Rollei RBM Black Magic 4 is pure photographic gelatin for the Black Magic system, 100 g.
What it is for
It does two jobs. First, preparing the support: a porous material like watercolour paper, wood or fabric is sized with gelatin first, or the emulsion soaks in instead of forming a layer. Second, adjusting the emulsion's viscosity to suit how you apply it.
How to dilute it
Storage
Sealed concentrate keeps well but dislikes air: the emptier the bottle, the faster it oxidises. Decant the remainder into a smaller bottle, or displace the air — that is what gets a concentrate to its last measure.
How it compares
A consumable for the emulsion rather than a product in its own right. On a smooth, non-porous support — glass, metal, glazed ceramic — you may not need it at all.
