828 HYDRAULIC CEMENT 



to impede tho draught too much. The lime-dust produced in burning the limo 

 becomes useful in making the hatches of tho oven air-tight, and if there is any 

 excess, it may bo ground down with tho cement without sensibly deteriorating its 

 strength. 



When the oven is loaded, the charging hatch is closed with a double 4-inch dry 

 brick wall, tho included space being filled in with lime-dust. The whole may be 

 made air-tight at top by means of an opening through tho crown of the arch. An 

 iron door can bo employed, and with saving of labour, but a brick wall will generally 

 bo found more convenient. 



The small hatch intended to assist in cooling the oven, in order to draw the charge, 

 is better closed with an iron door on hinges, tho whole being made tight with a mix- 

 ture of moistened clay and sand, technically termed ' pug.' 



If the lime employed is pure or feebly-hydraulic lime, it is advisable to allow it to 

 remain, with a current of air passing through it for a period which will vary, 

 generally, with the temperature of the air, and the draught through the oven. Tho 

 object of this is that the moisture carried through by the air may open or split the 

 lime a little, and otherwise facilitate the combination of the sulphurous acid during 

 tho subsequent process, without, however, causing it to fall abroad. Whatever tho 

 nature of the lime operated upon, such exposure renders the process more certain, and 

 should be employed whenever the tendency to slake with heat is not readily destroyed 

 without it. The period of exposure will vary from twelve to forty-eight hours. 

 After this exposure, a fire of coke or coal is kindled by means of a faggot or brush- 

 wood on the fire-bars, and a slow fire is kept up until the lime immediately over tho 

 arches shows a dull glow, such as is barely visible when viewed in the dark through 

 the pipes in the sides of the oven. The firing is then raked out, and after tho 

 lapse of a quarter or half an hour, according to the size of the oven, tho ash-pit is 

 closed, and the chimney-top covered with a slate-stone, or plate of iron, until it is 

 judged that the heat throughout the mass of lime is pretty well equalised. When 

 coal is used, there is sometimes a little difficulty in preventing the smoke escaping 

 through the fire-doors after each fresh feeding of the fire, but this may be obviated 

 with a little care and more frequent feeding. The flame of coal heats the charge more 

 equally. A mixture of coal and coke may also be employed. 



The time required for heating the lime depends on so many circumstances that no 

 definite rule can be laid down ; the dullest possible glow will sufficiently indicate when 

 the first point is attained, that a burner acquainted with the action of his oven will 

 not require the use of the viewing pipe at all. , 



When the heat in the oven is judged sufficient, and to be sufficiently equalised, the 

 ash-pit and the chimney are opened, andiron pots containing coarse unpurified sulphur 

 are pushed in on the fire-bars, and placed in such a manner as to distribute the fumes 

 of the burning sulphur equally. The pots may be made of wrought iron, of sufficient 

 capacity to hold about 25lbs. of sulphur each, the allowance being 15 Ibs. to each yard 

 of lime. A few red-hot cinders are thrown' into each pot as it is pushed in, to ignito 

 the sulphur. In ten or fifteen minutes the sulphur will be fully ignited, and its fumes 

 will begin to escape through the chimneys very freely ; the chimneys should then be 

 closed again perfectly, and a little clay or lime-dust used to make the cover tight. The 

 fire-doors and ash-pits are also to be plastered at the crevices, with the exception of a 

 small opening at the bottom of each of the latter, which opening is to be regulated by 

 the amount of air which may be required to carry on the combustion of the sulphur, 

 without allowing the sulphurous acid to escape, and such escape would be immediately 

 detected by the smell. 



When the sulphur is all consumed, which may be ascertained by looking from time 

 to time, after the lapse of some hours, into the oven fire-door, the wall closing the 

 hatch is thrown down, and all other openings made free to the passage of air, to cool 

 the charge and oven sufficiently to allow the former to be removed. 



The grinding is then commenced; a few trial samples are made up with wntor, 

 and an examination made at the top, middle, and bottom of the charge, to obtain 

 information as to temperature for future guidance. If the lime from the bottom of 

 the charge, when moistened, remains quite dead on the outside of the lumps, a sort of 

 crust being formed there, and at tho heart the lumps retain its usual properties, this 

 is a proof that the bottom portion, if not the whole charge, was too hot when the 

 sulphur process was commenced ; if the upper layers still retain tho usual character- 

 istics of the unprepared lime, BO crust being visible on the bottom portions, this shows 

 that the heat at the top was not great enough, and probably that the tempor.it un> 

 throughout was too low. Some pieces will always be found insufficiently prepared, l.nt, 

 their effect is lost in the mass, and the samples should bo three or four in number from 

 each part of the charge, top, bottom, and centre, avoiding a selection of pieces near 

 the walls. If in any case the mass is found insufficiently prepared, and the lower 



