Lids and Filters

When sterilizing grain spawn in jars, it is important to facilitate gas exchange (GE) while preventing foreign contaminants from entering the jars.  The best and most effective way of accomplishing this is by using synthetic filter discs (SFDs).  Synthetic filter disks are specifically manufactured to prevent spores and other contaminates from passing through the filter, while still allowing GE.  Linked below are a few methods to make SFD lids. 

Polyfil has been used for a long time with good results as a lid filter.   A simple way to make a polyfil lid; simply drill a 1/4" hole in a lid and stuff tightly with polyfil.  

Some cultivators simply use factory lids and slightly loosen them to allow GE.  This in an effective option when you don't have access to other filtration methods.  

In the past Tyvek was a popular method for lid filtration.  Now days it generally is considered inferior to the above mentioned methods.