[Alkaline glaze random scumming]

On 5/18/2023 at 9:33 PM, Babs said:

Is your kiln toploading? Do you use test cones?. Top shelves of top loaders can be cooler.

Before altering everything, if have no cones, add a soak to top temperature, Try 10 mins to start with.

Refire a failed pot in next glaze firing,  on a lower shelf.

I did another firing without changing anything and all pieces where fine. 

The kiln is a top loader and the  shelf  was near the bottom of the kiln. Going to have to get some more cones to test the temperatures. If it happens again I will try the soak! 

Thank you

 

 

 

[Alkaline glaze random scumming]

17 hours ago, Callie Beller Diesel said:

I’ve had something similar happen in a glaze that had dry epsom salts added to the batch. The salt crystals didn’t dissolve fully, so the bits left almost identical white patches.

So my question is, was your glaze lumpy, and could this have been caused by chunks of something (maybe the soda frit?) that weren’t fully dispersed? Did someone brush the lumps off of the pieces on one layer and not the other?

Hi Callie, no chunks in the glaze and only appeared on that one shelf, which is the curious thing. Thanks for the reply. 

[Alkaline glaze random scumming]

On 5/16/2023 at 11:08 PM, Babs said:

I have had a similar thing happen. I actually posted years ago.

I live coastally so the rainwater added to the sodium. I took the glaze higher and added a soak . Prob solved . 

My glaze was a frit ,nephsy high  C03 glaze, 

Didn't craze .

Hi Babs, thank you for your reply. Did the higher temperature affect the colour at all? The glaze tends to go greener the higher it is fired.  Do you remember what temperature and how long a soak?  Maybe you could share the recipe? 

[Alkaline glaze random scumming]

On 5/16/2023 at 7:21 PM, Min said:

There are a lot of possibilities of what is causing it. I can't recall seeing this issue before. It's almost like an Egyptian paste where salts form on the surface but at a much larger and localized scale. Could be soluble salts precipitating to the surface or a firing issue. It looks like the problem is worse on the rims which would make sense. 

We don't know the firing cone reached on the shelves with this problem versus those that fired okay so thats one variable, size of the kiln, how fast the problem ones cooled compared to ones that didn't. Was the shelf the problem ones on in a small or large kiln, shelf placement in the kiln? Is there a pattern with the firing and the results you can ascertain?

Glaze will be extremely high in R2O fluxes (sodium in particular) and very low in clay. This won't be a durable glaze insofar as being suitable for functional ware.

May I ask what the items are for? Non functional work? If you want to see how badly this glaze will leach out due to being a very unstable base with this amount of copper put a slice of lime or lemon on it (cut side facing the glaze) then cover with plastic wrap for a couple days or submerge part of a piece in household vinegar for a couple days then rinse and dry it and see how the glaze has changed. Also, with such heavy crazing and I'm assuming a claybody with a high absorption rate this would also be a very poor choice of glaze for functional ware.

Hi Mims, thank you for the reply, I will need to do some more firings to see if I can ascertain if any of the above mentioned factors may have affected that shelf in the kiln.

 The heavy crazing is because it is a raku glaze. We use it for that bight colour it gives. It is used for decorative pieces only!

[Alkaline glaze random scumming]

Photos

[Alkaline glaze random scumming]

Hello, thank you so much for all the replies!  Please see the recipe and answers to the questions below. 

Recipe 

High Alkaline frit   - 100

High soda frit - 100

China Clay - 20

Copper carbonate - 8 

Zirconium - 6

Bentonite - 4 

cmc 4 

Water 300

We also add a tbls of vinegar 

................................................................................................

1. It appears to be some kind of solid crystal in the glaze or on the surface of the glaze.
2. I don't know what caused it in the whole kiln loads. We have three kilns and in one the glaze was coming out fine so I just used that one kin for this particular glaze.  
3. It seems to disappear when wiped with a wet cloth and then it is visible again when dry. (Even when wiped with vinegar it does not go) 
4. In the 1st photo there is this whitish crystal thing in some areas. I will try to take a clearer photo. 
5. The firing temperature is around 1020 c. It is a low fire raku glaze
6. No controlled cooling cycle, the kiln was left to cool on it own.  
7. Items are glazed from green 
8. All  pieces where made, glazed and then fired at the same time in the same kiln. All pieces on one shelf only had the problem, the rest were fine. Not sure why or if that particular shelf was hotter/cooler. 

I can't seem to upload anymore photos!?! 

 

[Alkaline glaze random scumming]

Hello, 

We use a high alkaline glaze regularly and today opened the kiln to find all items on one shelf only, had what appears to be scumming on them.

(We have seen this before but normally on a kiln load rather than one shelf of items)

Any ideas of what caused this_  All items where made and glazed at the same time with the same batch of glaze. 

Photo  1 shows the pieces with defects and photo 2 the results from the shelf directly below which have no scum...