Alum water has been used to size rice paper to reduce its absorbency. The alum-treated rice paper is known as Shou Xuan ("mature rice paper"), which is used for gongbi(elaborate style) painting. In gongbi painting, alum water is used to seal the colors between different layer of washes so they will not run into each other. Alum is also widely used as water resistant to create special effects on the raw Xuan paper.
Alum as preservative is indispensable in wet mounting to mix with flour to make mounting paste. Tip: you may apply alum water to the dark ink calligraphy strokes or heavy color part in painting before wet-mounting to prevent bleeding.
Step 1: dissolve the amount of powder needed into a small amount of water;
Step 2: add more water to dilute it. To make mature xuan paper you may use 1/2 tea spoon alum for 1 caps of water and then adject it according to your needs (depending on how many times you apply the alum water etc).
Special Note: To make the alum water for Yu Zhixue style ice landscape, pour alum powder into hot water and stir it until it no longer dissolve. Do not reheat the water to dissolve the rest alum because it will be too thick.
As seen on Youtube: