So I was getting some water and noticed that I had a barrel with 1 liter of tar in it. I took the tar to a bucket and then filled the barrel with water. Being unable to do otherwise I took some water from the barrel to my waterskin and drank it. Then I thought that hey, this is veery dumb, why should I be able to reuse the barrel for water while it still must have some stains from the tar? Tar is very poisonous to drink.
When you empty the barrel of some liquid that it has, it should become not empty barrel but "barrel stained with x" x being the liquid that it previously had, tar in this case.
To make it empty barrel reusable for other liquids, you would have to wash it by right clicking it with 10 liters or so of water and then quickly scrub the inside with some fancy animation.
New item called scrub brush could be added to make the scrubbing more quick.