I have used and sterilised cork bark alot for my spiders over the last 2 years. I found the best way was to wash it in boiling hot water, give it a good hard scrub with a dish / plate brush, dry it off with a clean towel of some sort and then strerlise in the microwave for about 10 seconds. Much longer and the bark will start to go funny, crack because of the microwave making the wood expand. This method has given me no problems at all and is quick and easy to do. I hope it helps you out.