Making the switch from gas to electric..

[Lisa Nouvelle]

God morning friends. I am making the switch from cone nine gas firing to cone six electric. I use one glaze for most of my work and that is Leach White which I have reformulated to work beautifully in the lower temperature. In the gas environment I used iron oxide under the glaze to create great shading/shadowing effect along with the speckling and iron spots with the clay I was a happy camper. The oxide showed up any textures nicely as well. I also used the glaze heavier for a great shiny opaque white.

In the cone six firing I have no problem with the shiny white but I have not been able to recreate any interest at all with the oxide wash or iron spots or even letting the clay color show through. The clay with lots of spots in it looks like a cheap effect instead of an exciting or interesting development.

Any suggestions? Thank you so much in advance for any input you have.

[Norm Stuart]

Post some photos of the glaze at cone 9 compared with cone 6 so everyone can see what you're referring to.

[Lisa Nouvelle]

The first two photo's are of pieces done in cone 6-10 clay and have the oxide wash under the leach white, obvious in the crevices and around the edges. The third photo is my cone 6 clay, medium in color with some spotting that also had an oxide wash under the leach white. The oxide tends to color the entire glaze kind of a latte tan color. You see there is no personality, the spots are iron oxide that I tried dotting over the glaze before firing.

I am not stuck on doing things the old way, just have not figured a way to mimic it.

Thanks again.

[Norm Stuart]

These oil spot glazes are not what you're trying to achieve, but the concept is exactly the same.

http://www.flickr.com/photos/glazes/sets/72157605126678933/

If I make these glazes with gerstley borate the glaze viscoscity is thin enough to allow iron to flow through the top layer. Made with a frit base these glazes are so viscous thick they either don't allow iron from the base glaze layer to flow through at all, or when it does it becomes a large area which removes the top glaze completely.

In addition to thinning the viscosity of the top glaze, consider putting a thinner layer of the top glaze on. White Bird is a glaze which is completely white if applied too thick, completely brown applied too thin, and interesting applied just right.

http://www.flickr.com/photos/glazes/1084659163/

This is a gerstley borate White Bird compared with a frit based White Bird.

Slow-cooling can also provides enough time for iron to migrate through a glaze.  I think the largest difference between a cone 10 gas kiln and cone 6 electric is how fast the kiln cools.  Hard fire brick retains heat and cools very slowly. Soft fire brick retains little heat and cools quickly. We usually program our kiln to slow-cool between 1,800F and 1,500F over a six hour period (50F per hour).

Consider McCall's Warm Jade, made with Gerstley Borate. The first tile, taken from the creator's website, is a fast cool. The second tile is my example of the glaze with our 6 hour slow-cool giving the iron more time to migrate.

http://mccallsc.tripod.com/pottery-glaze-testing.html

[Lisa Nouvelle]

Wow Norm, you are amazing! So let me break this down for a beginners mind. I love the White Bird with the iron break, is the iron that is migrating through the glaze in the glaze? Or is it in the clay body?

If I look at the White Bird glaze recipe below the photo on the Flickr page, would I substitute the Frit for the GB? In equal measure? Any particular Frit? Would I need to use the Old Mine clay specifically?

Lisa

[Norm Stuart]

Snap a test tile in half and you'll notice that iron oxide preferentially binds with the clay. Once there's enough iron in the clay, only then is the surplus available for the glaze. So obviously a high iron clay will change this balance resulting in more iron in the glaze.

White Bird has 1% Red Iron Oxide which is a very low amount, not really enough to provide much for the glaze even with a high iron clay. So the iron will only be visible if the glaze layer is not too thick. Too thick and you only see white.

Vary the thickness of your glaze over the iron layer. You'll find a maximum thickness which allows the iron to migrate up through the glaze. You can also try adding more iron, but I don't think that will make a big difference.

Slow-cooling will probably allow the iron to migrate through a thicker layer of glaze. Likewise making your glaze less viscous will allow more iron to migrate through. If your glaze is already NOT frit-based, then less viscous means adding more flux so it's more fluid at melt temperature.

My assumption has been this type of iron migration occurs due to thermal currents from the bottom of the glaze to the top. If someone has a better understanding of this type of glaze that would help us all.

OM-4 clay has more iron than kaolin, so that's another idea.

