[MarianaM (OP)]

Tina is asking...

To help the process of making coffee in my cheap espresso machine, I heat the water in a 1/2 pint glass and my milk in its cup in the microwave before letting the coffee-maker do its thing...

I have found that if the water (in its glass) is (pre) heated in the microwave before I put in the cup/cold milk, then the water seems to continue to heat more than the milk!?

Is this all in my head OR is the glass better at taking-in heat (thus heating the water more) as it heats up and ostensibly becomes more susceptible to the microwaves (as a virtual liquid in itself)?

Can you help?

[Hayseed]

Have you ever rubbed two sticks together for heat?  A microwave rubs the sticks together, and when you remove from oven, it takes a minute for the sticks to quick rubbing.

It can appear that the water is still heating after removal.

Water and other molecules need some time to slow down.  And until they do, it's still cooking.

[alancalverd]

There are all sorts of hidden variables here!

The distribution of power in a domestic microwave is very uneven, which is why we have to "stir and leave" before eating microwaved curry. The cup and glass may have different shielding and absorption properties: some glazed mugs get very hot then transfer the heat to their contents. And they have different shapes so will put their contents into different areas of the microwave field. And milk and water have different microwave heating properties even if  you put equal quantities into the same vessel.

[Hayseed]

Very true.  My favorite ceramic cup gets much hotter than the coffee in it. No warm ups for that cup.

[Bored chemist]

it takes a minute for the sticks to quick rubbing.

No
Redistribution of energy from microwave absorption to heat is very fast- much quicker than you can open the door.

However,  the transfer of heat through things can be relatively slow.

It's also important to recognise that glass doesn't (normally) absorb microwaves, so it doesn't get hot.
(If you heat it until it is soft then it will absorb them).
If the glass feels warm, then it has been heated by the water- not the other way round and (usually) the same will be true of a mug containing milk.

[evan_au]

I have noticed that ceramics with a blue glaze sometimes get quite hot in the microwave.
Blue glazes often contain cobalt, which has a magnetic moment, and is likely to get pushed around in a microwave oven.

[Hayseed]

This old cup of mine is an ugly orange. It was probably made out of some foreign uranium mine tailings.  Time for a new one.

[Bored chemist]

Blue glazes often contain cobalt, which has a magnetic moment, and is likely to get pushed around in a microwave oven.

Many- perhaps most- ceramics contain iron...

[Petrochemicals]

I have noticed that ceramics with a blue glaze sometimes get quite hot in the microwave.
Blue glazes often contain cobalt, which has a magnetic moment, and is likely to get pushed around in a microwave oven.

I imagine pyrex is similar, it gets lots hotter than crockery

[Yaletha]

Glass will absorb microwave energy - that is the main function of the glass turntable - to absorb microwave energy and protect the magnetron. However, many consumers break these and don't replace them - a good way to end up with a fire inside the oven. But the magnetron is still protected by a thermal switch that shuts it off if there is too much reflected energy. But, if this happens too often the magnetron may end up being seriously damaged. By the way, we've melted a Pyrex beaker in a microwave oven ~ 1500 C!
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[Bored chemist]

By the way, we've melted a Pyrex beaker in a microwave oven ~ 1500 C!

Maybe 1500F but probably not 1500C

Glass interesting.
Cool glass doesn't absorb microwaves very well- in fact, it's a very poor absorber.
However, if you heat it  by some other means until it starts to melt then it becomes quite a good absorber of microwaves.
It doesn't take much to get it started. A bit of charcoal from something burning onto the glass would do.

that is the main function of the glass turntable - to absorb microwave energy and protect the magnetron

If that was true it would get hot, but it pretty much doesn't.

[vhfpmr]

One of my Pyrex casseroles gets quite hot in the microwave, you can see from the thin film interference that it's got some sort of coating on it.