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I've been using Continental B Clay bisque fired ^04  and fired to ^6, I use Potter's Choice glazes.  Most of my cups make my coffee and tea have a bitter aftertaste.   Does anyone know of a reason for this?

 

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JBaymore    1,432

Welcome to the forums, Julie.

Can we get a bit more information please.  It will help people possibly come to a correct conclusion.  There are a LOT of variables that could be at play in this pesky ceramics field.  Sometimes the "obvious" answer is not the real cause.  And sometimes it is more than one thing going on.

Are the "most" that cause this effect glazed on the interior with the SAME Potter's Choice glaze?

Do the ones that have the aftertaste all come from the same firing?

Do you use witness cones in your kiln when you are firing or just the automatic computerized controller?

Are the ones that cause the aftertaste crazed on the inside?

Are the ones that cause the aftertaste washed in an automatic dishwasher and the ones that don't are not?

Do the same ones that cause the aftertaste ALWAYS cause the aftertaste?

Do the ones that have the aftertaste seem to get less aftertaste over usage?

best,

................................john

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JohnnyK    87

It may seem silly, but have you considered the problem might be the coffee or tea?

I say this because I also use Potter's Choice glazes and have never noticed a difference in taste...could be because I use cream and sweetener in my coffee.

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Joseph F    866

Also which potters choice glazes are you using for the inside would be helpful to determine this as well. 

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Glazes used are Iron Luster, Deep Olive Speckle,  Blue Rutile, Blue Midnight,  Tourmaline,  Seaweed.   Glazes are used inside and outside, alone or layered, all dipping glazes bought dry in 5-gallon pails, mixed with distilled water.  Obsidian doesn't leave an after taste.   Several households with different coffee, tea, water and washing process have the aftertaste.

I don't  use witness cones, my kiln is a newer Skutt electric.   Cups from several firings.

I wash them in the dishwasher.  

The only glaze that crazes is Amaco zinc free clear.

The coffee tastes more bitter the longer it sits in the cups and gets progressively worse with each use.  I've tried refiring, which makes the taste issue worse.   I'm wondering if it's a clay/glaze fit issue, or possibly over firing the Potter's Choice Glaze??   At any rate, it's very discouraging.

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Min    784

My first guess would be the kiln didn’t fire hot enough.

 Really do need to use witness cones to verify the firings. Others will have other suggestions but this is what I would do if they were my pots. First off I would suggest to stop using the mugs. Get some witness cones, for cone 6 you will need cones 5,6 and 7, make some cone packs up and let them dry or use self supporting cones.

I would refire the mugs but put them on some thin scraps of rolled out clay (wasters) just in case the glaze runs and drips since it could be more fluid with the second firing.  You don't need to bisque the wasters. There is the distinct possibility that if the mugs are under fired then they will be somewhat porous and have soaked up some water. Place the cone packs (dry) in front of the spy holes in such a way as you can see them. Program in a hold at around 180 for a few hours to drive off any water that might have gotten into the clay then continue with the firing. Towards the end of the firing check the cones, wearing eye protection. Extend the time and or temperature of the firing until you see the middle cone with it’s tip touching the cone pack (or close to it). I would refire anything else that you use for food from the same loads also.

Re your comment about it being really discouraging…. working with ceramics can be like that no matter how many years you’ve been doing it for. There is always something that can come along and smack you in the face. Just gotta learn from it, move on and hopefully don’t repeat the same mistake too many more times.

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GEP    863

Continental B-Clay is a cone 6-10 body. At cone 6, it's probably not fully vitrified. I think the issue is with underfired clay, not with any of your glazes. 

Anecdote: at the studio where I used to teach, they used to stock a cone 6-10 clay. One of my students reported that she left water sitting in a mug, made with this clay, overnight. The next day, she refilled the mug with fresh water and put it in the microwave, and a black substance oozed out of the walls of the mug. An undervitrified vessel can grow all kinds of gross things inside its walls. The studio stopped buying that clay and switched to Little Loafers.

Here's a quick test to try. Fill one of your mugs with water, and put it on a paper towel on your counter. Give it a few days. Does the paper dowel become damp? If so, the problem is your clay, not the glazes. 

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Min    784

Having an underfired clay isn't going to help but a lot of earthenwares have high porosity and don't develop  a bad taste in foods because the glazes are fired to maturity. Agree on using cone 6 clay not 6 thru 10. Really would use cones and check what you are firing to also.

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Thank you all so much for the advice and suggestions!  I put a call into Continental Clay today telling them about the issues and was told they've not heard of this before and no one else has reported any issues with the B Clay and under firing at ^6.  He suggested that I fire to ^5 with a 20 minute hold.   I will need to order clay soon,  any suggestions for a safe white clay to use?  We live in Northern Wisconsin.

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GEP    863
1 hour ago, Julie Huebner said:

no one else has reported any issues with the B Clay and under firing at ^6. 

Notice that this is not the same as saying "the clay is mature at cone 6." You should still do the leak test, and let them know if you find leaking. And I agree with Min about witness cones, it would pay to find out exactly what your kiln is giving you. 

Anything with a max rating of cone 6 would be a good choice. 

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Joseph F    866

I agree with Mea. Thinking about that clay body absorbing that tea over and over would definitely make the next batch of tea bitter, particularly like you said that it gets more bitter the longer in the cup. 

