Kiln wash, wadding for soda, refractory coatings

I'm building a new soda kiln with my partner, Kim.  I've scrounged the best information I can find on refractory coatings for sodium atmospheres. I will update this post if I learn more.


Kiln wash recipes

	A	B	C	D	E	F
alumina hydrate	50	50	20	30		60
calcined kaolin		25
zirconium silicate			40	30	90
kyanite				30
kaolin	50	25	30			30
ball clay			10
bentonite				10	10	10

Notes
Kiln wash is nothing more than a layer of highly refractory material used to coat surfaces you wish to protect from glaze runs, kiln atmosphere, and ash deposits.  The wash should adhere, lie flat, not form dust, not form chips or flakes.  It should be easily removed with hand tools.

If the kiln wash shrinks too much on drying, it will crack and tend to fall off the shelf in flakes which can fall onto work being fired on a lower shelf.  To decrease shrinkage, increase the proportion of non-shrinking refractory material.  The choice of non-shrinking refractory material should be based primarily on cost of material.  It can be alumina, calcined kaolin, zirconium silicate, kyanite, etc.  The non-shrinking refractory material can be up to 90% of the kiln wash recipe.

You can buy calcined kaolin as Glomax or Molochite.  You can make your own calcined kaolin by heating to bisque temperature (red heat) in a bisqued bowl.  Calcining of clay eliminates the physical property of shrinkage without changing the chemical and refractory properties of kaolin.

To control swirling kiln wash dust in kilns with high turbulence, add 1-2% feldspar (source of info = John Britt).  If you add feldspar, make sure you don't add too much.  After firing, you should be able to scrape off kiln wash with a finger nail.  If you need a tool to scrape off the wash, it has too much feldspar.

Recipe D is essentially the same recipe posted on Clayart by Tony Clennell as the "working potter's ITC".  It has been applied to kiln walls and used for repairs (tony.clennell@gmail.com).

According to John Britt a soda kiln wash recipe should not contain silica.

1. Silica in kiln wash is ok at lowfire or midfire temperatures without salt.  Silica is not so good at high fire - especially for wood, salt, and/or porcelain.
2. Silica is a glass-former.  If a lot of glaze drips onto the shelf, it can melt the silica in the kiln wash and form a glaze on the shelf.
3. When you scrape your shelves to clean them, you create a lot of silica dust, which is a known carcinogen.
4. If you use a wash containing silica in a salt or soda firing, the kiln wash becomes a glaze.  This is because silica is a glass-former.  When sodium oxide, which is a strong flux, is introduced atmospherically, it can easily melt the silica in the kiln wash into a glass.

For applying kiln wash with a brush or roller, it should be mixed to the consistency of heavy cream.  For application with a sprayer, it must be mixed thinner.

Using a roller or spray gun are faster than using a brush.  If you are using a brush for doing a whole shelf, use a 4- or 5-inch house-painting brush.  If you are touching up bare spots after scraping off glaze drips, use a 1–inch glaze brush and just dab it on in the spots that need it.

If you use a brush, work very fast because the shelf will suck up the wash as soon as the brush touches it, making areas of uneven thickness.

When applying wash to clean shelves, apply several thick layers, allowing each to dry before applying the next.  Then, with a wet sponge, wipe the wash off the edges and a 1/4-inch band around the top border of each shelf to prevent chips of kiln wash from falling onto the ware stacked below.

The best way to clean shelves that have glaze drips or salt build-up that has eaten through the kiln wash is to use a "diamond grinding cup" on an angle grinder.  These cups come in a diameter of 4"-5" with prices ranging from $13 to $150.  Use a dust mask and goggles when using this tool.


Wadding
For WADDING... use the same recipe as for kiln wash with less water so it has the consistency of dough.  You can add 40% or more of sand and/or sawdust.  Sawdust burns out leaving crumbly material, easily crushed with pliers.

