80. Other Types of Stucco.-Aside from Portland cement stucco, which is the type most widely used, there are two others which deserve mention.

One of these is the lime stucco of earlier days, of which many splendid examples are still in existence. The question arises, why were these lime stuccos almost wholly abandoned in favor of Portland cement, when their quality and durability are evidenced by the fact that some of them have endured for more than a century? From examination of some of the well known lime stuccos of Germantown Pa., and elsewhere, the writer is of the opinion that the conditions under which these stuccos were built are conditions of the past (houses with 18-in. stone walls, lime slaked 12 months before use, mortar applied in thin coats, and each left to harden for an indefinite time); these were conditions highly favorable to lime stucco, but they no longer obtain. It is true that there are satisfacory stuccos of the present period in which varying proportions of lime and cement have been used, many of them no doubt containing more lime than cement, but so many failures may be traced directly to faulty combinations of these materials, and so much evidence has accumulated to demonstrate the superiority and greater reliability of Portland cement, that mixtures containing more than 20 to 25% of lime by volume should be avoided in modern practice.

Within the last 5 or 6 yr., a new and promising type of stucco has been marketed as a proprietary product by a number of competing manufacturers. This material, commonly known as magnesite stucco, is a mixture of magnesium oxide, sand, asbestos, and other fillers, and when gaged with a solution of magnesium chloride, forms a plastic mortar which develops a strength comparable with that of Portland cement mortar. As a stucco it has certain advantages over Portland cement stucco, the chief of which are greater plasticity, greater flexibiity, and the fact that it can be applied without injury in freezing weather. Its greater flexibility and low volume change also allow it to be applied to bases less suitable for Portland cement stucco,—e. g., wood lath of various types. On the other hand, its chief disadvantages are: intrinsically high cost, low resistance to the action of water and continued dampness, its tendency to corrode metal lath, and the present necessity for obtaining all materials and ingredients from the manufacturers. The lack of complete information regarding the properties and characteristics of the cementing materials, and the lack of specifications which will insure the quality of the finished product are the chief reasons for a conservative attitude toward magnesite stucco at the present time.

GYPSUM AND GYPSUM PRODUCTS

Pure gypsum is hydrous calcium sulphate,-i.e., calcium sulphate carrying water of crystallizaton. Its chemical formula is CaSO4 + 2H2O.

81. Gypsum Plasters.—Where gypsum is heated to a temperature between 250 and 400 deg. F., it loses about three-fourths of its combined water and the calcined product is known commercially as Plaster of Paris. This product, when finely powdered and mixed with water, takes up in combination as much water as it lost through calcination and becomes rigid, or" set", through recrystallization.

As found in nature, gypsum contains impurities, such as SiO2, AL2O3 and Fe2O3, CaCO3 and MgCO. If these impurities or substances such as fiber, added for the purpose, are present in the calcined product, they retard setting; and such gypsum plasters are termed "slowsetting." Cement plasters, so called, are gypsum plasters and may be either slow or fast setting. By calcining gypsum above 900 deg. F., a very hard flooring plaster is produced, which sets very slowly. Keene's cement is obtained by calcining pure gypsum at red heat, immersing it in an alum bath, then drying and calcining it again. Mack's cement is dehydrated gypsum mixed with 0.4% of sodium or potassium sulphate. This plaster takes a quick, hard and durable set and is used for floorings or for stuccoing on walls and ceilings.

82. Classification of Calcined Gypsum and Gypsum Plasters.—The report of Committee C-11 of the A.S.T.M., rendered in June 1919, sets forth in detail a classification of calcined gypsum as follows:

Calcined gypsum is divided into three classes, on the basis of its purity, as follows:

Class A.-Containing not less than 88.4% of CaSO4 + H2O if partially dehydrated, or not less than 87.6% of CaSO4 if totally dehydrated (these figures correspond to 90% purity in the raw gypsum rock).

Class AA.-Containing not less than 71.7% of CaSO4 + 1⁄2 HO if partially dehydrated, or not less than 70.4% of CaSO4 if totally dehydrated, nor more than the similar quantities specified for Class A (these figures correspond to 75% purity in the raw gypsum rock).

Class AAA.-Containing not less than 60.5% of CaSO4 + 1⁄2 HO if partially dehydrated, or not less than 59% of CaSO4 if totally dehydrated, nor more than the similar quantities specified for Class AA (these figures correspond to 64.5% purity in the raw gypsum rock).

Physical Properties.-Partially calcined gypsum (all classes) shall set in not less than 10 nor more than 50 min. Calcined gypsum (all classes) may be sold in one of the following sizes:

No. 4.-Material of this size shall all pass an 8-mesh sieve, and not less than 40 nor more than 60% of it shall pass a 100-mesh sieve.

