black method.

These are pictures of the same pot. On the left is the top view. The pot appears to be whole but on the right side of that picture you see a shiny black piece that has spalled off of the bottom of the pot, as shown in the picture on the right.
What makes a good black on black pot is a very high shine on the pot, contrasting with the matte black of the design paint. All clays do not make good pots for a black on black firings because some do not polish well. What you need to have is clay that will give you a great polish, the shiner the better!

This is a non-traditional method for black on black pottery, which some people may find interesting to try. To make this type of firing understandable, I have included a description of the equipment needed, the directions for setting up the kiln, a somewhat simplified explanation of what happens inside the firing chamber, and finally some pictures showing most of the process.

Equipment

Firing chamber – use a large stainless steel or iron pot with no holes. Do not use anything that
is galvanized because the coating will burn off and affect the pottery. I use the cheapest and largest stainless steel pot that I can find at the dollar stores. The pot handles need to be removed and the holes where they were attached plugged with a steel nut and bolt of the correct size. If this is not done the solder will melt and the handles will slump or fall off, opening up air holes into the chamber and ruining your firing. Avoid this by getting rid of the handles and holes before you fire you new pot!

Base for firing – use a flat non-burnable material such as tin or iron. I use a piece of scrap iron. It must be larger than the firing chamber. You need 4-5” of base around the firing chamber if possible.

Fine, dry sand – you don’t want a lot of organic stuff in it, but a little won’t hurt. Dry is necessary! For my chamber I will use 2-3 gallons of sand.

Small flat chunks of sandstone – this is what you set the pots on. Be sure it is dry so it won’t explode. The bottom of the pots may not be black because the sandstone blocks the carbon transfer.

Kiln stilts – Optional – I used them this year for the first time. I put them on top of the sandstone and the pots on top of the stilts. This allows the bottom of the pots to be black as well.

Wood shavings or sawdust – hardwood works best. Juniper works but pine is not so good. I like the wood shavings better than sawdust.

Bailing wire or similar wire – fine wire might break before the fire has burned down.

Wood – Juniper or some other wood that burns fairly hot. I like to cut the wood in pieces that are
just a little taller than the firing chamber.

Paper – you need this to help get the fire started.

Putting it all together

1. Clear a safe area of all grass and other things that will burn, and level the spot for the kiln.
2. Put the base on the leveled spot and make sure it does not rock.
3. Pour the sand on the center of the base, and spread it out evenly in a circle larger than the firing chamber. The sand should be at least 2 inches deep everywhere and smoothed flat on top.
4. Center the firing chamber on the sand and press it lightly into the sand to make a mark showing the diameter. This allows you to see where to put the pots so they won’t touch the sides of the chamber.
5. Remove the chamber and place the sandstones inside the circle mark and adjust them to fit the size of your pots. Remember the pots cannot touch each other or the sides of the chamber!
6. Place the stilts (if you use them) on the sandstones and then put the pots on the stilts. If you are not using stilts, place the pots on the sandstones.
7. Sprinkle the wood shavings around the sandstones staying inside the marked circle if possible. Do not get shavings or dust on the pots. Pots should be sitting right side up. Use enough shavings! It is better to have some shavings unburned than to not have enough to turn the pots totally black.
8. Carefully put the firing chamber over the pots and work it down deep into the sand to seal the bottom.
9. Stand the cut wood on end (vertically) all around the pot. The wood “stockade” should be at least 3” thick everywhere around the pot with no holes visible.
10. Wrap the wire around the wood to hold it in place around the pot. Add some shorter wood pieces on top of the firing chamber. If the upright wood is a little taller than the firing chamber they will help hold the wood on top in place.
11. Wad up the paper and stick it between the logs at the bottom. Do this in at least 6 places all around the pot. You want the fire to light on all sides at the same time.

Note: All this has been done with no fire or heat involved! You are now ready to light the fire, and a little lighter fluid on the paper certainly helps. Now it’s time to stand back, relax and let it burn!

Discussion

Any pottery fired in this type of kiln will turn black. Here is a quick explanation of how it occurs. In this process, the firing chamber acts as a sealed container. The only oxygen available is what is in the container when the fire is lit. As the fire surrounding the chamber burns, the temperature of the air inside becomes very hot, finally reached a temperature hot enough to ignite the wood shavings placed inside the chamber with the pottery. As they burn they produce smoke, which cannot escape from the chamber. The chips continue to burn until all of the available oxygen is consumed, forming a true reduction atmosphere. The result is a chamber filled with thick smoke. As the temperature continues to rise, the pottery reaches its firing temperature, and the free carbon reacts with the hot pottery surfaces forming a bond. When the fuel is used up, the temperature inside the container begins to cool, eventually reaching a level where the carbon bonded to the surface cannot be burned off, even in the presence of oxygen. The resulting black color will neither wipe off nor wash off.

Have fun and if you have any questions you may email me at jweldon@frontiernet.net

Jo Ann Weldon, Snowflake, AZ