If you get a jar like I posted, the air will escape and not let air in. The sourdough will keep longer and better that way.
I bought one from target in the $5.00 to $10 range. Burlington Coat Factory had one too.
Well I am glad that I posted here! It did not occur to me that the jar could be a problem! I have been leaving it partially threaded, not sealed. Should I tighten the lid completely shut? Better kept in the fridge or on the counter?
Thank you @ronott1, @sunflourand @appps!
What rottnot said!
Definatly don't seal it! It produces gas, if you seal it there is nowhere for the gas to go and it will crack the glass. Till you get a better one just sit it loosely on top without screwing it at all. When it's up and running you will find that jar is way too small anyway.