Vellum is animal skin. The vellum used for this Indenture was made out of goat skin. The animal skin itself contains two distinct surfaces: the hair side and the flesh side. The hair side refers to the skin's outer surface, presenting the most beautiful working surface. The flesh side, although workable, is uneven and greasy. You'll know which side of the vellum is based on the presence or absence of hair follicles.
Preparing vellum for ink is a whole process, which turns the vellum's hard, shiny surface into a velvet-like finish.
There are three problems that you need to address when you receive your vellum:
Remove the shiny, hard surface
Remove any grease or fat left over from the manufacturing process
Make the surface more accepting of ink, gesso or paint
First, identify which side of the vellum is the hair side. Do not lay the vellum directly your desk or table, wood surfaces with any distinct grain will imprint onto the vellum. Place the vellum on a smooth support surface, ideally on an acid-free board. Start sanding at the middle of the vellum, working with the grain. Figuring out the grain can be challenging on some samples of vellum because the grain will vary at the spine and body areas. Careful examination of the surface is necessary to locate each direction. It's easier to see the grain if you place the vellum onto a light table or something to illuminate it.
The next step after sanding the vellum is to remove the grease. Grease on the surface of the skin is normal and is typically caused by incomplete fat removal at the manufacturing level. Lightly dusting the surface of the vellum with ground pounce will remove excess surface grease and stop the oils left over in the vellum from rising to the surface. Brush off the excess!
PRO TIP: You can make pounce at home. White pounce is made with talc, grey pounce is a combination of talc and charcoal, and black pounce is made using charcoal.
The last step prepares skin areas reserved for calligraphy. Ground gum sandarac will help speed up the application of ink and assist in the execution of crisp lettering.
PRO TIP: Brush off the excess pounce and sandrac because excess amounts can make the vellum resistant to anything you apply to it. The ideal level of preparation depends on the condition, thickness, and purpose of the skin. The amount of work that goes into preparing a skin used for painting and gilding will not require the same tooth as a skin used exclusively for calligraphy. Personal preference and experience will determine the required surface for each vellum.