The rate of propagation of flame and the rate of propagation of smouldering in still air, have been investigated for several particle sizes, mixtures of coarse and fine dusts of the same material, and mixtures containing partially coarse dusts of limestone. Wood sawdust and grass dust were used as the industrial materials. Measurements were made by using a mold of acceptable dimensions. The results obtained indicated that the flaming and smouldering rates are dependent on the particle size and the depth of the layer and the values of the smouldering rates were found to be about 20% of the values obtained with flaming. Also, an admixture of fine dust of 50% of the same material to coarse dust for a 0.5 cm layer of both materials is sufficient to increase the values of the flaming rate by 61% for wood sawdust and the smouldering rate by 88% and 52% for wood sawdust and grass dust, respectively. The admixture of limestone as low as 10% was sufficient to produce zero propagation for both types of burning. (C) 2004 Elsevier Ltd. All rights reserved.