These bags are hand waxed. The wax is embedded in the cotton fibres making the fabric durable and water resistant.
If the bag gets very hot, the wax from the bag can mark surfaces - like leather car seats for example. This is more an issue with new bags where there is residual wax.
When caring for waxed canvas:
For light cleaning, anything that doesn't clean up with a damp cloth, try a bit of water and a soft bristle brush.
For tough stains use bar soap, flaked soap or saddle soap. Do this only if necessary and on specific areas only. Don't use detergent (liquid or powder) to clean this bag. This bag should not be dry cleaned. Dry cleaning will remove the wax.