Jump to content
Sign in to follow this  
Ginny C

Stoneware Mug Overheating In Microwave Months After Firing

Recommended Posts

My favorite mug, which has worked fine in the microwave for two years, suddenly gets too hot when re-heating coffee in the microwave. Especially the handle!  Now I believe my son who reported that same effect with two mugs I gave him several years ago.  Anyone know what's happening??

 

(Stoneware, commercial glazes, fired to cone 6.)

 

Ginny

Share this post


Link to post
Share on other sites

My guess would be from moisture getting into the clay through repeated dishwasher, hand washing and just plain usage. The accumulated moisture is getting hot as it's heated in the microwave. You could do an absorption test on the clay to see how porous it is. What clay are you using and do you know for sure it was fired to ^6?

Share this post


Link to post
Share on other sites
Guest JBaymore

My guess would be from moisture getting into the clay through repeated dishwasher, hand washing and just plain usage. The accumulated moisture is getting hot as it's heated in the microwave. You could do an absorption test on the clay to see how porous it is. What clay are you using and do you know for sure it was fired to ^6?

 

Min has it nailed almost for sure.  Common problem with bodies that are not actually highly vitrified. That heating of the water in the body's pores is also causing micro-cracks to form in the structure... weakening it.  Will fail faster than vitrified clays.

 

If it is happening to your cup... it is happening to your clients cups too.   Not "microwave safe".

 

best,

 

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

Share this post


Link to post
Share on other sites

I have never seen this in my world-it must be more common with cone 6 bodies???

I have seen shivering-bloating-exploding-running-hot handles full of water under a glaze? I guess I tend to over vitrify bodies.

I've always been one step over the line.

Mark

Share this post


Link to post
Share on other sites

No, folks, it's Laguna B-Mix 5 stoneware for cone 5 or 6. Fired to 6. Glazed with Robin's Egg first and Hot Chowder on the rim. And it's my favorite just because I like what it looks like. I don't use it everyday because I prefer larger mugs! And anyway I often just rinse my mugs out and hang them up again, rather than putting them in the dishwasher.   Somehow I think it's something else...but I DO appreciate everyone's attempts to explain it!

 

Some of my mugs have stains inside from coffee, but not this one. 

 

So I think the mystery continues!

Share this post


Link to post
Share on other sites

My guess would be from moisture getting into the clay through repeated dishwasher, hand washing and just plain usage. The accumulated moisture is getting hot as it's heated in the microwave. You could do an absorption test on the clay to see how porous it is. What clay are you using and do you know for sure it was fired to ^6?

 

I think without a absorption test which you can do with this clay easy-you will not know the cause.

Cone 5 B-mix WC-401 has an absorption of 2.3%

cone 5 B-mix with grog has a 2.75%

These figures are out of laguna's updated book 4/11

 

Just web how to test absorption and you may find the answer after a test.

Mark

Share this post


Link to post
Share on other sites
Guest JBaymore

It dies not have to be a dishwasher.  Just normal usage.

 

I'd put this as the cause at 99.9999999% likely.  This is common problem with non-vitrified bodies (anything over about 0.5% Apparent Porosity).

 

best,

 

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

Share this post


Link to post
Share on other sites

This may need a new thread, but ......

 

I am currently using standard 181, cone 6 to 10. I use many of the Laguna and Standard glazes, sold dry by the pound, all cone 5. Many or most of these glazes will not go to cone 6. This is the main reason I have started to mix my own glazes. How can the clay manufacturers work like this? Clay to one temp and glaze to another? Looking last week I only found one clay (of Standard, Laguna,and High water) that was stated to be cone 5. Does no one else see this as a problem?

Share this post


Link to post
Share on other sites
Guest JBaymore

It is a problem.  But many less experienced potters are not all that "up" on the technical side of things.  So the manufacturers get away with it.  And the potters think the issue is that THEY don't know what they are doing when they have issues.  This kind of stuff ties one hand behind your back in the pursuit of your work.

 

Note that a good clay body is designed for a pretty tight firing range.  There is no such thing as a cone 6 -10 stoneware body.  It is almost for certain underfired at cone 6 and overfired at cone 10.  It is probably a cone 8 body.  The Apparent Porosity will be high on the cone 6 side... and the MOR and brittleness will go up on the cone 10 side... and maybe also the Apparent Porisity if it is really bad.

 

The suppliers do this so that they do not have to have so many clay bodies in stock.  With the recent shift to more people firing to cone 6.... I expect more suppliers to have specific cone 5-6 bodies in stock soon.

 

If you are firing to cone 6 ... and selling your work........ then search for a body that is specifically a cone 5-6 body....... DON'T use a cone 6-10 body.

 

And test, test, test.

 

best,

 

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

Share this post


Link to post
Share on other sites

I noticed this trend of long range firing clays in the 6-10 range in the 70's. Back then I was teaching HS looking for a viable clay body for the classroom. I did not like the red earthenware we were using, and after looking at Standard Ceramics catalog, called them.  They spoke at great length with me about what my expectations were, where I would get my glazes, what I was going to use the clay for and so many other things. After this talk, they suggested a couple of clays and I chose their 225 that had been listed at 5-6, but is now 4-6. Over the years it has worked well for classroom, and the 112 version worked well for me in functional ware. Both of these have absorption at 2.5 at ^6.

 

I have used some other clays from them with lower absorption, and still talk to them about what my concerns, or needs may be. They never seem to steer me wrong, and seem to know their clays well.

 

Final jest of all this. . . . call your clay manufacturer, not your distributor, and get an opinion from them based on you needs and expectations.

 

Best,

Pres

Share this post


Link to post
Share on other sites

I believe John is exactly right about the Standard 181.  I used to use it for larger pieces, before I went to all porcelain, and I fired it to 8.  It was very functional at that cone.

Share this post


Link to post
Share on other sites

Our little studio had been using standard 181 for years.  When I started teaching there, I said that I wanted to switch clays for just that reason...we fire to cone 6 so why use a clay that is formulated for up to cone 10?  I decided on standard 553 which is a buff stoneware rated 4 - 6.  The added benefit for the studio is that it is cheaper than 181, so we save a little money there.  Still testing glazes on it, but it throws and fires very similar to 181, and personally, I like a fired body that is not so white.

Share this post


Link to post
Share on other sites

I have had two coffee mugs that started getting hot at the handle. I found that they both had a tiny crack at the top end of the handle where it attaches to the cup. Barely visible. My theory is the crack stops the flow of the molecules, or whatever they are so there is a fast back and fourth up and down the handle. And with the handle being thinner than the body, it heats up.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.