Silicon Carbide #40
All of our abrasive grains and powders come in sealed jars. This keeps them safe from contamination during shipping and later when you are using them. We recommend that you have only one abrasive size at your work space at a time. This will lower the risk of cross contamination of grain sizes which can lead to scratches on your mirror (not to mention lost time having to go back a few grain sizes back to correct the problem). We have also labeled both the jar and lid to make sure each pair is kept together.
When you have finished with this abrasive be sure to clean your yourself, your work space, and your jar thoroughly. You do not want a stray spec of grit lodged in your fingernail or stuck to your shirt to find its way to your mirror after you have started working with the next size.