I had a tube of watercolor that had dried up and was rock hard. It wouldn't squeeze out of the tube. That doesn't usually bother me because I can just cut open the tube and rewet the paint with my brush. However, my husband put some of these empty paint tubes in my Christmas stocking, and I have been dying to use them.
So I put the hard cakes of dried up paint in a little dish and soaked it in water. I covered the dish with plastic wrap for several days and left it. It absorbed all of the water but was not completely soft. So I put some drops of gum arabic on the cakes and mashed it all together with several makeshift tools. I did not have a muller, but found some rounded objects and ground it all together. Then I used a palette knife to blend it until it was a smooth texture. I scraped it into the bottom of the empty tube. I poked down in the tube with a toothpick several times to release air bubbles. Then I used a paper towel to clean up around the bottom opening. I folded the bottom several times and then took it to my husbands vice. I pinched it with the vice, being careful not to pierce the tube. Voilà- a new (ish) tube of Ultramarine! Not necessary but Fun.