Nikawasui (literally, glue water) is used in the production of Japanese painting, mainly for fixing paint onto the surface and is a solution made by dissolving glue in water.
The basic method of making this glue solution from sanzenbon nikawa (hide glue) is to take a strip of glue (approx. 10g), wrap it in a towel and break it into fine pieces, place this in a container (nikawanabe), add 50ml to 100ml of water and heat this indirectly in a bowl or container of hot water. Keep stirring the mixture because the fixing power of the glue solution weakens if it is allowed to boil. When the glue has melted completely, the solution must be strained with gauze to remove any impurities before it is used. This glue solution is actually the undiluted liquid from which sizing solution is made.
Because glue solution may break down or become moldy if it is kept in a high-temperature environment, in summer, it should be stored in the refrigerator and used within a few days. In winter, it can easily become hard even at room temperature, so it should be heated from time to time during use. Freezing the solution will destroy the collagen and will weaken its adhesive strength. Adhesive strength is also affected by temperature and humidity so a slightly stronger solution should be prepared in summer and a slightly weaker solution in winter. However, this should be done carefully because if the glue solution is too strong, it could cause the surface to crack and if it is too weak, the paint may peel off easily. Using a weaker glue solution produces better paint colors so ideally, it should be prepared in as weak concentration of glue as possible that will still allow paint to fix properly.