Bitter aftertaste from mugs

[Phlotsam]

Hi, quick introduction, I've just thrown myself into the ceramics world (fairly late) and trying to make as many mistakes as possible, as quick as possible. I suspect this may prove to be one of them, I just can't get to the bottom of it and would appreciate a few pointers.

In short I'm throwing with a stoneware clay and made myself quite a few perfectly serviceable mugs, or so I thought. The problem is I'm getting a bitter metallic aftertaste everytime I try them.

I bisqued and then fired to 1245°c cone 7? (Turned out it was more like 8 according to my cones) in a shared electric kiln. The glaze inside the mugs was an unbranded lead free (just add water type) from a local supplier. This particular problem happened in 2 glaze firings I did so I thought I'd change the glaze on the inside of my next batch to rule that out. I bought a ready mixed Botz food safe, brush on job but I still get the taste.

Now, being a newbie I couldn't wait to get some oxides and start throwing them at my pots, and this I'm thinking could be the source of my error. In all 3 glaze firings I had decorated some of the outside of the pots with a copper oxide slip, in various quantities, sometimes it was just one or 2 examples, another kiln had pretty much everything decorated with it.

Obviously the copper oxide had boiled through the glaze and I hasten to add I hadn't used it on the inside and sparingly on the outside of the mugs. It was mostly on other items in kiln where it boiled most.

I've tried testing the glaze with lemon juice and it seems stable enough. As far as I can see it's been fired high enough so now I'm thinking that the copper has in some way fumed the kiln to such a degree that it's condensed onto the pots and contaminated everything, is this possible?

I've scrubbed, soaked, scrubbed again and still the taste persists. At this rate they may all end up as plant pots or meet the quality control hammer.

Any advice/pointers gratefully received. 

[Phlotsam]

Apologies, realised I've prob stuck this in the wrong section!

[neilestrick]

Brushing copper directly onto the pots will most likely be too thick for the glaze to lock up the copper, and it does fume out quite a bit. It could definitely be the cause of your problem. Are the glazes you used just plain white or clear, or are they a saturated color?

[Denice]

Try glazing some mugs with just white glaze on them.   I don't know if your kiln is new or used,  but I have read that some chemicals leave a residue on the kiln and fumes when you fire.   I think Neil could tell you if it can be cleaned.      Denice

[Min]

5 hours ago, Phlotsam said:

The glaze inside the mugs was an unbranded lead free (just add water type) from a local supplier. This particular problem happened in 2 glaze firings I did so I thought I'd change the glaze on the inside of my next batch to rule that out. I bought a ready mixed Botz food safe...

Which glazes, could you add a link to the unbranded one from your supplier plus the ready mixed one? Cone 10 clay or one of those wide firing range claybodies? Post a picture so we can get a better idea of the copper slip application.

Welcome to the forum.

[liambesaw]

Definitely sounds like you've got some copper flavored mugs.  Copper is pretty foul tasting.

[Phlotsam]

Thanks for all of your replies people. In answer to some of those questions...

The clay is a white stoneware 1120-1280°c

First glaze is a leadfree s/w transparent 1220-1280°c (no breakdown of ingredients listed)

The second glaze I tried was a Botz 9876 glossy white brush on.

In addition on 3 of the mugs/cups I added a coloured slip inside (I used a slip made from the body and a blue green stain.

The pic shows 4 of the mugs affected, the 2 on the left have copper oxide and cobalt carbonate slip decoration on the outside, a slip on the inside and Botz white inside the mugs and clear glaze outside. The 2 on the right contain no oxides, just slip and underglaze decoration with the transparent glaze inside and out.

Left and right were from two different firings, but both firings had items with copper on that has boiled through the glaze. They all taste metallic, as do the bowls that were in the same kiln.

I'm now thinking I need to make 2 identical pieces and fire one in my kiln and one somewhere else to check.

Everyday's a school day it would seem!

 

[neilestrick]

Lots of variables at play here, but I think you need to start by firing some pieces without any of the oxides or slip  in your kiln and in another kiln to make sure it's not something coming from the kiln itself. Then start weeding out the other possible culprits one at a time- colored slips, copper on glaze, etc. What % stain did use in the colored slip?

[Phlotsam]

Thanks Neil, I've got another firing lined up free of oxides and slips fairly soon so we'll see how that goes. The stained slip was about 10% but one of the mugs didn't have the slip inside and still tasted bad so this led me to think fuming oxides as the culprit. The 3 constants in all firings are, clay, kiln and copper.

A friend of mine uses the same clay with no weird taste so for now I'm ruling that out.

On the subject of kiln firing. The kiln is electric and has been recently serviced and a new bentrup controller fitted. However I know little about using it other than what I've been told. The only opening, as far as I know is a chimney at the top in which rests a kiln brick. Currently, I set the controller, leaving the brick in place, until the kiln is cool enough to open. Occasionally I have taken the brick out at about 70°c to aid cooling a bit, other than that I don't touch it. I wonder if the kiln should be vented in some way during firing by taking out this brick and then replacing it to allow the kiln to get to temp?

[neilestrick]

2 hours ago, Phlotsam said:

I wonder if the kiln should be vented in some way during firing by taking out this brick and then replacing it to allow the kiln to get to temp?

Can you post a picture of the kiln? Normally you would leave the top peephole out to allow fumes to escape. This could definitely be part of the problem- the copper fumes can't escape. Is there good venting for the kiln, so the fumes don't fill your studio?

[Phlotsam]

Here's a few pics of the kiln and the 2inch square opening at the top, this is the only opening on the kiln, the spyhole at the front being just a glass peephole. Done a lot of reading round this today and thinking I could vent that top chimney out of the small window behind the kiln without too much trouble. Question is should it just be left open or do I need to leave access to it to replace the brick at any point during firing. Have read people who close after 600°c and those who don't. 

The kiln is actually in the studio next door to mine. It's a fairly large room but one in which 2 other people who use the kiln work so not sure just venting into the room would be viable or particularly healthy.

Thanks for help with this, I don't think the other people using the kiln know much more than I do about this but as they work in the room with the kiln in it's probably even more in their interests to vent this properly.