No. 5.-Material of this size shall all pass a 14-mesh sieve, and not less than 60 nor more than 80% of it shall pass a 100-mesh sieve.

No. 6. Material of this size shall all pass a 28-mesh sieve, and not less than 80% of it shall pass a 100-mesh sieve. Partially calcined gypsum (all classes) when mixed with water to form a paste of normal consistency, which is molded into briquettes and allowed to set until dry, shall have a tensile strength of not less than 100 lb. per sq in. Partially calcined gypsum (all classes) when mixed with water to form a paste of normal consistency, which is then molded into cylinders 4 in. high by 2 in. in diameter and allowed to set until dry, shall have a compressive strength of not less than 1000 lb. per sq. in.

The same committee has also submitted a tentative report on gypsum plasters, a portion of which is quoted below.

Ready Mixed Gypsum Plaster.-Ready-mixed gypsum plaster is a plastering material in which the predominating cementitious material is calcined gypsum, and which is mixed at the mill with all the constituent parts in their proper proportion. It requires only the addition of water to make it ready for use.

(a) Scratch or First Coat.-Ready mixed gypsum plaster shall contain not more than two-thirds by weight of sand. The remainder shall contain not less than 75% of its weight of calcined gypsum. The other 25% of this remainder may be hydrated lime, ground clay, retarder, or fiber.

(b) Browning or Second Coat.-Not more than 75% by weight of the ready-mixed gypsum plaster shall be sand. The remainder shall contain not less than 75% by weight of calcined gypsum. The other 25% of this remainder may be hydrated lime, ground clay, retarder or fiber, but not Portland or other hydraulic cement. (0) Scratch or First Coat.-This plaster shall set in not less than 11⁄2 nor more than 5 hr. (b) Browning or Second Coat.-This plaster shall set in not less than 2 nor more than 6 hr.

(a) Scratch or First Coat.-Briquettes made of this plaster shall have a tensile strength of not less than 50 lb. per sq. in.

(b) Browning or Second Coat.-Briquettes made of this plaster shall have a tensile strength of not less than 40 lb. per sq. in.

Neat Gypsum Plaster.-Neat gypsum plaster is a plastering material in which not less than 85% of the cementitious material is calcined gypsum, mixed at the mill with other material in their proper proportion.

Neat gypsum plaster shall contain not less than 85% by weight of calcined gypsum. The remainder shall be hydrated lime, ground clay, asbestos, retarder, or fiber. It shall contain no Portland or other hydraulic cement. Briquettes made of neat gypsum plaster shall have a tensile strength of not less than 100 lb. per sq. in. Gypsum Wood Fiber Plaster.-Gypsum wood-fiber plaster is a gypsum plaster in which fiber is used as the aggregate.

Gypsum wood-fiber plaster shall contain not less than 80% by weight of calcined gypsum and not less than 1% of wood fiber made from a non-staining wood. The remainder shall be composed of hydrated lime, ground clay, asbestos, sand, or retarder. It shall contain no Portland or other hydraulic cement.

Gypsum wood-fiber plaster shall set in not less than 11⁄2 nor more than 6 hr.

Briquettes made of gypsum wood-fiber plaster shall have a tensile strength of not less than 100 lb. per sq. in. Calcined Gypsum for White or Gray Finished Coat.-(a) Calcined gypsum is the product made by mechanical process resulting from the partial (incomplete) or complete dehydration of gypsum by means of heat.

(b) These specifications cover two grades of calcium gypsum, white and gray.

Calcined gypsum for white or gray finishing coat shall all pass through a 14-mesh sieve, and not less than 60% shall pass through a 100-mesh sieve.

Calcined gypsum for white or gray finishing coat shall set in not less than 20 min. nor more than 3 hr. Briquettes made of calcined gypsum for white or gray finishing coat shall have a tensile strength of not less than 200 lb. per sq. in.

Molding Plaster.-The requirements for molding plaster shall be as given in specifications for calcined gypsum for white or gray finishing coat, with the following exception: Molding plaster shall set in not less than 10 nor more than 40 min.

Casting Plaster.-The requirements for casting plaster shall be as given in specifications for calcined gypsum for white or gray finishing coat.

Finish Coats.-Material for trowel or smooth finish shall be composed of lime putty and calcined gypsum, white or gray. The proportion of calcined gypsum and lime putty shall be varied according to the season of the year and the only practical method is to permit the experienced mechanic to use his judgment as to the proportion

of each according to the time of the year when the material is being applied, to make a proper finish. As a guide to the mechanic, 75% by volume of lime putty and 25% by volume of calcined gypsum is recommended.

Material for sand float or rough finish shall

be composed of lime putty and calcined gypsum in the following proportions: Lime putty 1 part by volume, sand 3 parts by volume; these two materials are thoroughly mixed; before applying, mix 1 part by volume of calcined gypsum with not more than 9 parts of this mixture.

