臺灣日治時期建築「孔雀壁」的材料分析與修護建議

Material Analysis and Conservation Recommendations of“Kujyaku-Kabe”of Architecture Heritage in Taiwan During Japanese Colonial Period

在臺灣日治時期建築修復工程中，常發現由黑砂所作成之砂壁（すなかべ，sunakabe）。砂壁是以天然色砂、石材、礦物碎片為原料，使用海藻熬煮成的糊或動物膠作為黏著劑，施作於壁體的日式泥作工法。砂壁依材料不同而有不同的名稱，因其缺乏耐久性導致保存不易與修復困難。黑砂壁常見施作於床之間等建築體中重要的空間。臺灣總督府檔案之「仕様書」中曾有砂壁規範與圖說，但其作法與原料並無記載。化學分析證實，黑砂成分為煉銅爐渣。此材料在日本左官稱為「孔雀砂」，以其施作之砂壁稱為「孔雀壁」。
孔雀砂與市售煉銅爐渣最大的差異在於顏色與產狀。前者顏色七彩具金屬光澤，後者色黑無光澤。回火是應用於鐵合金的熱處理技術之一，伴隨溫度及氧化程度不同而產生不同回火色。然而，煉銅時產生爐渣的過程並未經回火，色澤應與市售煉銅爐渣相似。孔雀砂可能是經人為刻意回火產製。本研究對市售煉銅爐渣拌炒加熱，證實回火後確實產生色澤變化。由於市面上已不再販售孔雀砂，修復時建議回收已剝落之舊材料再利用。如遇回收不足之情形，可購入市售煉銅爐渣進行回火來製造「孔雀砂」。

Sunakabe (literally“sand wall”in Japanese) made from black sand was commonly discovered in architecture heritage of Taiwan in the Japanese colonial period, as evidenced by recent efforts to repair these buildings. Sunakabe is made using Japanese plasterwork techniques, with natural color sand, gravels, or mineral fragments serving as the base materials and a paste made by boiling seaweed or animal glue serving as the adhesive. The name of sunakabe changes depending on the base material that is used, and it is commonly found in the major spaces of a building, such as the tokonoma. However, the limited durability of sunakabe poses challenges when attempting to preserve and repair it. Specification documents from the Government-General of Taiwan archive include specifications and construction drawings for sunakabe; however, the documentation on its construction techniques or materials is limited. A chemical analysis revealed that the black sand used in sunakabe is copper slag, which is referred to as kujaku (literally“peacock”in Japanese) sand by Japanese plasterers. A wall made from the material is known as a kujakukabe (literally“peacock wall”in Japanese).
Kujaku sand and commercially available copper slag primarily differ in terms of their color and appearance. Kujaku has a colorful and metallic luster, whereas commercially available copper is dark and lacks luster. Tempering is a heat treatment technique used to make iron alloys, and the color of the final product in tempering changes with the temperature and oxidation levels employed during treatment. In general, copper smelting for copper slag production does not involve tempering. However, the production of kujaku sand could involve tempering because kujaku sand would otherwise have a color similar to that of commercially available copper slag. In this study, commercially available copper slag was heated by stirring it in an iron pot, and this process demonstrated that a change of color occurs following tempering treatment. Given that kujaku sand is no longer available on the market, old kujaku sand detached from kujaku-kabe should be collected and reused in repair works. In the event of a material shortage, commercially available copper slag may be used to produce kujaku sand through tempering.