Testing and developing glazes

I've been gearing up for some serious glaze testing. I've been watching the struggles of my friend, Kim, who is developing slips and glazes for soda firing. 


Building on multiple sources, I have a plan for how to proceed with my own testing. In this blog, I will share my ideas and any good glazes I develop.  Comments or suggestions are welcome.


Why is testing necessary?

Glaze recipes do not travel well - often need to be adjusted for local conditions. Making a single test tile of a recipe is a shortcut that often misses the mark.  Likewise, trying to use glaze calculation software to create a one-shot cure is also a shortcut that often misses the mark.


What sort of local conditions can affect glaze features?  Hot oxides interact with each other, the kiln atmosphere, and the clay body ...  and have complex effects on the glaze melt.  There is a lot of variability in raw materials, clay bodies, solubles in the water, kiln atmosphere, and firing schedules - any of which can affect the glaze.  Glaze calculation is no more accurate than the analyses of the materials - should be taken as a rough approximation.  Your working recipe will be the result of systematic testing.  Use the software to narrow the scope of the testing.

When to test

Test all new glazes and test working glazes any time something changes, such as a change in raw material, glaze density, water supply, clay body, firing schedule, kiln atmosphere, colorant combinations.  You also may need to test for suitability for leather-hard, bone-dry, or bisque application. 

How to test

Instead of testing a zillion recipes to find a few that work for you, your time will be better spent if you select a few glazes and test them systematically ... on the faith that you can probably get any of them to work.


You will want to do either a line blend (varying one oxide) or a square blend (varying two oxides). Rather than weighing out all the ingredients of each test, you weigh out just the ingredients for each boundary point of the blend (such as the corner glazes of a square blend or the ends of a line blend), add water to make them have equal volumes, then use volumetric means to proportion out the boundary mixes to create the mix for each test tile.  


After the tiles are fired, glaze software can instantly provide the recipe for every tile in the blend based on the recipes of the corner glazes and the number of intermediate points. 


The commonest variables that require testing are alumina and silica - something that should be done with any untried or problematic recipe.

You have to keep things simple in order to make sense of the results.  In every test (even a square grid that changes two variables) you are really always examining a change in single variable at a time while you keep everything else the same.  


A Currie blend (or grid) is a type of square blend that fixes the proportions of a set of glaze ingredients while systematically varying the alumina and silica to create a family of different glazes.  This grid is usually 5x7 (35 tiles).  Silica increases by 4 increments left-to-right.  Alumina increases by 6 increments bottom-to-top.  Using the same size grid helps you visualize a tile's position in the grid and understand how the glaze changes as you vary alumina and silica.


A more general concept is the square blend in which you systematically vary one oxide in each of two axes.  You create corner recipes that determine the oxide levels in the corners.  You determine the number of increments in each axis.  You can customize the scope and granularity of the grid any way you want.


I believe that the best use of glaze software in solving glaze problems or in developing new glazes is in the design of tests.  Reality is what is recorded in the tiles.  Your working recipes will be the recipes of the mixes you actually put on the tiles - not something you came up with using software to juggle ingredients while you check the calculated effects on oxide levels.  


You can design the corner glazes of your test to make the range of oxide levels being tested as broad or narrow as you wish.  You can determine the granularity of your testing by defining the number of intermediate points in each axis of the test.  


If you are trying to fine tune an almost-working recipe you want to narrow the focus of your test.  If you are trying to discover a glaze nobody that has never seen before, you can make the scope as wide as you want.  If you are a functional potter, you can use the software to keep the oxides in the four corners close to the known limits for durable glazes.


In constructing a square grid, you define a set of ingredients that will be constant in all 4 corner glazes.  Then you add ingredients to the corner glazes that will systematically change only one variable in each axis.



Labels, registry, computer records

Each test set and each tile must have unique identifiers. The computer can be used to define the tests before they are run ... as well as the labels that identify each test and it's corresponding set of tiles. There should also be a hand-written registry for recording date, test, and tiles.

The hand-written registry will only show what tests were done and when. The details are all in the computer in records that can easily be backed up. If the registry book were lost, it would be no big deal because the information originated in the computer records. (If the computer records were to be lost, the registry information would be nearly useless.)

The test set identifier will consist of a letter corresponding to the year plus a sequential number representing the number of the test set during that year. For example, if "A" represents "2010", then "A46" would be the 46th set done in 2010, "B19" would be the 19th set done in 2011, etc.

There are difficulties involved in labelling tiles being tested in salt/soda firings. Identifiers painted on the bottoms of these tiles are easily obscured by kiln gunk. One solution to this problem is to stamp or inscribe the identifier on the tiles during the leather-hard stage. Another way would be to paint the identifier onto an unglazed area on the back or side of the tile, rather than on the bottom.

The most precise and error-free way to identify the tiles in a set would be to paint a label on each tile immediately after it is glazed, using a combination of set identifier plus the sequential number of that tile in that set. For example "A46-21" would identify the 21st tile in the in the 46th test set of 2010.


Glaze software

Glaze software is indispensable for understanding and developing glazes, and for keeping records.

The different available glaze software products vary considerably in their ability to store and organize information. GlazeChem is a good one for storing and retrieving records.

The records in GlazeChem can include images of glazes and detailed notes.  You can keep all your tests in special folder.  Each test can be kept in a separate glaze datebase file on the computer. Each database can contain detailed notes, the recipes of the boundaries of a blend, and the recipes of all the tests in the set. It is possible to predefine and label each test set and print out the corner recipes before you go out to the studio to mix the glazes. The printouts can be used in the studio to check off glaze ingredients as they are added to a batch. After the tests are run, the print-out recipe lists can be discarded. You can use the program to determine the recipe of any of the tiles that interest you.  You can go back to the computer and record your results ... even including images of selected tiles.  You can give descriptive names to tiles/recipes that worked well and copy them to a collection of working glazes.  

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.