Wadding is used in salt/soda/wood/gas reduction firings to allow the kiln atmosphere to reach surfaces that would otherwise be excluded.  Wadding is necessary in salt/soda to prevent pieces from being glued to kiln shelves by atmospheric sodium or glaze runs.

The kiln atmosphere will reach around the wadding everywhere except the surfaces in direct contact with the wadding.  The surfaces in contact with wadding are masked from the atmosphere.  This will result in pale wad areas surrounded by expanses of atmospheric glazing and/or flashing.

It has become part of the mystique and tradition of these firing techniques to use wadding in creative and decorative ways.
Examples:

• Wad bottoms in order to glaze/flash bottoms.
• Lay tall pieces sideways on wadding to get ash/glaze runs to pool on the gravity side.
• Stack bowls rim-to-rim to get atmospheric effects on the rims and inside ... while conserving kiln space.
• Wad lids in place so that they are exposed to the same atmosphere as the nearby pot surfaces ... and allowing flashing of lid contact surfaces.
• Stack plates or bowls separated by wadding ... resulting in interesting flashing effects on inner and outer surfaces, as well as conserving kiln space.
• Apply wadding purely as a decorative masking technique (i.e., regardless of whether wadding is required for placing or stacking the piece in the kiln).

Refractory Coatings

We are thinking of building a larger kiln.  According to Nils Lou, a kiln made of hard brick will use four times as much fuel as the same size kiln made of insulating firebrick.  In the past, most salt/soda kilns have been made of hard brick because uncoated insulating brick does not hold up well in the soda atmosphere.  If you could put a protective coating over soft brick, there would be a huge savings in fuel costs.

We are aware of various refractory coatings that have been recommended to enable insulating firebrick to withstand the erosive effects of a salt/soda atmosphere.  Thus far, I have no personal experience with any of these coatings.  I would greatly appreciate comments from others who have experience with any of these coatings.

In The Art of Firing (1998 edition), Nils Lou devotes a chapter to refractory coatings and discusses his experience using these coatings in salt/soda kilns.  It is now many years since he reported his preliminary findings with HUC and ITC coatings.  I wrote to Nils Lou to find out what his current thoughts are about these materials.  I contacted him by email ... and he patiently answered my questions.  I summarized this correspondence and have put it on this blog with his permission (link).

Recipe D above is essentially the same recipe posted on Clayart by Tony Clennell as the "working potter's ITC". It has been applied to kiln walls and used for repairs (tony.clennell@gmail.com).  Whether or not it can protect IFB from soda vapor, I dunno.

Here is the contact information for International Technical Ceramics ("ITC"):

Delkich International Technical Ceramics Inc

325 Mealy Drive

Jacksonville, FL 32233-6901

Phone:(904) 285-0200

(ITC has no website.)  

Here is a link to an ITC vendor named anvilfire.  

Nils reports that a salt kiln made of 23K brick and coated with ITC100HT has held up well in repeated cone 9 soda firings over many years.  He also recommended applying a topcoat of ITC296A which is a denser material which will raise the emissivity.

Instead of paying an arm and a leg for these ITC coatings, I wonder if I could substitute Tony Clennell's working potter's ITC for commercial ITC100HT.  I wonder if I can mix up my own working potter's HUC (containing silicon carbide, clay, and maybe some sodium silicate) as a substitute for ITC296A.

Here is what Kim and I have talked about doing....  In our current little kiln lined with hardbrick, we will include a little piece of IFB coated with working potter's coatings ... along with an uncoated piece of softbrick as a control.  We will place these pieces of IFB right in front a burner and blast them with heat and soda.  If the softbrick seems to be holding up compared with the control, we will eventually use these coatings on a larger kiln made of softbrick.  If the Clennell mixture doesn't work so good, we can use ITC.  Another thought: Maybe we should not work so hard trying to develop our own kiln technology and just use what is commercially available; that way we will have more time to make pots.