I'm getting ready to cover my Electric S-1 Ringmaster with silkspan and wanted to determine the grain so it would run span wise for the extra strength. I tried searching this forum, there is a lot of how to apply but I didn't see anything about determining the grain direction. In some material it is very easy to determine by looking at it. Silkspan is a bit less obvious. I've heard you can do a tear test in which you take a small piece of material and tear it straight down. If you are tearing in the direction of the grain it will be a smooth tear line, if against the grain it will be ragged. For me this was not cut and dry, both tears were similar.
What I then did was a pull test. For this you take a small piece of the silkspan (noting the direction you cut it from the sheet) and simply grab it with your thumbs and forefingers near the middle of the test piece (I used a piece about 1" sq) and pull apart. If you are pulling in the direction of the grain you will find it takes about 2 -3 times the pressure to pull it apart then if you are pulling across the grain. The results were very obvious. This should work for other materials like doctor paper or other tissue.
Best, DennisT