Cone 6 tests with Digital Fire Bases

[Tom Humphries]

Hi everyone,

I've been busy being a new dad for a few months, finally got chance to do some glaze tests I've been wanting to do.

With help form this forum I reformulated the digital fire glossy G1214W and matt G1214Z base glazes to use the ingredients available to me in the UK.

I then tried to make a 'semi-matt' version between the two.

I guess they won't be very interesting for most of you but someone may find this useful I guess. 

Overall I'm happy to have some decent results to work with.

Notes :

• Glazes weren't sieved - hence some white specks on some of them, obviously I plan to sieve them in future, it just seemed a bridge too far after all that weighing
• The red iron oxide additives I originally expected to produced some reds. Having read the recent posts on this I think failure to create red may be due to firing schedule - no hold time. My kiln is a kiln sitter type and I don't have a pyrometer. Not sure If I can achieve red colours without having to do holds at final temp?...
• The 'Semi - matt' Is actually more like a gloss. And the Matt seems too Matt for my liking so I will have to work on achieving the desired result.
• Photos were done in natural light but it was a bit cloudy - I live in England 

Any thoughts would be appreciated, am really happy with some of the greens, browns and yellows but will need to work on the reds, and will investigate blues in the future.

Can seem to paste excel data here, the recipes are on this sheet, on tests tab

GlazeChem2.xls

1 to 5

6 to 10

11 to 15

16 to 20

21 to 25

errr.. 25?! again to 28

[Norm Stuart]

Tom - Tin-Chrome colorants are probably the most particular about the type of base glaze which can be used to avoid a red or pink turning brown or grey. 

But red iron oxide glazes have some chemistry requirements of their own.

Supposedly red iron oxide is helped with the addition of Phosphorous (PO4), especially as a ratio to Calcium CaO and my own experience supports this.

Look at the chemistry difference between G1214Z Silky Matte with Orange Street which always provides gratifying amounts of Red Iron Oxide, even if your red iron oxide source is not very pure.

The Silky Matte base contains no Phosphorous compared with 0.064 for Orange Street, even though Orange Street has somewhat more Calcium.  Most use Bone Ash as a source of phosphorous, so you'll always be adding some calcium with your phosphorous.  I would adjust the G1214Z with Bone Ash in place of the Wollastonite, with more silica to make up for what you lose in the Wollastonite.

G1214Z Silky Matte

27  Wollastonite
36  Frit 3124
35  EPK

  5  Silica

Orange Street

46.8%  Feldspar - Minspar Soda
17.9%  Gerstly Borate
15.2%  Silica
13.8%  Talc
12.0%  Bone Ash
  8.1%  Dolomite
  4.5%  Kaolin
12.0%  Red Iron Oxide Precipitate 98% pure
  6.2%  Yellow Iron Oxide

Using a pure red iron oxide with large crystal size like Bayferrox is already a pure red iron oxide pigment, so you get what you start with unless:

1.)  you have an unfavorable glaze chemistry, which breaks red iron oxide down into black iron oxide or other iron compounds;

2.)  you fire hot enough (Cone 7+) to break down this beautiful red iron oxide into black iron oxide or brown.

Looking at your tiles with red iron oxide, it's possible it's still all there but near the clay body, which attracts iron out of a melt, and partially hidden by the opacity of the glaze, rather than laid on the surface in a lavish display like Orange Street does.

Orange Street

Phosphate Glass

Iron Phosphate glass acts the same way as Silica glass does in a glaze, so phosphorous in an iron glaze can end up replacing some of the Silica.  Neodymium Phosphate glass is very expensive and used to construct high-powered lasers.

http://en.wikipedia.org/wiki/Phosphate_glass

Sodium Calcium Phosphate glass Na3Ca6(PO4)5.  These glass pellets actually dissolve in water over time - so not a very good glaze, but it has a lot of other uses.

