Firing the PrattMWP salt kiln

Loading the salt kiln


Previously, we talked about prepping the salt kiln for firing.  This involves cleaning any debris from the last firing and getting the kiln furniture ready.  Wash the shelves and make sure the posts are in condition to make it through another firing.


Loading the kiln is relatively simple.  It is a bit more time consuming. The pots must be set so they will not need excessive clean-up after the firing.  Since the shelves are coated with a wash (see earlier post) that will be unaffected by vapor, the pots can be place directly on the washed and dusted shelves.  But as an added precaution, we place a clay and alumina wadding directly on the foot of the pots.  There are many recipes for wadding out there.  Some inexplicably call for the addition of flour.  I think it is added as a binder.  All it does is cause the remaining wadding to go sour and stink.  No need for that.  Follow any of the recipes you might find.  Leave out the flour and you will be fine.  Ours is simple.  Kaolin, Alumina, grog and sometimes pyrophylite.  We use something close to equal parts by weight EPK and alumina (hydrate or oxide) with enough grog and water to make it stiff but workable. Add some pyrophylite if on hand.


We make marble sized balls of wadding, dip one side in a water based wax resist and stick that side on to the bottom of the (bisqued) pots.  On occasion, we brush the bottom of some pieces with a wax resist that has alumina mixed in.  But this approach is more for porcelain than stoneware.

The pots are wadded as we load.  Pots should be stacked a bit more loosely than a reduction or electric firing.  Leave enough space for the vapor to flow around the pieces.  With experience you get to know what part of the kiln gets more vapor and load accordingly.  Loading does alter the flow of flame and vapor through the kiln.  Shelf placement has a greater affect.  Most kilns have at least two banks of shelving.  Make sure the shelf heights in banks are staggered as much as possible.   You don't want to create heat zones or sinks.  Remember that the vapor is in the kiln chamber for a very short time.  It is simple fluid dynamics- baffle and flow.  Keep the vapor moving inside the chamber as long as possible.  Adjust the bag wall if necessary.  For example, adding height to the fire wall may push heat to the crown.  Think of fluids moving through the  kiln and around the  pots.  But this is a bit more involved than we will get into now.  More on that in another post.

Our kiln is a downdraft.  The fire boxes are on both sides with the salting ports directly over head in the front wall about midway up the kiln.  The ports are high enough that the salt will have a distance to fall and vaporize (few feet), but not too high for placing the salt in the kiln.  We use a five foot piece of inch and a half angle iron as a trough that we slide into the port, parallel to the side wall of the chamber, and rotate, dumping the salt over the firebox.  But, again, I am getting ahead of myself here! More on this later.

Place cone packs throughout the kiln.  We use ∆ 8,9, and 10.   Use more cones if you want to see heat gain or need a marker for knowing when to start reduction.  You can never have too many cones.  Our kilns have more spy ports than you might see in other designs.  In addition to the usual ports in the door (3), we have two in each side wall.  You need to be able to see what is going on in the kiln!  Some potters believe that the vapor affects the cones end point.  I am not convinced that this is true in any measurable, significant way.  So we don't protect the cones.  The cones and the test rings are the only tools that give an accurate sense of heat work.

Place draw rings in front of the cone packs at each front spy ports.  Both cone packs and rings should be in far enough to get a good read on the chamber temperature and vapor gain, yet still visible with the rings within easy reach of of the draw rod.  Make sure the rings are sized to fit through the ports.  Remember they will shrink some in the firing.

We brick up the door with straight (high duty, fire) bricks all laid up in a bonding course, or short side facing into the chamber.  I learned this from Peter Sohngen some years back.  Only one end gets glazed.  I am not sure which is a better door, one of bonding courses or a door of stretcher courses (which still need a bonding course every 5 rows or so).  In either case, you will have to replace bricks often enough.  I am planning to experiment with a door design that would use straights tapered ends in a bonding course door.  We will let you know how that goes.

Make sure the cone packs and test rings can be seen at the ports.  Leave a few bricks out at the top of the door for early draw when you start the burners.We usually place an offering of sake (or schnapps or whatever is on hand!) and rice in bowls on the kiln top and start the firing.  In the next post of this series we will talk about starting the firing, the optimal firing schedule, controlling the ramp and atmosphere, and salting the kiln.  Happy New Year to all!