355. Staff, a material used for the exterior covering of all the buildings of the World's Columbian Exposition at Chicago, may be considered as almost a new material in this country, although it has been in extensive use in Europe for many years. A large part of all exterior decoration of buildings, both public and private, in the provincial cities of Germany, whether ornament, columns or statuary, is made of staff, and in instances a period of fifty years of existence will testify to its enduring qualities. Staff was first used extensively in the construction of buildings at the Paris Exposition of 1878, and it was also adopted in work on the much grander buildings of the exposition of 1889. The methods of application at these expositions were, however, widely different from and much more expensive than those employed at the Columbian Exposition.
The staff for the World's Fair buildings was made on the grounds at Jackson Park in the following manner :
The ingredients were simply plaster of Paris, or Michigan plaster, water and hemp fibre. Hemp was used to bind together and add strength to the cast, and the New Zealand fibre was preferred, as both the American and Russian fibres were found too stiff. The first step in making staff ornaments is the creation of a clay model. The model is heavily coated with shellac, and a layer of clay separated from the model by paper is put on its face and sides. This layer of clay is oiled or greased and a heavy coating of plaster and hemp is put over it. The thickness of this coating is dependent upon the size of the model; sometimes it is 5 or 6 inches thick and contains heavy battens of wood to strengthen it. In less than twenty-four hours this coating is hard and is taken off the clay covering the model. The coating thus removed is called the box. Next the clay is removed from the model and the model is thoroughly oiled. The box is oiled and put over the model, leaving the space between model and box formerly taken up by the clay coating a free space. Holes have previously been made in the box, and upon a large centre hole (sometimes two or three in large pieces) a plaster funnel is placed. Molten gelatine is poured through these funnels, which fills every space, air being allowed to escape through small holes in the box. In from twelve to twenty-four hours the box is again removed, placed hollow side up, and the now hardened gelatine is removed from the clay model and placed in the box, which it fits perfectly. The clay model has now served its purpose, for the gelatine, which has become a matrix of the cast desired, is used in the further stages of the work. In case of large moulds the gelatine matrix is sometimes cut into as many as eight pieces. All these, of course, join perfectly in the box and are cast from as if from a single matrix. The gelatine mould is washed a number of times with a strong solution of water and alum, and after oiling is ready for the operation of casting.
* The following description of this material is taken from an article by E. Phillipson, published in the Engineering Record of June 4, 1892. Mr. Phillipson had charge of this portion of the work on the World's Fair buildings.
The plaster for the staff is thoroughly stirred in water, and the hemp, cut into lengths of 6 to 8 inches, is bunched loosely, saturated with the plaster and put in the moulds in a layer of about I inch in thickness. Succeeding handfuls of hemp are thoroughly interwoven with the preceding, the hemp being expected to fill in all the corners of the cast. When the mould is filled the back is smoothed over by hand, and later the cast is removed from the mould. The time consumed from starting a cast to removing it from the mould, is for a cast 5'x2'6" in size, about twenty-five minutes. After the removal of the cast care must be exercised in either standing it up or laying it down that it shall not collapse or lose its form by warping. During the summer months a cast of the dimensions given will dry thoroughly in about thirty-six hours and is then ready for application. In the winter months there is danger of casts freezing before they are dry, and in that event they are apt to go to pieces when warm weather comes. A good workman can make as many as seventy-five casts in one mould, and then the gelatine is remelted and a new mould made of it, the box being good for use for an indefinite length of time. In making pilasters or mouldings, etc., not ornamented or under-cut, plaster and wood moulds are often used, the latter material being especially preferred, owing to its durability.
"Applied to a frame building, staff is simply nailed on to the rough construction, and a cheap brick wall covered with it can, at a comparatively small expense, be made to assume a classic appearance. In building a brick house with the employment of staff in view, it is advisable to insert wooden furring strips in the brick, as these simplify the labor of putting it on. For cornice work it is claimed that a strength and boldness of design are possible with staff which cannot be realized with other materials.
"At the Paris Exposition the buildings were constructed almost entirely of iron, and nearly all the staff was cast in panels, which were set in iron frames. While this method was considered excellent both in finished effect and in durability, it was far too expensive and tedious to be employed in covering the much more extensive structures to be built for the World's Columbian Exposition. Accordingly, after many weeks of study, the construction department decided to construct the buildings of wood and to nail the staff directly to the furring.
"The name 'staff' properly applies to material that is cast in moulds, and not to ordinary plaster or cements that are put on with a plasterer's trowel. Work with such materials is subject to well-understood limitations by the temperature and weather, but atmospheric influences have practically no effect upon staff. This has been demonstrated by the acres of staff that has been standing all winter outside the various casting shops in Jackson Park. No attempt has been made to keep off the rain, snow or frost. Several pieces of it have been submerged for over a month at a time, allowed to freeze and thaw, and freeze again with the water, and when taken out they were found to be perfectly intact."
While this material admirably answered its purpose on the Fair buildings, it became considerably deteriorated, and evidently would not answer in such a climate for permanent buildings unless kept well painted. In fact, it appears to be generally conceded that Portland cement is about the only material that will endure permanently under the trying conditions of our northern climate. In warmer and dryer climates compositions of plaster are largely used on the exterior of buildings, and in many instances they have lasted for centuries.