Test Tiles Work Like a Charm

Last week, I unloaded a kiln full of only glaze tests which I did on the new test tiles we made with the extruder.  I have to say that I am very pleased with the performance of the tiles themselves.  The glaze tests are another matter.  Let’s just say that out of a few hundred combinations and recipes, I think I’ll keep or expand on maybe five.  I keep telling myself that you never know what a recipe is going to look like until you fire it in your kiln under your conditions.  That’s my justification for appearing to waste so much time and energy.

In addition to 12 line blends, I also took all of my existing glazes and tested them over and under each other.  That’s 8 x 8 = 64 tiles right there.  In general, my glazes do not play well together in terms of appearance.  However, this was very educational, and now I know which ones combine to do something interesting, if not attractive.  I can also safely say that none of the combinations seem to crawl off one another or anything else catastrophic. 

I did manage to generate some interesting purples, a new silica white, a medium waterfall green, a floating green-on-brown, and a floating blue-on-black (à la Midnight Peacock, but simpler because it’s one glaze not two, woo hoo!!!).

I also learned the following tidbits which might help you in some of your testing.  Remember, everything is dependent on the chemistry of the base glaze and your firing schedule. 

1.  Although lithium (from lithium carbonate, spodumene, or petalite for example) contributes to a lovely glaze finish in some base glazes, it can also kill certain color responses.  For example, at our temperature, I can get some great iron greens in certain feldspathic base glazes.  However, adding an appreciable amount of lithia-containing mineral to these base glazes make them turn an unimpressive brown. 

2. Maybe this is obvious:  Two base glazes which have the same oxide analysis but use different minerals to get there do not necessarily look the same when fired.  For example, you can get calcia from whiting or from wollastonite (and others).  If you substitute wollastonite, you obviously have to decrease your flint content, to keep the silica the same, but once everything is adjusted and the molar amounts of all the different oxides are similar (within 0.1) you aren’t guaranteed that one base glaze will fire like another.  Fascinating, isn’t it?  From a purely chemical standpoint, one might assume that oxide source wasn’t nearly as important as final oxide content.  However, it seems that source does matter.  So, choose wisely.

3. Calcium does indeed have a bleaching effect on iron.  I don’t know if you would see this in a (high) iron-containing glaze, but in a clear glaze, increasing the calcium content will bleach the tan appearance coming from the iron in the clay body.  This is particularly noticeable on our white stoneware.  We use Phoenix from Highwater Clay which is pretty darn white in oxidation at cone 10.  However, there is a noticeable difference when clear glazes that differ only in calcium content are used on this body.

4. Black iron oxide (FeO) and red iron oxide (Fe2O3) perform differently in the same base glaze.  Now, I’m not talking about intensity of color, since by weight, red iron oxide is not as “strong” as black as there are more iron atoms in a given weight of FeO than Fe2O3.  At cone 10, Fe2O3 breaks down to FeO and thus, if correcting for molecular weights, one should be able to obtain the same color from FeO as Fe2O3 if you adjust the percentage correctly.  However, red iron oxide, in my experience, consistently generates a warmer brown than does black.  Browns made from black iron oxide tend toward a grey-green cooler hue versus those made with red which are warmer, more like burnt sienna.

5. Very small amounts of chrome in the appropriate base glaze are awesome and can do some miraculous things.  In the wrong base glaze it turns out cat-sick green.

6. Magnesium causes copper to tend toward a warmer and softer green whereas lithium generates yellower greens.

7. The titanium dioxide in rutile can have a lovely effect on basic cobalt blue, however, you have to be careful of the pinholing that is associated with adding TiO2 to a glaze.  I have worked on ameliorating this phenomenon by extending what I refer to as the “open” time of a glaze.  By that I mean, the amount of time a glaze is molten in your kiln, before a skin has formed on the glaze surface through which bubbles have to pass contributing to pinholing.  Logically, you can extend the open time of the glaze by maintaining its maturing temperature for longer either through heat or chemistry.  If you’re trying to extend the open time via different fluxes, you may need to take into account the temperature at which compounds such as oxygen or carbon dioxide are evolved from the molten glass.  Please note that “open time” is my own special word, meaning if you use it in front of someone other than me, they probably won’t know what you’re talking about!

Please enjoy the pictures in this post.  I’ve only shown you the pretty stuff as there are only so many cat sick greens and poo browns my ego can bear!

This isn’t all of them!

Black Iron Oxide (FeO) 10% Line Blend

Black Iron Oxide (FeO) in a Feldspathic Base Glaze II

Black Iron Oxide (FeO) in a Feldspathic Base Glaze

Black Iron Oxide 10% Line Blend no. 2 – The Base Glaze Makes a Difference

Midnight Peacock in One Glaze instead of Two!

Midnight Peacock in One Glaze instead of Two! – II

New Medium Waterfall Green

New Silica White in Feldspathic Base Glaze

Same Turquoise Blue Color, Terrible Munchy Surface

Cobalt quickly overwhelms a chrome-tin pink.

Impact of Spodumene on Copper Green

Spring Shows are Rolling In

We are pleased to announce that we will be participating in the

Melbourne Art Festival

April 24 and 25, 2010

Please, visit their website for more information.

http://www.melbournearts.org/

For a continually updated list of our shows, please see: http://prometheanpottery.wordpress.com/upcoming-shows/.

