Glossy glaze coming out Matt

[Brenda Outram]

I am testing my various porcelains, I'm using tried and true recipes that I have used before. I checked my cones and they are showing 6 fully down and 7 just starting to bend at the tip. I have used these glazes on at least some of these clays before with no problem.

Firing is a slow warm up and controlled cool with a 30 min hold at max temp.

The only thing I have changed is that I now bisque at cone 08 instead of 04 could this being causing some of my glossy glazes to come out Matt?

Any advice would be greatly appreciated, I so miss my shiny Amber.....sigh

[Brenda Outram]

Norm do you have a source for synthetic Red Iron Oxide?

[Norm Stuart]

Iron Oxide-Red (High Purity) from http://www.uspigment.com/chemicals.shtml has fired the reddest for us.

It costs $3 per pound, or less depending on volume.

Iron Oxide, Red, Precipitate from http://www.standardceramic.com/Materials.html costs $3.80 per pound, but has not fired quite as red. I was expecting better results from this Bayferrox product which is no less pure. So particle size also appears to be important to the red iron oxide remaining red.

Anything ceramic suppliers sell as Crocus Martis is also a synthetic red iron oxide as the crocus martis mine in Africa closed 30 years ago.

How synthetic red iron oxide is made: Large quantities of sulfuric acid are used to clean steel and iron in metal processing plants, which creates a lot of metal sulfates. When this used acid bath liquid is concentrated, very pure green iron sulfate crystals form, excluding other other sulfates or chemicals from the crystals. The iron sulfate is then fired in an oxygen-rich tunnel-kiln to extremely pure red iron oxide, and the sulfur dioxide gas is reprocessed into sulfuric acid.

[Brenda Outram]

I purchased 98% red iron oxide from Seattle Pottery, would that be the same as "high purity"?

[Norm Stuart]

You need to ask your vendor what they're selling, or ask them the MSDS (Material Safety Dsiclosure Sheet) which will usually be pretty specific about what you're using.

I had thought that one of the Bayferrox red iron oxides would have produced the reddest reds, since Bayer Chemical invented the process of making red iron oxide from iron sulfate. The product is primarily used to give a red color to cement and printers ink.

U.S. Pigment, Syed Abbas in Elgin IL, is purchasing a knock-off red iron oxide made in China. But unless I switched the bags somehow, I was very surprised to see this is producing the reddest reds. I assume this is because the particle size is larger and less uniform. And I can't complain Syed's pricing. It starts low and gets lower with quantity.

I just wish I could get Laguna Clay to start stocking this product instead of their partly recycled product mixed with barium carbonate, because that way we wouldn't have to pay for shipping as we're less than 30 miles from Laguna's main warehouse.

Red Iron Oxide doesn't readily decompose into black iron oxide until cone 7R.

So if you only fire to cone 6 or below and start with a red iron oxide of fairly large particle size, your should still have a lot of that during the glaze cool-down. Any loose molecules of black iron will have lots boulders made of pure red iron oxide to crystallize onto without much of an extended cooling hold.

I just added Pete's Tomato Red #13 and Post's Tomato Red to the Insight-Live glaze database. Both can be made with Dolomite and a little Whiting (calcium carbonate), or the more costly way with Magnesium Carbonate and more Whiting.

One of the members on Cone6Pots, Sylvain Boucher, got very red results with Tomato Red.

http://cone6pots.ning.com/photo/img-0380

As did another potters on Cone6Pots, Shine Chisholm has a tile of this glaze on her TheDelicion.com Flicker site. As you can see, the glaze is quite red.

This is the same Tomato Red #13 using Laguna recycled red iron oxide with barium Carbonate. As you can can see it's the color of an unripe tomato, so the quality of your red iron oxide matters a lot.

At our studio, Kate Magruder Red is popular, as is Orange Street, both in the free Insight-Live glaze database.

Magruder Red made with U.S. Pigment Iron Oxide-Red (High Purity).

Magruder Red made with Laguna impure red iron Oxide

Two samples of Orange Street made with U.S. Pigment "Iron Oxide-Red (High Purity)".

Orange Street

http://www.uspigment.com/chemicals.shtml

There's additional Red Iron Oxide discussion attached to Shine Chisholm's photo of "Penland Red-Orange".

http://cone6pots.ning.com/photo/penland-orange-red-red-orange

[Norm Stuart]

The real mystery is why Yellow Iron Oxide remains yellow after a long cone 6 firing.

