Psychotic coffee cup

My favorite coffee cup is ceramic and looks almost hand made. I’d usually fill it with water, heat the water in the microwave for 2 minutes, then let a tea bag steep for a while. After a few months of this, the inside of the cup became stained; gross, but functional. I decided to finally clean it by letting a weak solution of Clorox and water soak in it for several hours. Worked like a charm… however, now after I heat up the water, the cup itself is painfully hot, and the water mostly lukewarm. This is distressing me. Has the bleach somehow changed the composition of the ceramic to do this? This is my favorite cup, it holds almost a pint and a half! How do I fix this?

thanks
mark

Clorox™ is a solution of sodium hypochlorite. Chances are it created a matrix of conductive sodium compounds and nonconductive ceramic, resulting in a microwave-absorbing effect, called interfacial polarization:

[quote]
This mechanism is important for systems comprised of conducting inclusions in a second, non-conducting material. An example would be a dispersion of metal particles in, say, sulphur. Sulphur is microwave transparent and metals reflect microwaves yet, curiously, the combination forms an extremely good microwave absorbing material (So good, in fact, that interfacial polarisation effects are reputed to be the basis of ‘Stealth’ radar absorbant materials). From here. This explains why the cup heats up so much and why the coffee doesn’t. You can try soaking vinegar in it for a day or so and see if that fixes the problem.

Ceramic is porous,when you used bleach on the inside you removed or etch the glazing that was sealing these pores. Now the ceramic absorbs the water and that is what is heated with the microwave

You can’t reglaze it so its probably ruined for good.There may be some type of foodgrade sealer you could use but I’m not aware of it

That doesn’t quite work. First off, water isn’t that efficient at absorbing microwave radiation, so even if it were the case that the porous ceramic was retaining water, plenty of microwave energy should pentrate to heat the coffee in the cup. Secondly, ceramic glazes are essentially pigmented glass and are virtually impervious to bleach. I suspect that the inside of the cup is unglazed, as many handmade ceramic items are.

No, the inside of my cup was glazed, and still is. I’m not saying that there aren’t any microscopic cracks though. I’ll try the vinegar tonight.

Microscopic cracks are a definite possibility too. To help the vinegar along, it might help to stir it occassionally while it soaks.

You know if the surface of the cup has been rendered that microwave-absorbent by the bleach treatment, you might wanna sell it to the Air Force. You might have the next generation in antiradar Stealth coatings there. :wink:

Hmmmm. Can the effect be duplicated?

A new practical joke is born! Chemically modify ceramic coffee mugs so that the microwave oven heats up the mug, which also shields the coffee and keeps it nice and cool. Gotta try it!

[QUOTE]
*Originally posted by Q.E.D. *
**Clorox™ is a solution of sodium hypochlorite. Chances are it created a matrix of conductive sodium compounds and nonconductive ceramic, resulting in a microwave-absorbing effect, called interfacial polarization:

not sure if you are right here. According to the site, this mechanism works for metallic materials. Sodium in solution or in a ceramic would be ionic. Ionic materials are heated by microwaves as mentioned in the site, so it could be something like that blocking the microwaves, but the amounts of sodium etc absorbed into the cup would seem to be quite small. I have noticed that some cups seem very good at getting hot in the microwave rather than heating the water, but don’t know of any explanation for it.

Well, I filled it with vinegar and let is soak overnight (~9hrs). It may have helped, though only a little, I think. I will report back later with specifics after I conduct a couple more trial experiments. I’m on my second cup of tea already…

Not only is my coffe cup psychotic, but I think my plates are going insane as well!

Some ceramic glazes contain metals, particularly lead. Though these have been banned for food use in the US, it is still possible to find them in imported dishware. You can buy a simple testing kit from a hardware store.

Lead would account for your cup heating up, though I don’t understand why it didn’t before the bleaching. Just thought I’d make the suggestion.

Some ceramic glazes contain metals, particularly lead which can leach out during use. Though these have been banned for food use in the US, it is still possible to find them in imported dishware. You can buy a simple testing kit from a hardware store.

Lead would account for your cup heating up, though I don’t understand why it didn’t before the bleaching. Just thought I’d make the suggestion.

Ok, I think I’m on to something. I just heated up my 3rd cup of water today but accidentally left it in the microwave about 30 seconds too long. As I took the searing cup out, I noticed some liquid bubbling out in several places on the top rim of the cup that smelled like bleach. Methinks that when I soaked the cup, some of the bleach solution may have seeped into the porous “clay” material underneath the glaze through microscopic cracks. The moisture is what is heating up the cup. Does this make sense?

I hate to have to find a new cup, but seeing as I don’t want bleach to leach in my tea…

Hmmm… although on second thought, I’ve let a half cup of tea sit in this cup overnight, and didn’t have this heating problem.

One other thing I’ve noticed is that the parts of the cup that get hot are the parts that are not touched by water, i.e., the inch or so of the rim above the waterline, and the handle…

…now I must pee…

Ceramicist checking in here.

Lead is used in some glazes in the US, usually in the form of lead silicate. Lead can be used in ware that is used for food, provided that it does not leach out into the food (there are kits available through ceramic suppliers to test this.) However, use of lead is fairly uncommon in vessels used to hold liquid. Use of lead in these cases is unpopular because it has a very low melting point, which is not very compatible with clay that is often used for functional wares (stoneware or porcelain–they have a high firing temp.)

The reason clay that fires hotter is used for drinking vessels is that the clay itself is not very porous after firing. A glaze in itself is often not entirely waterproof, especially when it is put on a porous clay. When fired wares are cooling, the glaze normally cools faster than the clay, which can create small, often invisible cracks. If the clay underneath is porous, the clay will suck the liquid out of the cup and onto the counter. Even less porous clay can do this to a degree. Continuous use of the cup will eventually solve this problem, because the minerals from the water will fill the pores. This makes the cup denser, and perhaps more susceptible to microwave heat. Interestingly, letting tea sit in vessels is an old potter’s trick to make them less porous.

Perhaps that is more information than you ever wanted.

No knowledge of chemistry here, just a household tip: coffee and tea stains can be removed with lemon juice; it works almost as well as bleach, with fewer aftereffects.

I will keep this in mind for the future. I’m gonna soak my cup over the whole weekend in a last gasp attempt to save it, but I’m figuring Monday I’m going cup shopping. The one I’m using now is a teeny tiny little thing with my work’s logo (ugh) emblazoned on it. :frowning:

http://www.csgnetwork.com/clothingwomenconv.html

This site has a javascript US <-> European clothing size converter. I cannot vouch to its accuracy, but it may help.

Oops! Sorry! Wrong thread!