Traditional ceramic production in central and southern Madagascar: first insights from mineralogy and petrology of bricks

The traditional buildings of Madagascar are mainly built by using bricks. The huge brick production is mainly organized in local workshops close to the clayey deposit outcrops where the sediments are extracted, moulded in bricks, dried and then fired in open-air furnaces. Samples of bricks from four workshops located in central and southwestern Madagascar were collected to establish the firing technology of the Malagasy traditional brick “industry”. For each site, the samples consist of one unfired brick and two or three fired bricks collected at different distance from the fuel source. Mineralogical and petrographic analyses have been performed to achieve a technological characterization. The unfired bricks from central Madagascar are made up of lateritic soils formed from in situ alteration of intrusive or metamorphic basement rocks. These deposits experienced low transportation rate, as showed by the unsorted granulometric distribution of subangular quartz, altered feldspar, opaque oxides, detrital rocks fragments, with rare amphibole, hematite and olivine grains. Siderite is found in central Madagascar samples. The clayey sediments from south-western Madagascar have well sorted grain size distribution, with quartz as prevailing constituent. Calcite and fossils also occur, attesting a marine depositional environment. After moulding the clayey deposits are fired in open-air furnaces and the type of fuel mostly depends on wood availability in the area. In fact, the inner areas of central Madagascar suffer a strong deforestation, thus the bricks are fired by using peat fuel. The time of firing is quite longer for peat (up to 15 days) with respect to for wood furnaces (1-2 days). The samples from central Madagascar are fired at ca. 550 °C, as confirmed by the lack of the dehydroxylation peak (TG analysis) of kaolinite (detected in unfired samples) between 450 and 550 °C. The absence of siderite in fired samples from central Madagascar with respect to unfired ones, allowed confirming a firing temperature of 500 - 550 °C given that decomposition of siderite to Fe-oxides takes place within this temperature range. Little differences have been recorded in phyllosilicates content among the samples thus inferring a variable diffusion of heat. Samples from south-western Madagascar are fired at slight lower temperatures because since kaolinite in fired samples still persist (weak endothermic peak at 470-500 °C in TG curves). Anomalous heat diffusion within this kind of furnace is here supposed due to significant variations of kaolinite content among the investigated samples.