The question you need to ask the manufacturer is: What is the absorption at cone 6 for that clay? It is probably too high for ware that is being used for food and drink. Ideally you want <1%. But most people settle for <2%. 

 

Edited by Joseph F

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JBaymore    1,432

If the inside glazes are crazed, AND the clay body is porous (a 6-10 body fired at 6... almost for sure not mature), AND if the mugs are washed in a dishwasher, AND if the dishwasher uses a "finish rinse" to enhance drying and water spotting........ THAT could be the issue.

Sometimes crazing is there but you can't see it easily.  Take a mug that exhibits the issue.  Paint some India Ink liberally on the interior glaze in a big swath.  Wait a couple minutes but not long enough for the whole surface ink to dry.  Then wipe the ink off the surface.  Look for tiny darkened lines.  If there....... it is crazed.

 

Absolutely do GEPs test for the weeping.  You should do this with an unglazed but fired to cone 6 cylinder also.  I will be SHOCKED if this is not weeping.   Clays doe NOT have that kind of firing range for reasonable vitrification.  It is either a cone 6 body mature at cone 6 and then overfired at anything above about cone 8......... or a cone 8 body underfired at cone 6 and overfired at cone 10 ....... or a cone 10 body underfired at anything below about cone 9.  The manufacturers do this 6 to 10 garbage to not to have to have so many clay bodies in stock.  They get away with it because so many people do not know better...and think the issues they experience are their own.

best,

.......................john

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glazenerd    816

John:

I would like to add a caveat to this topic and to the "weep" test. Stoneware clay bodies hold moisture on their inner platelettes, whereas porcelain only hold moisture on it.s surface. An example: OM4 ball clay will hold 35 grams of water in every 100 grams of clay material: without showing any signs of weeping. That number is much lower after a firing: if immature it would be in the 3-5 grams of water per very 100 grams of material. Simple math: an 1lb. Mug could conceivably hold as much as 12-20 grams of water without showing any signs of weeping. Compound this by the exterior glaze being vitrified, and trapped water unable to pass through it.

So while the weep test is one method of testing, it is not a definitive test for a piece holding/releasing moisture in the clay. If a scale is accurate enough: fill the piece with water and let it sit 24 hours and empty the water and towel dry. If the weight before and the weight after differs: it is holding water internally. Immature stoneware would be highly susceptible to this problem.

i would recommend making a 4x4 tile using this (or any) clay and fire it to maturity unglazed. Let it sit in water 24 hours and towel dry before weighing. If the weight differs before and after the soak period: then you know with certainly the clay is holding water. This differs from the standard boiling test: but it will give a quick insight into the problem.  Hate to be offensive, but I suspect the after taste is from fungus growing inside the clay walls. 

Nerd

Edited by glazenerd
Correcting the auto corrections- as usual

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I filled a few cups with water and let them sit over night on paper towels, no moisture.  Also tested for crazing on the inside of the cups,  none.   I also filled one with water and microwave it for 5 minutes, the water remained clear without seepage.  I have used these same glazes on 5 different clay bodies using two different electric kilns. There have been taste issues with various cups from all firings.  As for the dishwasher I understand that this could be part of the issue, but not all of the offensive cups have been in a dishwasher, a few have been rinsed in clear water after firing still having the taste.  All clay ^o4 bisque and ^6 glaze fire,  Maybe it's just how pottery tastes.

Clays used:

Laguna BMix

Standard 112

CC Dark Iron Stoneware

CC Bclay

CC Mid range oxidation with manganese

 

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glazenerd    816

Julie:

then borrow some porcelain from a potter friend and make a few small cups . Glaze using those mentioned: if there is no after taste- you are down to a clay issue.

strange question perhaps, but it has a purpose. Does the water you use to throw develop a pungent smell in a short period?  The darker the clay body: the higher the sulfate/ carbon content. Organics in a clay body develop a musty/rancid smell in the throwing water in a short period. If you change water everyday then you probably have not noticed it. If you let it sit a few days, you will.

Nerd

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JBaymore    1,432

Julie,

Make about 5 -6 test bars with the clay that is exhibiting this "aftertaste" business.  Put a number on each tile.  Typical size might be something like 14-15 cm long, 1-1.5 cm thick, and maybe 4-5 cm wide.  Bisque them and then high fire them unglazed.  IMMEDIATELY after the firing while they are still warm from unloading.... weigh them VERY carefully.  Go crazy with accuracy.  If you have access to a scale that is accurate to one HUNDREDTH of a gram... use that.  Record the weights of each bar.

Then take the tiles and place them in a pan of water and boil the water for a few hours with them in there.  Take them out and dry off the outer surface with a nice soft fluffy towel.

The  IMMEDIATELY weigh them VERY carefully once again.  record the weights.

Post the weights from both ends of the tests here.

 

This aftertaste business is NOT normal.

best,

...................john

 

 

 

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JBaymore    1,432

Hum.......... do you have cups form ANYWHERE that DON'T make the coffee and tea have the aftertaste?

Side by side test....... one cup that does and one cup that doesn't.  Same way to heat the water.  Same tea or coffee. Simultaneous test.

best,

..................john

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GEP    863

@Julie Huebner Overnight is not long enough for a leak test. It can take 3 or 4 days. 

In the anecdote I mentioned above, "overnight" was the time frame of that particular story, but keep in mind that mug was months old and had probably been soaking up moisture for a long time. 

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