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 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 http://www.efunda.com/formulae/heat_transfer/home/overview.cfm 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. http://www.mileskimball.com/shopping/product/detailmain.jsp?itemID=8246&itemType=PRODUCT 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 http://fantes.com/butter.htm#keeper 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 http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1144.pdf 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. http://www.amazon.com/exec/obidos/tg/detail/-/B00005NN9B/002-9468604-7590402?v=glance Blocking UV also helps slow aging, and marble again will outperform a plastic box. Hedgie```
 ```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?```
 ```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?```
 ```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.```