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We use plaster of Paris (POP) to cast and lift foot prints/tyre prints from the crime scene for evidence purpose. While preparing cast, we follow standardized protocol and add a pinch of common salt to the POP solution. It catalyzes thickening of the POP. I am keen to know chemistry behind it. How does salt react with POP?

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  • $\begingroup$ Interesting question. Dissolving POP in salt water will make POP set faster too, so it isn't solid NaCl that increases the set rate. An interesting paper is found here (I have no idea if it the current state of the art...) ia801704.us.archive.org/17/items/someaspectsofset755clif/… $\endgroup$ – MaxW Feb 11 '16 at 4:35
  • $\begingroup$ As a side comment I'll point out that exactly how crystals form, and how they grow into different forms is still an active area of research. Many minerals have multiple forms influenced by temperature, pressure concentrations, flow rates and so on. So as you peel the onion on this question it quickly gets very complex. $\endgroup$ – MaxW Feb 11 '16 at 5:13
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My guess would be that it absorbs the water in the plaster of paris. Plaster of paris normally has what's called 'water of crystallization' in it. However, it sets too fast with just this, so extra water is added. The salt absorbs that extra water, thereby making it set faster. Here's a website with details about plaster of paris chemically: http://collections.infocollections.org/ukedu/en/d/Jg16foe/7.4.html and a book called 'Science in the Real World: A simplified story of how technology using chemistry and physics is used in the real world of industry' has good information on it as well.

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