Gravel board

The so-called Japanese torpedoes appear to be an American invention. They contain a paper cap placed between two masses of gravel, and in general require to be thrown somewhat harder than silver torpedoes to make them explode. The same torpedo board is used as in the manufacture of silver torpedoes, but a gravel board which holds only about half as much gravel. After the gravel has been put in the paper cups, a paper cap, still moist, is placed on top of it, more gravel, substantially equal in amount to that already in the cup, is added to each, and the tops are twisted. 

Some physical properties of sands and gravels with special reference to their use in filtration. 24th Annual Report, Mass. State Board of Health 1892. Pub. Document 34, pp. 539-556. 

Previously, municipalities and county boards allowed road spreading without cost to the oil companies (at times, even providing additional gravel). In effect, it had been a cheap paving and dust control method for these jurisdictions. Consequently, oil companies found it to be the least costly method of produced solids and slop oil disposal. However, there is concern with this method that heavy metals may leach into the surrounding groundwater. 

The concrete was prepared by first spreading out the gravel on a platform of rough boards, in a layer from 8 to 12 inches thick, the smaller pebbles at the bottom and the laiger at the top, and afterwards spreading the mortar over it as uniformly as possible. 

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