More Treasuries at ETSY!!!

Go See!

http://www.etsy.com/treasury_list_west.php?room_id=81904

http://www.etsy.com/treasury_list.php?room_id=104332

Treasuries expire after just a few days, so get ‘em while they’re hot!!!

Random Picture No. 3

Here’s an impressive soup tureen thrown on the wheel by Robert and glazed by Emily.  Done in Em’s Honan Tenmoku Black.  I particularly like this piece because it really shows off Rob’s throwing skills.  The foot and lid were thrown separately and then turned and joined to the body of the piece.  Emily’s tenmoku is a deep rich black that breaks to rust orange on the ridges and grows small grey-green crystals where it puddles.  Photo credit to R. L. Graham.

Soup Tureen by Promethean Pottery in Em's Honan Tenmoku Black

New Treasury

Treasuries on ETSY expire fast so have a look before it’s gone!

http://www.etsy.com/treasury_list_west.php?room_id=80723

Thanks Again Dorothy!  (Go see Dorothy’s great pottery at her ETSY shop too link under blogroll –>)

Random Pottery Picture No. 2

Presenting one of Rob’s fantastic lidded canisters in Emily’s Persimmon Glaze.

Large Persimmon Canister by Promethean Pottery

Random Picture No. 1

Rob thought it would be a good idea to start posting more pictures of our stuff.  So, I am going to start a series called “Random Picture No. X”.  This is number 1.  It happens to be a vase we made for a dear friend’s mom.  It’s done in our more traditional style and that fabulous Midnight Peacock glaze.  Tee Hee…

Midnight Peacock Traditional Vase by Promethean Pottery

Thoughts? Consensus?

Please choose one from each list:

Hand Made

Hand-made

Handmade

and then there’s this one:

Wheel Thrown

Wheel-thrown

Wheelthrown

I have seen and used all versions of all of these.  I have a nagging need in the back of my skull for consistency.  Help me decide!

Pugmill Wonders – Test Tiles

So, I do a lot of glaze testing.  I still haven’t posted all the pictures I plan on posting, but it’s getting there.  But believe me, there are boxes of little pots and tiles with nasty avocado green and poo brown (for example) glazes on them littering our house.

One major hurdle to glaze testing is having enough of the thing to which the glaze is going to be applied.  Well, this is true for me at least.  I like to do line blends.  Years of scientific research have taught me that the one condition you think will be worthless is usually your best test subject.  Typically, a line blend for me is a 10% step down so there are 11 total tests in each series (100% à 0%).  We started out by having Rob throw lots of little miniature pots.  He hates this, I think, and truthfully it does take away from the amount of time he is able to throw real pottery.  So, we started rolling out slabs of clay and cutting them into tiles.  There was a lot of folding, breaking, bad language, etc and of course it was very slow.  The process was involved because I need tiles that are self-supporting but also thin enough to mimic Robert’s throwing thickness.

So, a while ago we got a pugmill.  It’s a fantastic invention, and I don’t really know how we lived without it.  We had the forethought to order the “tile nozzle” attachment.  This reduces the size and shape of the cylindrical pug to a flat 2 inch x 5 inch tile.  It also came with a few pieces of plywood.  These one uses to cut little shapes out of to make “dies” for the pugmill through which clay can be extruded.  So, in principal it’s just like a hand driven extruder except it has a motorized auger and vacuum pump – YES!

So, we thought about it and came up with a self supporting shape – sort of an upside down 7.  We cut this shape out of the plywood and affixed the negative die to the end of the tile nozzle extension.  After a little fiddling with clay of different consistencies, I managed to produce about 50 test tiles in less than half an hour.  Woo Hoo!!!

Now that I can pug these little puppies out, the limiting step is how fast I can mix each dilution in a line blend.  I’ll have to figure out some way to automate that next.  When I do, I’ll be rich!  The kiln is on a bisque cycle right now and has about 400 little tiles in it.  I am so excited I could barf.  Too bad I already have enough tests planned for all 400 tiles.

Pugmill as Extruder for Test Tiles

Pugmill as Extruder for Test Tiles

Pugmill as Extruder for Test Tiles

Emily at the Pugmill Making Test Tiles

Emily at the Pugmill Making Test Tiles

Emily at the Pugmill Making Test Tiles

Emily at the Pugmill Making Test Tiles

Rob Slicing Test Tiles

Rob Slicing Test Tiles

Finished Test Tiles

Finished Test Tiles

Finished Test Tiles

Finished Test Tiles

New Glaze Glossary Up!!!

Hi Everybody,

I’ve been working on a Glaze Glossary and Resource page.  The first part of this multipage post is now up.

The parent page – Glaze Glossary and Resource – is here http://wp.me/PxVXW-3w (or at the top of the blog).  And the first sub-post – Glaze Colorants – is here http://wp.me/PxVXW-3F.  There are already some pictures up in the colorant post, but you’ll see the places where I plan on putting in more pictures as I take them.  So, you might want to check back frequently for more photos.  This post constitutes a few months of book and internet based research as well as things I have learned on my own as I test glazes.  I hope you find it particularly useful.

Later I will add a sub-page about fluxes and glaze components as well as one about general terminology.  I guess the terminology one should have come first, but the colorant one was definitely more interesting to write!

Enjoy!