According to Digitalfire and the MSDS, Yellow Iron Oxide is simply a water hydrated Red Iron oxide Fe₂O_{3.  Supposedly it decomposes it decomposes into Red Iron Oxide at 1800C, between Cone O5 and O6, well below Cone 6.}

Yet somehow Yellow Iron Oxide, Fe₂O₃.H₂O, or  FeO.OH, comes out with the clay stained yellow, even after a cone 6 firing. Yellow Iron Oxide supposedly lose it's water at 1800C, between Cone 05 and Cone 6) and reduces to red iron oxide. But is come out of the kiln yellow, so the water is still there or the water has been replaced by another glaze material which allows red iron oxide to still reflect yellow.

Even Digitalfire doesn't have an explanation for this.

http://digitalfire.com/4sight/material/iron_oxide_yellow_876.html\

http://digitalfire.com/4sight/material/iron_oxide_yellow_876.htm

An example of this is

Strontium Crystal Magic Warm is an example of the peculiar heat resistant nature of the Yellow Iron Oxide. Unfortunately, I don't enough chemistry to explain why the Yellow Iron Oxide doesn't change color to red at Cone 6..

118.9%	Strontium Crystal Magic ^6 -10 Warm
45.9%	Feldspar Custer
17.3%	Whiting
15.2%	Kaolin
13.8%	Titanium Dioxide
12.6%	Strontium Carbonate
4.5%	Ferro Frit 3124
4.5%	Lithium Carbonate
2.3%	Bentonite
2.8%	Yellow Iron Oxide

Because of the Temperature stability i use Yellow Iron in place of Crocus Martis, which gives red glazes a brighter yellow coloration. I prefer that look, as in this Yellow Street glaze using Yellow Iron Oxide in place of Crocus Martis.

http://www.lagunaclay.com/msds/pdf/3rawmat/adry/Iron_Oxide_Yellow.pdf

[Jeff Poulter]

So Norm,

    The Neph Sy is the culprit.  I was wondering 'cause there were some things that I layered that should have been killer coming out of a slow cool & they were butt ugly instead..  I will have to go back & check the recipes.  I am wondering about the 3269 & 5301 frits & using them in a macro crystalline glaze.  Would they inhibit macro formation or enhance it?  For macros the glaze has to be very fluid for the crystals to form, but enough has to stay on the pot so that the ingredients are there to form crystals, but you also don't want to many nuclei or it will just be a mass of crystals.  I'm just thinking there has to be a happy medium in there somewhere.  It seems like the 5301 makes for great aventurines & 3269 makes some killer metallic glazes, so I was wondering if they could enhance macros.  After all, when doing macros, it's a very long cool and/or holds in the magic crystal growing range.  Just wondering.... jhp

[Norm Stuart]

The one macro-crystalline glaze I have, but haven't used, is on New Mexico Clay's website.

http://www.nmclay.com/Customer_service/crystals.htm

One of our studio member uses this recipe at home replacing the Ferro Frit 3110 with Ferro Frit 3134 for the first glaze coat, with the recipe above as a second coat. So it would seem the more fluid glaze underlies the stiffer glaze. Maybe he's just winding me up.

Melting Cone

3110 ^08

3134 ^015

3269 ^016

5301 ^013

So the fluorine frits 3269 and 5301 melt like the 3134, but all will be more fluid than the 3110 glaze.

The more I use fluorine as a flux the better I like it, so I'd certainly try it as a macro-crystalline base.

But I do have one recipe with 3269 Frit which turns an ugly opaque mustard when slow-cooled, rather than a crackly lime green glass. It's my version of Laguna Clay's Tang Lime, which also turns an opaque mustard when slow-cooled. It also doesn't take kindly to too much Bentonite.

102.5%	Tang Lime  ^6 - Cannot be Slow-Cooled
80.0%	Ferro Frit 3269
12.0%	Silica
8.0%	Kaolin
2.0%	Bentonite
0.5%	Chrome Oxide

The only aspect of nepheline syenite chemistry which much different from Gerstley Borate or frits is it has more than double the amount of alumina.

http://digitalfire.com/4sight/material/nepheline_syenite_1069.html

It's also the primary ingredient in "snowflake crackle" recipes which in my experience are very unreliable in the way they turn out.

http://ceramicartsdaily.org/ceramic-glaze-recipes/mid-range-glaze-r...