Re: Are stoneware or glass cups better for keeping water hot

I don't know that I can answer the question exactly because the thermal 
properties of a ceramic will depend a lot on the glazing and how dense it 
is (full of air bubbles or not).

Let's start with what happens when you pour hot water into a vessel of any 
type.

The water starts out hot.  The cub starts out at room temperature.  Just 
after you pour the water into the cup, the heat from the water starts 
flowing into the cup and trys to raise the temperature of the cup.  
Shortly after being poured, the cup is raised to a temperature somewhat 
lower than the initial temperature of the water because the water has 
cooled off by giving up some of its heat to the cup.  This temperature 
will depend on how much water you have, the mass (or weight) of the cup, 
and the thermal capacity of the cup material.  For glass, it takes about 2 
million joules per cubic meter of glass to raise the temperature by 1 
degree celsius.  For water, it takes about 4 million joules per cubic 
meter of water to cool it by 1 degree C.  I couldn't find a value for 
ceramic.

Once the glass reaches the temperature of the water, the water will start 
to cool by evaporation (steam coming out of top surface) as well as 
conducting heat from the glass to the air surrounding it.  In addition, 
the cup will conduct heat to the surface it is resting on.  I supsect most 
of the heat will be lost from evaporation.  If you poured two identical 
cups of hot water and covered one, the covered one would probably stay hot 
a lot longer.

The thermal conductivity of glass is about 1 watt per meter of thickness 
per degree C.  I found all kinds of values for ceramics, from 0.5 to 20 or 
more.  If the conductivity was 20, then the ceramic mug would conduct heat 
much faster than the glass. (Making the glass mug better for keeping the 
drink hot.) The thicker the mug would make the heat travel slower, and so 
would having more air bubbles in the ceramic.  But a metallic glaze on the 
ceramic would probably make the heat trevel faster.  I suspect that the 
evaporation of the water out the top would be the overwhelming effect, 
making the choice of glass or ceramic a minor contributor.

If you could find a glass mug the same construction and size as a ceramic 
mug, you could perform an experiment by pouring hot water into the two 
cups from the same source and put a thermometer into each one.  if you 
watch and record the temperature versus time over 20 minutes or more, you 
would be able to answer the question.

In reality, finding two glasses that similar would be difficult.  You 
could try several ceramic and glass mugs and average the results.  Just be 
sure you use the same hot water temperature and amount to start to keep 
things even.