cut a mask with this ruby lith stuff, and put the negative ON TOP of the mask, IOW, the mask sits between the negative and your paper. I learned the hard way that you can get refracted light out the edges of your negative which will leave a nice thin unsightly black border on your pristine white paper. Using a mask is also the best method to check and make absolutely sure your clearing procedures are working properly. If you see stain or other stuff in the masked-off area, then your clearing methods are not adequate.