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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