Going back to the mineral oil post, Kadiane asks
1. Does mineral oil help in avoiding shrinkage? The length might stay longer if it keeps humidity outside of the hair.
This is a pretty good theory. Treating hair with oil has been shown to reduce the amount of water vapour that the hair can pick up. In addition, mineral oil has been shown to let in less water vapour compared to coconut oil (International Journal of Cosmetic Science, pg135-145, 2007). Shrinkage is related to the hair taking up water and therefore applying oil to it will reduce it. Yes mineral oil will be better than coconut oil for this, but they both DO seal the water in (However, I must state that as a personal preference, I would never use mineral oil for my hair).
2. How long can moisture be sealed in. Does it not evaporate after a while?
Another excellent question. First yes moisture can be sealed in. The oil film on the hair and/or oil within the hair shaft slows down the exit of water from the shaft compared to hair which is untreated with oil (International Journal of Cosmetic Science, pg135-145, 2007).
Yes, the water evaporates after a while. This is dependant on many factors such as how much water you placed on the hair initially, how much oil you used and which type of oil, the temperature and humidity of your environment, etc. One study on skin hydration (not hair but similar) using glycerin and jojoba oil suggests that you can keep the moisture for upto 24 hours at room temperature (Cosmetic Dermatology, pg 268-278, 2008).
3. Do i need to avoid sealing in moisture if i want to stretch my hair and want to minimize shrinkage for few days?
The scientific answer is yes. This is why when you have your hair blowdried or pressed you do need to keep the water off it, no misting or wetting unless you want your hair to revert. I would however just add another answer for those who may be trying to grow their hair longer........moisture is your friend. Moisture allows your hair to be more pliable (flexible) so that it doesn't break prematurely (meaning it doesn't snap off unless you yank it too much). Shrinkage does come with moisture but there are loads of shrinkage proof styles for loose hair such as knotouts, twist outs, pin up styles etc. If you do want longer hair, keep the shrinkage.