In response to @Fiona Robertson's questions about sea water in Zany's No-Slime Castile thread, I thought I would start a thread about this. Fiona, you state that you are worried about plastic waste on your beach and consequently the amount of pollution/contamination in the water. One of the advantages of living in New Zealand is that we have a small population (only 4 million in a land slightly larger the size of the entire UK) and thus, less pollution. The beach I go to is by a very small township, and they are a very 'green' community. So before I even collect the sea water, I know it's probably a lot less polluted than the water surrounding the Edinburgh area (nice city BTW - I've been there). Once collected, I sieve it first to remove anything major, then I boil it for a short time ( about 5 mins max) to remove any impurities. I then pass it though a coffee filter and freeze it once cooled (thanks to @SaltedFig for that idea). I often use small 'snack sized' plastic zip lock bags and label the quantity - smaller quantities are better so you can use one, two or three for a batch of soap. Or you can ice cube them all and store them that way. Hope that helps. It would be interesting to find out what other people do with their sea water.