When doing kintsugi repair on porcelain or light-color clay, many got frustrated when the ki urushi got soaked into the porous surface resulting with a brown stain along the sides of the crack or the unglazed footing. We are sharing a surface-sizing technique, that prevents this urushi brown stain.
The idea is simple, you would want to use a good, natural material to seal-off the porous surface to prevent the urushi from bleeding into the clay body. The technical term is called "sizing", which is commonly practiced in watercolor, oil color painting, textile and book printing in both the western and eastern art world for centuries. Material requires "sizing" before any liquid pigment is applied. (common example is white gesso on canvas).
The traditional material we use for sizing in Japan is called nikawa. The word translates to "glue", and these are actually collagen extracted from various animal skins and sometimes bones. Nikawa in Japan has extensive history and is commonly used in nihonga (Japanese water-color painting). The gel-like collagen is kneaded with dry color pigment to create the traditional water-base paint for Japanese nihonga paintings, and are thinned to be used to size painting surface such as Japanese washi paper or silk . They are also being used as sealing material for handcrafted fine instruments including violins.
Just paint thin coats of nikawa on all unglazed edges of the shards or even hairline cracks before you apply any urushi base mixture. Ensure all surfaces are properly covered. The collagen will act as a barrier to prevent the natural lacquer from seeping into the porous surface to become the urushi brown stain.
Liquid-form nikawa collagen is a water-base natural material which can be stored in the fridge for about a week. We recommend the dry grain form for ease of storage, so you can adjust and dilute a small batch when needed.
These days, some of the nikawa or animal skin glue have mixed in with chemicals, so research on the suppliers and manufacturers if you are keen on using only all-natural, organic material in your practice. If you prefer an animal-free product, you can experiment and make your own diluted starch glue with rice. But the strength as a barrier may not be as pungent as animal collagen.
As requested, we have made a few short videos to explain the material and technique to prep your kintsugi pottery before repair. Check out our videos for more explanation on the material and how use use them (and thank you for being patient with our slow roll-out). If you haven't yet, sign up to access our videos, it's free.
So no more unsightly urushi brown stain after many precious hours of kintsugi repair work! For those who are interested in trying this material, we have just added nikawa in our kintsugi supplies store. Snatch a small pack when you order your kintsugi supplies next time!