Preparing the lacquer

The first stage in the production of lacquer is obtaining the raw materials. Sap is collected from mature trees through incisions made in the bark (Quin, 1882). When first collected, the sap is a double emulsion of water in oil in water containing 27-50% water (Figure 4.4). During maturation the sap converts to raw lacquer, a water in oil emulsion. Raw lacquer contains urushiol (60-65%) and glycoprotein (2-5%) in the oil phase of the emulsion and polysaccharide (5-7%), laccase enzyme (1-2%) and water (20-25%) in the water phase.

Chemically, urushiol is a catechol - a benzene ring with two adjacent hydroxyl groups -which can have a variety of C15 or C17 carbon chains attached to it (Du, 1988) (Figure 4.5).

Raw lacquer is used as an adhesive for the priming layers in the manufacture of lacquer objects. However, if sap is used for the upper layers as well, the coating will be found to have very changeable properties and is not at all durable. The mechanical properties of a coating made with refined urushi remain unchanged over long periods of time

Water

Water

Water

Water

Urushiol Chemical Structure
Figure 4.4 Diagrammatic representation of the double water-in-oil-in-water emulsion nature of lacquer sap when first collected

Urushiol polymer

Urushiol polymer

Urushiol Polymer

Polysaccharide

Polysaccharide associated with glycoprotein

Figure 4.6 Diagrammatic representation of a proposed structure for an urushi grain. The high molecular weight urushiol polymer has outer polar hydroxyl groups. Polysaccharides and glycoproteins are absorbed onto the urushiol molecule to form an outer layer on each urushiol grain (Kumanotani, 1988)

Polysaccharide

Polysaccharide associated with glycoprotein

Figure 4.5 The chemical structure of the main urushiol constituents of Chinese and Japanese lacquer (Du, 1988). The zigzag line represents the C15 or C17 chain attached to the primary urushiol molecule, and the double lines indicate the position of double bonds on these chains

Figure 4.6 Diagrammatic representation of a proposed structure for an urushi grain. The high molecular weight urushiol polymer has outer polar hydroxyl groups. Polysaccharides and glycoproteins are absorbed onto the urushiol molecule to form an outer layer on each urushiol grain (Kumanotani, 1988)

(Kumanotani, 1983). The process of converting sap to true lacquer is therefore very important.

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Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

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