Portland Gray, 9992 was developed to fill a hole in the borosilicate palette. There were no gray rods available and the pre-forms went almost white... how could you make a believable shark? So we made the first opaque gray; we chose neutral #5 on the gray scale as a starting point.
 
Working Tip: Per-heat in outer flame.  Avoid rapid heating.
 
Artist Description by Mike Shelbo:
"This medium gray opaque crayon works amazingly well compared to other opaques and is perfect for solid sculpture when used without encasement.

It melts into itself very nicely and can be worked in a variety of flame mixes and regions that allow it to be versatile in all applications. When blown thin enough, the gray will become slightly translucent, this effect is best achieved sandwiching between clear and blowing thin or drawing down.

Treat Portland Gray as you would other GA Crayon colors. This is the best gray you can find for monochromatic color schemes." - Mike Shelbo