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

Date: Wed Oct 20 19:17:54 2004
Posted By: Jim Stana, Mechanical Design/Analysis Manager, Lockheed Martin Orlando
Area of science: Engineering
ID: 1094045707.Eg
Message:
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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.

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