Japanning and Enamelling Methods With Natural Japanese Lacquer.
Urushiol, the principal constituent of Japanese lacquer, does not according to the Japanese investigator, Kisaburo Miryama, dry by itself at ordinary temperatures, but can be dried with ease at a temperature above 96° C. In the same way, lacquer that has been heated to a temperature above 70° C. and has entirely lost its drying quality can be easily dried at a high temperature.
In this method of japanning the higher the temperature is, the more rapidly does the drying take place; for instance, a thin layer of urushiol, or lacquer, hardens within 5 hours at 100° C., within 30 minutes at 150° C., and within 10 minutes at 180° C.
Japanning at a high temperature with natural lacquer does not require the presence of the enzymic nitrogenous matter in the lacquer, and gives a transparent coating which is quite hard and resistant to chemical and mechanical action; in these respects it is distinguished from that dried at an ordinary temperature.
During the drying, oxygen is absorbed from the atmosphere and at the same time a partial decomposition takes place.
This method of japanning has its application in lacquering metal work, glass, porcelain, earthenware, canvas, papier-mâché, etc.; because the drying is affected in a short time, and the coating thus obtained is much more durable than the same obtained by the ordinary method.
For practical purposes it is better to thin the lacquer with turpentine oil or other solvent in order to facilitate the lacquering and lessen the drying time of the lacquer.
Since the lacquer-coating turns brown at a high temperature, lacquers of a light colour should be dried at 120° to 150° C.; and even those of a deep colour must not be heated above 180° C.
Most pigments are blackened by lacquer; therefore the varieties of coloured lacquers are very limited.
You might Also see :