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?

  • $\begingroup$ 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
    Commented Feb 11, 2016 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
    Commented Feb 11, 2016 at 5:13
  • $\begingroup$ I tried using brine instead of just a pinch of salt and the gypsum plaster didn't set at all. Not sure why just an observation. I guess too much salt could cause it to set too soon whilst still particulate and so the particles can't set together $\endgroup$ Commented May 20, 2023 at 18:30

1 Answer 1


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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.