近世におけるベンガラの製法に関する復元的実験

Subthetic Bengala (iron oxide red: Fe2O3) is one of the most popular redpigments used in the Edo period. In the general process of Bengala manufacture at that time, workers used roha(FeSO4・7H2O) as raw material. They roasted the roha, and refined it in the water. We made some experiments in making Bengala,in order to understand its character. The results we obtained are as follows.(1)During the roasting process, the color of the raw material changed gradually from 1to 6:1 green or blue ~2 yellow ~3 yellow orange~4 red~ 5 reddish brown ~ 6 black .(2)The chemical character and crystal from changed at the same time. The phenomenon is as follows. Raw material (FeSO4・7H2O)[Melanterite] ~ intermediate material (Fe4.67(SO4)6(OH)2・20H2O and Fe14O3(SO4)18・63H2O) [Copiapite etc.] ~ red color material (Fe3O4) [Magnetite].(3)It was found that the adequate roasting temperature is about 650-700℃to produce iron oxide red.But this condition is so delicate that it is very difficult to keep this range and produce oure iron oxide red by hand-made method.(4)Much of sulfur component was remained in the roasted materials,and made the quality unstable especially under the high humidity condition(about more than 80% relative humidity).(5)To obtain the high quality Bengala, the sulfur component should be washed way. During the washing process, we confiremed precipitation of iron hydroxide (Fe(OH)3)in the water. It was found that,the synthetic Bengala ( as a red-color pigment) is composed the mixture of iron oxide (Fe2O3) and iron hydroxide(Fe(OH)3).