Shape of metal to be heated in consumer microvave oven? I tried to use a common 900 W microwave oven melt some alloy with low melting point (138 deg C, eutectic bismut/tin).
The metal did not heat up, neither did the metal container, which makes me think the waves got reflected from it just like on the device inner walls.
The oven is working at 2.45 GHz, about 120 mm wavelength.
I expect it is possible to build some kind of antenna that would heat up?
What would be good shapes - thin and 120 mm long? Half and quater wavelength could work too?
Can the individual long parts be connected - like at the center?

There are microwave kilns available commercially. It's a heat resistant foam material container with an inner graphite-containing coating. I do not know whether they work based on a specific geometry of the coating.
 A: For safety reasons, as pointed out in the comments, you should be using a kiln.
Looking  at microwave kilns one finds a youtube DIY.

I found loads of videos on how to use this type of kiln on youtube but none on how to go about making it. So I decided to do it. Now if you are skilled enough to use it you are certainly skilled enough to make it and probably do a better job than me. It uses fire cement, vermiculite, magnetite, sugar and graphite and the only thing I had to buy was the fire cement. If you don't have any vermiculite yo can use perlite just as easily and this can be got from the net or a garden center for about £2 for a huge bag of the stuff - certainly more than you will ever need. Incidently the same process can be used to construct the tube of a muffler furnace - for those out there who are looking for ideas on how to go around Chemical Vapour Deposition at home. I might even get round to this at some future date. This thing can be used for sintering metal powders, ceramics, glass and high temperature chemistry - which is what i want it for..

Note, metal powders . That agrees with this reference 

Because of small  (order of micron or less) penetration depth of electromagnetic
   field into metals, temperature of a bulk metal cannot be raised 

It is the Eddy currents on the surface that heat metals , so in that sense the geometry of powder allows heating.
