A Difference between cotton and Linen

I batch tannin and mordant my fabrics for eco dying so that I have a selection of fabric to use. Usually it is just two or three metres of cotton but this time I also had a metre of linen. I have been using Sumac as a tannin bath because it doesn't discolour the fabric like chestnut tannin or tea and is very saft. Sumac is a middle Eastern condiment. The worst part about it is that it stays in suspension so rinsing at the end can be a bit more difficult. I was very surpris3ed to see that the linen turned a light grey although once line dried this was less noticeable. You can see the difference in the photo above. I have never had a problem ecodying linen but I have been told it needs to be mordanted even more carefully than cotton to get good results. We will see what happens with my experiments over the next few days