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Q: thermodynamic properties of marble ( Answered,   3 Comments )
Subject: thermodynamic properties of marble
Category: Science > Physics
Asked by: tkk2-ga
List Price: $25.00
Posted: 25 Aug 2005 10:23 PDT
Expires: 24 Sep 2005 10:23 PDT
Question ID: 560354
Please envision a hingedly topped marble box with 3/8" walls and top
whose interior dimensions slightly exceed those of a bar of butter. 
When a bar of butter is placed inside said box and the top closed, the
butter will remain "fresh", will not discolor or melt for 4-5 days.

What is going on here?  As marble has a higher K value (heat
conductivity) than the air trapped inside the box, the butter and the
air outside the box, is the marble box exhibiting thermodynamic
properies by a) absorbing heat from within the box, or b) absorbing
heat from outside of the box?

Clearly the marble box blocks two of the major causes of butter
degradation; 1. UV rays and 2. ambient (convection) air currents but
as the marble, the butter and inside air will all reach the external,
ambient temperature within say an hour, are there really any
thermodynamics at work here?

Many thanks

Tim King
Subject: Re: thermodynamic properties of marble
Answered By: hedgie-ga on 27 Aug 2005 08:43 PDT
There are factors which will affect the aging of a  stick of butter

1. Light, particularly UV light
2. Oxygen
3. The temperature of the surface of the butter

The third one is the most important in normal situations (room and fridge).

The increase in temperature inside the box (after it is taken out of the fridge)
happens by 3 heat transfer precesses: Conduction, Convection, Radiation

The box has two effects

(a) It limits the circulation of the air - limiting convection
(b) The other effect is called thermal inertia.

re (a)
  If you leave stick of the butter, when just taken out of the fridge,
  unwrapped on the table, the conductivity of the air is unimportant.

In a normal, room most heat (about 1000:1 depending on drafts, fans in  the room)
will be brought to the butter by convection (air currents), not by conduction.

It is for this reason butter stick keepers like this one are made airtight.

 The other effect on aging, mentioned by Moarin in a comment,
 limiting the supply of oxygen also exists, but is secondary.

re (b) Keepers made of ceramic or, even better, of marble, like these

add a second beneficial  effect: Their specific heat is high, meaning they absorb
a lot of heat before their temperature will increase by one degree.

Thermal inertia is a combination of mass, low condutivity, and high specific heat.

A metal container would not work (high conductivity). Plastic (being light) tends
to have low  specific heat.  Marble is best because it has high specific heat 
and relatively low conductivity.

 More on thermal inertia (a search term) can be found here

 This second effect, that of thermal inertia, only works if you first
chill the butter keeper in the fridge.
 Depending on conditions in the room, it can take hours to days for a marble
 box to equilibrate to room temperature. You can test this by putting a 
thermometer in the keeper after taking it out of the fridge and
checking from time to time
to see how long it takes for the temperature to rise. Checking will
affect this process if you keep
opening the box, but the effect will still be observable. For better
results use a remote sensor.

 Blocking UV also helps slow aging, and marble again will outperform a plastic box.

Subject: Re: thermodynamic properties of marble
From: qed100-ga on 25 Aug 2005 12:35 PDT
Well, not to be facetious, but under what conditions did you observe
this? Did you measure the actual temperature as a function of time
within the marble box? Did you measure the external surface
temperature of the marble? Did you try it under a variety of thermal
environments? For example, inside a refrigerator, inside an oven, out
in the sunshine, down in the dark basement? Did you try it with other
materials having other melting points?
Subject: Re: thermodynamic properties of marble
From: zodiacman-ga on 25 Aug 2005 14:46 PDT
Dear sir, one of the few things I remember from my college
Thermodynamics class (its been 20 years!!) is that Heat-transfer
always flows from the Hotter/Warmer surface to Cooler/Colder surface
or area, and said heat transfer will continue until an interim
temperature is reached.

if you just bought the Butter from the grocery store, pulling it from
the Dairy section, its likely to be quite cool... lets say 50 deg F.

Then you take it home, and you put it inside the Marble Butter Casket,
which, if it was sitting out in the open for a while, might be 75 deg
F (if you have Air Conditioning) or even higher if you dont.

Assuming that the air inside the Marble Butter Container is same
temperature as the butter itself.... when you put the cooler butter
inside, and close the door, then Heat Transfer will begin.... heat
will flow from the Marble surface to the bottom of the Butter, making
it warmer.  Heat will also flow from the air surrounding the butter
into the top and sides of the butter, again, making it warmer.  The
temperatures of the butter would come up some, and the temperature of
the marble would go down some until they reached equilibrium. At that
point, theoretically, no more heat would transfer from Marble to
butter or vice versa.

If you left the Marble Butter Dish where it was, and didnt move it,
over time, the Marble and the butter inside would gradually assume
same temperature as the sorrounding ambient. Again, heat transfer is
taking place, and heat is flowing from the room to the marble to the
air to the butter.

Or, if you placed the Marble dish inside the refridgerator, over time,
the Marble would become the same temperature as the air inside. This
time, heat transfer is taking place in the other direction, and the
refridgerator's cooling system is remiving the extra heat from the
inside of the refridgerator.

If you did not place the butter in the refridgerator befire too long,
heat transfer would cause the butter to get warmer than recemmended,
and the butter will turn to yellow mush! YUK! Then your wife will come
up, and tell you to stop experimenting in the Kitchen.  :)

Funny, what things you remember from College, huh?
Subject: Re: thermodynamic properties of marble
From: myoarin-ga on 25 Aug 2005 15:43 PDT
I believe discoloration of butter is mainly if not solely due to
oxidation, which is precluded with the the small confines of your
marble butter dish.  Obviously the dish was kept at a temperature that
did not cause the butter to melt.

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