Glaze sometimes matte/ sometimes glossy?

[studiosoda]

Hi everybody!

I have been testing glaze Nr. 255 from the book "The potters book of glaze recipes".  It's an iron glaze for 1200°-1260°. On all my test tiles it came out glossy. When I did a firing in my studio (cone 6), some pieces from the same kiln came out glossy, some matte, depending on the shelf. I presumed it was that specific clay. I did further samples (in a tiny kiln) and on that same clay it came out glossy this time. The firing schedules could have been a bit different, I'm not sure as my studio does it. 

Now I did a firing in my own toploader kiln (1240°). All the pieces glazed with it came out matte - no matter which clay. They were all on the bottom floor. I'm wondering if they might be underfired? How come a glaze is sometimes matte, sometimes glossy? Does it mean it's underfired? Can the firing schedule play such a huge part on the outcome?

I hope somebody has some advice  Thanks!

left is mate/right is glossy - same clay

Edited June 1, 2023 by studiosoda

[Hulk]

Could be the cooling rate?
Faster cool, more gloss; slower cool, more matte.

The tiny kiln almost certainly cools down faster - more surface area to volume (and mass).

While waiting on more replies, please post the glaze recipe.

[studiosoda]

@Hulk

 

This is the recipe:

Feldspar 50

Whiting 20

Zinc Oxide 4

China clay 11

Flint 15

Red iron oxide 10%

 

Good to know, thank you. If it's the cooling; when I fired in a huge kiln in the studio the same glaze came out matte (lowest shelf) and shiny (other shelf) from the same firing... Also I noticed another glaze I use come out fine (green) on the middle shelf and completely another color (blueish/grey) in the bottom shelf of my kiln... Therefore my question, if it's normal that glazes do that sort of thing.

[Hulk]

Good question!

Perhaps some glaze experts will have something to say about your recipe's gloss/matte behavior.

I'd noticed that top shelf (my old kiln, given away; now waiting on setting up new Studio and, eventually, a new kiln) would have glossier finishes, so I started doing a controlled cool to 1850F, then shut down the powered kiln vent and heat at the same time.

As for color, for sure my glazes will sometimes pick up fuming from neighboring pots. The amount of heat work, thickness of the glaze, cooling rate, any underglazes, and the clay all influence color as well. 

[Kelly in AK]

In my soda kiln (c6), it both gets hotter on top and cools more slowly there. Selsor’s Temmoku is almost always matt on my top shelves and glossy down low. It’s not because they’re under fired. The slow cool is the obvious variable in my case. I believe the high percentage of iron has an effect, because other glazes using the same base don’t behave that way. 

[Bill Kielb]

6 hours ago, studiosoda said:

I hope somebody has some advice  Thanks!

left is mate/right is glossy - same clay

Generally if a known glossy glaze fires very matte, it is usually under fired. Clays and glazes melt by cone or heatwork and not necessarily peak temperature. Different places in the same kiln can fire to a different cone. So my best guess, underfired.

[Min]

Need to rule out one of the variables, do you have access to cones to see what each shelf in each kiln is reaching?

 

[studiosoda]

9 minutes ago, Min said:

Need to rule out one of the variables, do you have access to cones to see what each shelf in each kiln is reaching?

 

Not yet, but I just ordered some and will test it. I just read through the specifics of my oven and found out that it is single zone, maybe that could explain temperature difference...

[Min]

11 minutes ago, studiosoda said:

Not yet, but I just ordered some and will test it. I just read through the specifics of my oven and found out that it is single zone, maybe that could explain temperature difference...

It is common for single zone kilns to fire unevenly if they are not loaded well. I would suggest putting a cone on each shelf until you get to know how it fires. Also, kilns generally fire cooler at the bottom and top and hotter in the middle (where the thermocouple usually is) so to compensate for this place your tallest pots on the bottom of the kiln, short pots in the middle and tall again on top area. By having the shorter pots in the middle there will be more mass (kiln shelves) to heat up so that area will in effect be slowed down in the heating allowing the top and bottom to hopefully reach the same temp by having more elements per volume of space and mass.

[studiosoda]

Thanks for your help guys. I will check with cones & load the kiln differently to see if that makes a difference and report back if I make any progress

Edited June 2, 2023 by studiosoda

[Kelly in AK]

Yes, there is no substitute for cones in the kiln. At school our bisque kiln gets hotter on the bottom, unusual. Heavily packed loads will cool markedly slower than light loads. Cones give you a lot of information.

[studiosoda]

Hi everybody, so I have an update: Turns out that when my boyfriend made a new batch of this glaze he accidentaly took the recipe from the next glaze in the book  which is a matte brown and marked it wrong.  Ups.... mistery solved.... It's still an issue though as I have seen this glaze come out matte and glossy from same kiln, which seems to be because of different heat in kiln.  Thanks for you help!