[Tom Humphries]

Norm - as always you are a fountain of knowledge! Thanks for all the info. It looks like bone ash is the way forward. I will adjust my recipes and pick some up next time I resupply. I will let you know how I get on. Many thanks

[Tom Humphries]

I've been busy the last 6 months building a house extension, just getting back into pottery now slowly. I plan to do some more glaze testing and have a couple of questions before I go and resupply my glaze materials - I still need to get the bone ash to do my reformulated red bases.

I actually used quartz not silica in my previous tests, I'm not sure why. My supplier only stocks 'silica sand 60' which I may have thought to be too coaurse for my sieve... I'm not sure if the 60 refers to particle size though? I think the quartz I used was called 'quartz 300 high purity'

The glazes appeared to work fine, I just wondered what the difference would be as they appear to be both 100% silica ?

My other question is I plan to get some cobalt to try out some blues, is there anything to bear in mind that might prevent me from achieving blues, as was the case when I tried to get reds with RIO?

[Norm Stuart]

Many potters in England prefer to call SiO2 Flint while most Americans call it Silica or Quartz.  Raw materials in finer mesh size melt more evenly.

When you're melting Silica it doesn't much matter if the Silica is quartz, cristobalite, tridymite which are crystallized under heat, or flint (which is a sedimentary version of silica made into rock under pressure), or very fine precipitated amorphous silica (which you can create by mixing some hydrochloric acid into sodium silicate (aka waterglass).

In other uses like hydraulically fracturing a gas well, you'd be concerned with crush resistance and particle shape in addition to mesh size, but you're not.

As an example Laguna sells 325 mesh Silica produced by US Silica under one or more of these brand names: ASTM testing Sands; Glass Sand; Flint Silica; Foundry Sands; Hydraulic Fracking Sands; Min-U-Sil; Mystic White; #1 Dry Special; Penn Sand; Q-ROK; Sil-Co-Sil; Supersil; Mason Sand.  —  Laguna Silica

They guarantee this minimum chemical analysis:

Crystalline Silica (quartz)  CAS-14808-60-7  98.7% to 99.9%
Aluminum Oxide CAS-1344-28-1  <1.1%
Iron Oxide CAS 1309-37-1 <0.1%
Titanium Dioxide  CAS 13463-67-7 <0.1%

If you're interested in your supplier's content, ask for their MSDS sheet for the material.

I don't know of any glaze chemistry which changes cobalt blues into another color.  But if you find one, please post it.  You get more cobalt for your money with cobalt oxide than cobalt carbonate, even though cobalt carbonate costs less per pound.

[Tom Humphries]

Thanks for the info Norm, hopefully I can pick up some glaze materials over the next couple of weeks, I will post my results here. One other thing is, last firing a number of pots had small patches where the glaze was missing after firing - I didn't notice any dry patches before firing. I wonder if my glazes might be a bit thick, and maybe thining them out, and doing a double dip might be a better way of ensuring full coverage?

[Norm Stuart]

Glaze applied thick all at once can definitely peel off the bisque as it dries, ultimately not flaking off the bisque until it's in the kiln and expanding as it heats up.

If I'm in a hurry to get a new glaze test tile into the kiln as it's heating up, I've occasionally had part of the thick cake of wet glaze I just slathered onto the tile fall off in my hands while I take it over to the kiln, the test tile being too wet to adhere the glaze. So I've seen that first hand.

Many ceramic books suggest the most common reason for this problem is a dirty bisque with dust or oily fingerprints on it - anything which interferes with the glaze soaking into the porous bisque.

Many at our studio swear by thining down the first coat of glaze and wait until it dries completely before applying glaze to the other side, and again waiting for it to dry before applying additional coats.

You can also get similar bare patches, once the glaze is molten, through crawling when a glaze has a high surface tension at melt. High magnesium crawl glazes do this by design. "Bisque-fix" which is something like sodium silicate with ceramic fibers and grog, does a good job of forming a glassy surface which repels glaze once it's molten. Calcium phosphate cement which is similar to bone or tooth enamel makes an even better "glaze teflon" - all molten glazes just bead-up over the areas of calcium phosphate.

I'd bet there's other mechanisms which can cause areas unintentionally devoid of glaze.