The ground layers (shitaji)
Ground layers (shitaji) is a basic structure that influences the durability and quality of lacquerware.
This kind of layers have also a protective function for the object base and sometimes is used as a decorative element.
The structure of the grounds depends on the lacquer decoration, on the nature of the object base and on the cheapness of the lacquerware.
As already explained in the urushi lacquer types page urushi lacquer is part of all stages of decorative production and this is also effective in the case of the preparation of the groud layers.
To explain it very simply, it is necessary to consider the ground layers as a buffer that connects the base support to the properly called “lacquering layers”.
Many different clay powders are used for the ground layers.
They can be different in color, hardness and grain size, according to the region and tradition origin.
There is a first base distintion for the clay powder used between jinoko and tonoko.
Jinoko is a coarse clay dust while Tonoko is a fine clay dust. These powders, combined with the raw lacquer (ki urushi), become part of the preparation layers, sometimes mixed in different percentages with each other.
There are various different ways of preparing the foundation layer but in all cases several urushi coatings, initially rough and then of increasing fineness, are applied before polishing.
Only after the foundation layer has been prepared does the application of urushi proper begin.
Summing up we can explain the basic sequence for the preparation of the ground layers as follows:
- lacquer layer (suikomidone) made up with ki urushi;
- coarse groung layer (jitsuke) made up with jinoko, water and ki urushi;
- medium ground layer (kirikotsuke) made up with jinoko and tonoko, water and ki urushi;
- first fine ground layer (sabitsuke) made up with tonoko, water and ki urushi;
- second fine ground layer (sabitsuke) made up with tonoko, water and ki urushi.
- last lacquer layer (suikomidone) made up with ki urushi;
All layers are applied with a spatula in coats from thicker to thinner.
The layers are dry or wet sanded how is illustraded in the imagine below.
There are several urushi lacquer types and each of them is dedicated to a different use.
The Urushi lacquer is part of all the steps of the decorative production, from the preparation of the grounds to the final coatings.
In addition to this the first major distinction in the case of urushi types is between raw and refined lacquer.
Refined process of urushi
There are two important steps in the refined process of the lacquer. They are generally done together.
The first, called kurome, is a dehydratation process. In this process is very important that the water content in the raw lacquer must reduced very slowly.
The second, called nayashi, is a process of homogenization. Oils (sesame, linseed, princess tree and rapeseed oil), resins (pine, gamboge) or coloring agents (pigments, iron filings) are added to the raw lacquer during the homogenization stage, depending on the viscosity to be obtained and the use of the finished lacquer.
Black urushi lacquer
Iron hydroxide (mixture of iron sulphate and caustic soda in a 10:3 ratio) reacts very rapidly with urushi and should be addeed at the end of the dehydratation process. It is mixed with water and then the precipitate is washed and filtered several times before it can be used for coloring.
Lacquer types diagram
A simplified diagram of the distinctions of different types of urushi lacquer:
Heckman Gunther, “Urushi no Waza, Japanese Lacquer Technology“, Nihon Art Publishers, Ellwangen, 2002. ISBN – 3980575527
What is urushi?
Urushi lacquer is a latex extracted from trees growing in some Asian countries (in areas 500 meters above sea level), with a temperature range from 8 to 20° C and in which the rainfall exceeds 0,6 m a year.
The urushi lacquer is the most fine because the climatic conditions in Japan are the most favorable to obtain a high quality product.
The urushi harvesting process (Urushi kaki)
Lacquer is obtained from the sap of the Rhus Verniciflua, a tree of the Anacardiaceae family.
Lacquer trees originally grew in the wild while for many centuries they have been cultivated.
The main harvesting time of urushi is between the middle of june and the end of september.
First tapping, called hatsuhen, is made from may to june.
Second tapping, called sakarihen, is made from june until september.
Third or late tapping, called osohen, is up to the end of november.
The sap of the lacquer flows in channels inside the plant. If the tree is damaged, the channels break and the sap the comes out.
