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

[Norm Stuart]

A controlled-cooling will result in more glaze crystallization and a more matte result. I would guess you haven't previously slow-cooled these glazes.

If slow-cooling not your problem I'd like to see the glaze recipes and photos of the glazes, before and now.

Glazes which turn matte with a slow-cool can be made glossy with a slow-cool by adding more flux and glass. Adding 50% Ferro Frit 3269 can make almost anything glossy, even though it may not necessarily look like the original glaze. 

As an example, this is Pete Pinnell's Weathered Bronze Green, intended to look very matte like the photo on the left. The right-hand photo has 50% 3269 added, and is clearly glossy at slow-cooling, even though it looks nothing like the original.

If your bisque firing is fairly slow, most bisque gas generation will be finished half-way between ^07 and ^08,  but immature bisque would be producing pin holes rather than a matte glaze.

[Brenda Outram]

Thanks Norm

This would make sense as I never did a slow cool before the last two firings and that is when I noticed the change in my glazes. I will do another firing and eliminate the slow cool or maybe just speed it up a bit. Below is what I did last firing, it's a slightly altered Steven Hill schedule. I wanted to try it to see if I would get something more from my existing glaze palette.

1 200 deg/hr to 220 hold 1 to 3 hrs depending on wetness and thickness
2 100 deg/hr to 500
3 400 deg/hr to 2100
4 100 deg/hr to 2160 hold for 1 hr 
5 9999 deg/hr to 1700
6 50 deg/hr to 1600 hold 30min 

7 50deg/hr  to 1500 no hold kiln off

Brenda

Norm Stuart said:

A controlled-cooling will result in more glaze crystallization and a more matte result. I would guess you haven't previously slow-cooled these glazes.

If slow-cooling not your problem I'd like to see the glaze recipes and photos of the glazes, before and now.

Glazes which turn matte with a slow-cool can be made glossy with a slow-cool by adding more flux and glass. Adding about 50% Ferro Frit 3269 can make almost anything glossy, even though it may not necessarily look like the original glaze. 

As an example, this is Pete Pinnell's Weathered Bronze Green, intended to look very matte like the photo on the left. The right-hand photo has 50% 3269 added, and is clearly glossy at slow-cooling, even though it looks nothing like the original.

If your bisque firing is fairly slow, most bisque gas generation will be finished half-way between ^07 and ^08,  but immature bisque would be producing pin holes rather than a matte glaze.

[Norm Stuart]

This is Ron Roy's Sapphire Blue, with increasingly slow-cooling, which is typical of the changes you'll see.

Controlled-cooling emulates a cone 10 gas kiln because the hard fire-brick of a gas kiln retains heat and cools very slowly. The result is often more sophisticated, and sometimes disappointing.

Clear Base Blue (below) was a glaze which needed some extra frit to become a beautiful glaze at slow-cool.

I think Fluorine is an under-appreciated flux because of overblown concerns about free HF gas. Ferro Frit 3269, 5301 and Cryolite are great ways to free up a glaze which becomes over-crystallized with a slow-cool.

[Brenda Outram]

Wow what a difference!

Here are the recipes that I noticed a biggest change in:

Sun Valley ( Conrad Li)

GB 12.12

Talc 10.10

Whiting 16.16

Neph Sy 30.30

EPK 15.15

Silica 16.16

Add:Rutile 7.07

[Brenda Outram]

Sorry Norm I don't have a pic of the glaze as it was when I saw it, it's new to me but I have seen it glossy.

The other is a Jade recipe that I layer over another that usually comes out very glossy.

Feldspar- Custer 40

frit Ferro 3124 9

Whiting 16

Talc 9

EPK 12

Silica 16

Add: Copper Carb 5

Rutile 6

This is the old firing of Jade over my Cream Brulee glaze

This the Jade on it's own that I just fired

[Norm Stuart]

I haven't spent the time to think through why this is . . ., but as a general rule glazes with a big percentage of Nepheline Syenite don't often produce a beautiful glaze with a slow-cool, without adjustments to the glaze recipe.

Low-temperature melters like Gerstley Borate or frit don't produce the sort of problems Nepheline Syenite does with slow-cooling.

The 7% rutile you're adding, is close to enough titanium dioxide to matte most glazes.  Titanium matte glazes use 8% to 16% titanium dioxide. Slow-cooling will increase the titanium crystallization.

[Brenda Outram]

and this is the Jade over the cream brulee now:

Although there is still shine, it's not the same, a lot of the green is gone.

[Norm Stuart]

Slow-cooling Pete Pinnell's "Weathered Bronze Green" also brings out a mustard color and diminishes the green.

This is the original recipe for Weathered Bronze Green,

60  Nepheline Syenite
20  Strontium Carbonate
10  Ball Clay
 9  Silica
 1  Lithium Carbonate
 5  Copper Carbonate
 5  Titanium Dioxide

This is my revised recipe for slow-cooling at 50 F per hour between 1800 F and 1500 F

60.0%	Nepheline Syenite
20.0%	Ferro Frit 3269
20.0%	Strontium Carbonate
10.0%	Ball Clay OM4
9.0%	Silica
1.0%	Lithium Fluoride
5.0%	Titanium Oxide
5.0%	Copper Oxide  --  7% Copper Carbonate Notice Pinnell's recipe is 60% Nepheline Syenite - which suggests bad news for slow-cooling

[Norm Stuart]

This is Alisa Clausen's tile of Weathered Bronze Green, showing one of the many ways this glaze can go wrong.

It is a ^10 recipe used at ^6 without adjustments.

Adding the additional 3269 frit helps, as does replacing the Lithium Carbonate with Lithium Fluoride. Like any ^10 glaze it needs more flux to become a reliable ^6 glaze.

[Brenda Outram]

 Does it have to be 3269? and I don't have any Lithium Fluoride. I have to laugh, just when I think I have every glaze chemical I will ever need I find out that I don't.

[Brenda Outram]

Hey Norm

Where do you get Lithium Fluoride? I just checked the Clay Art and Seattle Pottery catalogues and couldn't find it.

[Norm Stuart]

Lithium Fluoride is a relatively new raw material for art ceramics, being distributed by Laguna Clay and their Axner subsidiary.

http://www.axner.com/lithium-fluoride.aspx

It costs more than lithium carbonate but I like it a lot.

If I didn't have access to lithium fluoride I'd be using Cryolite or Ferro Frit 5301 in addition to the lithium carbonate. Lithium fluoride is ever so slightly more "fluxy" than lithium carbonate, but the flow patterns it creates in the glaze are typical of fluorine - you don't get that with lithium carbonate alone.

Ferro Frit 3134 or 3110 can be used to add more glass to a glaze that over-crystallizes with a slow-cool.  But Ferro Frit 3269 is fairly fluxy having 1.6% Fluorine in addition to potassium and sodium, while 5301 has 9% fluorine in addition to the potassium and sodium, so even fluxier still.

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

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

At $3 a pound Cryolite is probably the cheapest was to buy fluorine flux.

http://www.axner.com/cryolite.aspx

http://digitalfire.com/4sight/material/cryolite_250.html

The drawback to 3110 frit is it adds the sodium, thus increase glaze expansion and crackling/crazing, without adding a lot of other flux so you have to add relatively more of it.

This is an example of what fluorine adds. These are tiles of Magruder Red made with impure and relatively brown Laguna Red Iron Oxide.

The first tile is Magruder Red with 10% Ferro Frit 3134 added.

This second Magruder Red has 10% Ferro Frit 5301 added - notice the metallic crystal sparkles.

This tile is Magruder Red, but made with synthetic Red Iron Oxide.