CRAMPERN, or CRAMP-IRON, an iron bent at each extremity, towards the same side of the middle part, used to fasten stones together in a building.
When stones are required to be bound together with greater strength than that of mortar, a chain, or 'bar of iron, with different projecting nobs, is inserted in a cavity cut in the upper side of the course of stones across the joints, instead of single cramps across the joints of each two stones. Cramps are generally employed in works which require great solidity, as in the piers and abutments of bridges. and the voussoirs of large arches. They are also employed in uniting the stones of copings, and cornices, and generally any external work upon the upper surface, or between the hods of the stone. External work liable to the injuries of the weather, to be ..rainded. The most secure manner of fixing cramps is to let ,item into the stones their whole thickness, and run them ith lend ; but in slight works, it is sufficient to bed than in plaster, as is the practice in chimney-pieces. Iron is used in modern buildings, hut the Romans, who were accustomed to employ cramps in the greatest profusion, used bronze, a material much more durable than iron, as it is not so liable to rust.
Bronze or copper is preferable to iron, not only on account of its own durability, but likewise because it is not so liable to destroy the masonry which it connects : the rust which accumulates round iron cramps, tends to rupture the masonry to a much greater extent, than the cramps to keep it together ; besides this, if placed near the surface, iron is sure to discolour it. In general work, if the masonry be well put together, there will be but little need of cramps, especially if the separate masses be of moderate size ; nor even if they be or small dimensions, is there any absolute necessity for their employment, as we may gather from the works of the ancients. Mr. Murphy instances the spires of Salisbury
cathedral, and of the church of Batalha, Portugal, which though not more than seven inches in thickness, are for the most part connected without the aid of iron cramps. These observations apply with greater emphasis to wrought than to cast iron.
Sir Christopher Wren used a large cramp or chain below the springing of the dome of St. Paul's, in order to distribute the pressure equally. This architect, however, seems to have been fully aware of the caution requisite in the use of iron in stone buildings, for lie observes in his Parentalie, ‘• It has been observed in removing cramps from masonry at least four hundred years old, which were so bedded in mortar that all air was perfectly excluded, that the iron appeared as fresh as when it came from the forge. In cramping stones therefore, no iron should lie within nine inches of air if possible ; for air is the menstruum that consumes alI materials. When for want of large stones the use of iron is requisite, care should be taken to exclude the air from it." Copper, bronze, or gun-metal, form excellent and incorro sive substitutes for iron in cramping masonry ; they are more expensive at the first outlay, but will be found in reality more economical ; the first material may be mixed with a small portion of tin, which imparts to it greater durability.
The above objections to iron, do not hold good as regards brickwork ; the only inconvenience in this case arises from the stain with which the rust is apt to disfigure the work, if' the iron be placed too ni ar the tiicing.