This is what happens when a cut is made on the bark of the tree and a substance similar to latex leaks out: the raw lacquer.
It is a rather rare substance because even under ideal conditions mature lacquer trees produce a small amount.
The lacquer harvest takes place in mid-June and continues until the first cold weather (mid-October).
The trees used for extraction are carefully chosen, so that, in order not to damage the plant at all, each tree is used every three years and only four or five times during its life.
Modern harvest is carried out by making a series of horizontal incisions in the bark. The shape and style of the engravings varies from area to area.
Raw urushi lacquer (ki urushi)
Fresh lacquer has a milky-white or gray-yellow color that darkens as soon as it is exposed to the air.
After harvesting, the lacquer is immediately placed in a cup and covered to prevent it from hardening.
Once extracted, the lacquer needs to be clarified and processed. Twigs and other impurities must be filtered before further refinement.
Different degrees of purity of lacquer are used for different purposes.
The one used for the base layers does not require further refinement.
Raw lacquer is a protective, non-corrodible, acid and heat resistant, waterproof, luminous, shiny and long-lasting coating.
Freshly harvested lacquer contains 25 to 65% water, an amount that depends on where and when it’s extracted.
The presence of water is shown by a beige color, a milky appearance and a greater opacity of the viscous liquid.
Excess water can be removed from the lacquer to help harden, to improve the transparency and strength of the final product. For this purpose, the lacquer can be heated to temperatures around 40 ° C.
Chemical properties of lacquer
The main component of the lacquer is urushiol.
Urushiol is a mixture of phenol derivatives suspended in water with a small percentage of proteins, or rather an oil in aqueous emulsion.
The lacquer does not harden by evaporation of the solvent but by polymerization in the presence of humid air.
The molecule polymerizes under the influence of an enzyme (laccase) which forms very resistant chains with a cross-linked structure, according to a hydropolymerization process, that takes place in the presence of oxygen.
Urushiol is in fact responsible for the cross-linking that makes the lacquer so resistant and durable.
It is qualified as a thermosetting polymer due to the irreversibility of the hardening process.
Since lacquer curing depends on water and oxygen in the air, the surface layers are the first to start the reaction, sealing the underlying layers.
The curing process of urushi
Urushi starts to hardened under defined conditions.
A fundamental parameter is the humidity which must be very high, from 75 to 85 %. The temperature is around 23-26 °C.
Many people show a more or less strong allergic reaction (Toxicodendron dermatitis or Rhus dermatitis) to not hardened urushi lacquer.
The extend, degree of severity and duration of the symptoms depends on the personal sensibility level, the quantity and duration of exposure to urushiol.
For this reason please always use thick gloves and covering clothes to protect your skin!
Urushi lacquer experience with UrushiLAB
UrushiLAB is a project born dfrom the desire to introduce people to the Japanese lacquering technique. It wants to show, through direct experience and a continuous path of study and experimentation, the world of lacquers.
After years of research on lacquerware, I want to share my experience and invite you to explore the secrets of this ancient technique. Discover with me a special material: urushi.
Work in progress
Much of my experimentation is still ongoing. This makes UrushiLAB a continuous working progress because, as the saying goes “You never stop learning“, a slogan that in the case of the lacquering technique is more true than ever.
UrushiLAB will also talks about the conservation of Japanese lacquers starting from the background of the author who discovered the love for lacquers from the study of their conservation.
Together we’ll discover the conservation needs and the processes of decay of lacquerwares, learning how to take care of these wonderful objects that, for centuries, have enriched the Western collections .
Modern urushi design
With the intention of spreading knowledge of Japanese urushi lacquering technique, UrushiLAB offers the possibility, through its articraftsmen, to create modern design objects decorated with urushi lacquer.
Keep in touch and stay connected with the UrushiLAB pages!
For further information on restoration work please visit the site of my conservation of cultural heritage company: RestART (site in italian).