[Lisa Nouvelle]

So if I were to make up a small test batch of the White Bird that will be frit based, do I just substitute the Gertsley Borate? And is there a preferred frit?

I will try the slow cooling process as well.

BTW, I lived in So Cal for over 20 years! Still go back to visit friends every year.

[Norm Stuart]

In the frit-based  White Bird tile I posted I simply added 10% Ferro Frit 3269, in addition to the gerstley borate. Ferro Frit 5301 has even more fluorine so results in more flow and can result in more interest.

http://digitalfire.com/4sight/material/ferro_frit_3269_371.html

http://digitalfire.com/4sight/material/ferro_frit_5301_468.html

This is because the increased crystallization at our slow cool eats up so much of the melt I need to add more flux and silica.  This tile shows the original White Bird recipe, without the added 3269 Frit, after the six hour slow-cool between 1,800F and 1,500F - as you can see the glaze looks very dry in appearance with iron coloring making up most of the glaze with very little white area.

If I need more glass without adding much flux, Ferro Frit 3134 is better.

http://digitalfire.com/4sight/material/ferro_frit_3134_351.html

But, as I mentioned the thickness and viscosity provided by frit often work against you, as you can see in this frit based Oil Spot glaze with a mason yellow top coat.  Notice how the iron flow-throughs are larger than a gerstley borate top coat, often simply pushing the top coat glaze out of the way, and on vertical section the top coat can slide off the ware onto the kiln shelf.

Compare this to Kathy Ransom's Oil Spot with a gerstley borate base.

I find modifying ^10 glazes to ^6 interesting and challenging.  After a few changes the test tiles help you get to know what's really producing the look you're trying to maintain. There really is no substitute for small batches of test tiles to give you the insights you need to reformulate the glaze for ^6.

[Lisa Nouvelle]

Thanks Brian, I too 'cheated' using dots of iron oxide on top of the glaze and it did a fair job of mimicking the real deal. People really like the shading effect of the oxide from my gas firing days so I am paddling hard to get that as well.

So Norm, I mixed up the White Bird glaze to test and I also added the Old Mine clay to my white recipe to see if the extra iron would help. My test tiles are firing as we speak so I will let you all know how it goes. Getting out my book to see how to program the slow cool too! Having fun, thanks for all the assistance.

[Lisa Nouvelle]

Do you dip your pieces? I do all spraying and am wondering if the magnetite is small enough to get through the gun. Do you have a percentage that you add to a recipe??

[Jeff Poulter]

Lisa,

     What kind of spray gun are you using?  If it is an HVLP from Harbor Freight you should be fine.  Any larger particles will always accumulate in the chamber, but as long as you clean your gun out each time you should be fine.  I have some spherical titanium that I have to brush on with a little glaze as it will not pass through the nozzle.  I have never been one to sieve glazes.  I mostly use a blender or a blender stick depending on how large the batch is.  I came across one of these on e-bay & they fit perfect on top of the cup on the Harbor Freight sprayer.  That way you don't get any missed chunkies into you glaze sprayer.  jhp

[Norm Stuart]

Lisa - Glen Filer's use of ^6 Frosted Melon over iron-rich ^6 Tenmoku is the same iron plume technique you're working to migrate from ^10.

It's often been surmised that the plumes are black iron oxide rising on an air bubble as red iron oxide loses an oxygen. But this decomposition doesn't greatly occur until temperatures above ^7. This is why I assume iron migration through the top glaze is caused more by thermal currents, but there may be other reasons.

Glen Filer's  iron-rich glaze covered by a pale green glaze on the arms and legs. In this piece the iron migration through the top glaze is extensive.

http://cone6pots.ning.com/photo/man-01

Frosted Melon: http://www.brackers.com/wp-content/uploads/2011/07/pc49-application...

Your iron-rich glaze covered by a matte white

[Lisa Nouvelle]

I have been testing like crazy. I got my kiln programmed for the slow cool and it did work with the White Bird glaze and the iron oxide wash. While I like the look of the glaze very much it is very different from the color that i have been using for my 'line' of work. It is white and very orange, again, glassy, shiny, great color variation and speckling from the clay,etc. I will give it a go again with posting a photo for you to see.

I was wondering if black iron oxide would give me 'cooler' tones. Next on the test agenda.