Gypsum Products.-Gypsum plaster board, gypsum wall board, gypsum wall tile, and other formed building products are made from calcined gypsum mixed with various aggregates, such as fiber, wood-pulp, etc. (see pp. 934 and 935). They are molded as units by various processes and supplied ready to place on the job.

83a. Gypsum Plaster Board. Gypsum plaster boards are used as a sheet lath or base for gypsum plaster on walls, ceilings, and partitions on the interior of buildings.

Gypsum plaster board shall consist of sheets or slabs composed of one or more layers of hydrated gypsum plaster, with or without fiber, reinforced on the surface with chip board, or felt. The thickness of plaster boards shall average not less than the following:

1⁄2 in. thick with permissible local variations of 6 in. plus or minus, and the thickness at any point in the board shall not be less than 4 in.

16 in. thick with permissible local variations of 16 in. plus or minus, and the thickness at any point in the board shall not be less than 316 in.

4 in. thick with permissible local variations of 16 in. plus or minus, and the thickness at any point in the board shall not be less than $16 in.

The width shall be 32 in. with a permissible variation of 4 in. less than the dimension specified, and the length shall be 24, 36, or 48 in. with a permissible variation of 1⁄2 in. plus or minus.

Unless otherwise specifically stated in the order, plaster boards of the widths specified and in lengths of 18 and 30 in. may be included in amount not exceeding 5% of any single car load.

The weight per thousand square feet of plaster board shall conform to the following:

For 3 in. thick, not less than 1500 nor more than 2000 lb. For 16 in. thick, not less than 1250 nor more than 1600 lb. For 4 in. thick, not less than 1200 nor more than 1500 lb. Strength test samples shall be 12 in. wide and approximately 18 in. long, and when tested shall be supported on parallel knife edge bearings spaced 16 in. and loaded through a similar bearing inidway between the supports. When tested as described, samples taken from the plaster boards shall carry not less than the following loads:

The minimum acceptable strength shall be not less than 5 lb. below the average given. Samples tested shall fail by rupture of the surfacing and core and not by the breaking of the bond between the surfacing and the core.

Gypsum plaster boards shall be shipped so as to be kept dry and free from injury. Each board shall be plainly labeled with the name of the brand and of the manufacturer.

Gypsum plaster boards shall conform to the foregoing requirements and shall be tested as provided for above when determining their strength. Plaster boards may be rejected upon the failure to conform to any of the fore

going requirements.

83b. Gypsum Wall Board. 1

-Gypsum wall boards are used without plaster coatings, as a finish on walls, ceilings, and partitions on the interior of buildings.

Gypsum wall board shall consist of sheets or slabs composed of a layer of hydrated gypsum plaster with or without fiber, and a surfacing of chip or manilla board on both sides.

The thickness shall average not less than 3 in. with permissible local variations of 32 in. plus or minus, and the thickness at any point in the board shall not be less than 56 in.

Where the wall boards are to be laid with joints butted, the width shall be 32, 36, or 48 in. with a permissible variation of 332 in., plus or minus. Where the joints are to be filled with joint filler, the width shall be 314, 354, or 473⁄4 in. with a permissible variation of 32 in. plus or minus. The length shall be 4, 5, 6, 7, 8, 9, or 10 ft. with a permissible variation of 3% in. plus or minus.

The weight shall be not less than 1500 nor more than 2000 lb. per thousand square feet of wall board. Strength tests samples shall be 12 in. wide and approximately 18 in. long, and when tested shall be supported on parallel knife-edge bearings spaced 16 in. and loaded through a similar bearing midway between the supports. Such samples taken from the wall boards shall carry a load of not less than 80 lb. when the line of the supports is at right angles to the direction of the fiber of the surfacing, and not less than 32 lb. when the line of the supports is parallel to the fiber of the surfacing.

Samples tested shall fail by rupture of the surfacing and core and not by the breaking of the bond between the surfacing and the core.

The cores shall consist of hydrated calcined gypsum plaster to which may be added not to exceed 15% by weight of sawdust or other vegetable fiber intimately mixed. Cores shall be of sufficient thickness throughout to make the finished product.

The surfacing material shall be composed of plain chip, manilla filled news, or other stock of the same general character containing sufficient sizing to meet the following conditions:

Samples of the finished wall board shall sustain a static head of 1 in. of water (confined within a 2-in. ring on either of the surfaces of the board) for a period of not less than 2 hr. without penetrating the surface sufficiently to stain the core.

1 From report of Committee C-11, A.S.T.M., June, 1919.

The surfacing material shall completely cover the two larger faces of the core and shall be securely bonded to it. The surface designed to be exposed on erection shall be true and free from imperfections that would render the wall boards unfit for use with or without decoration. The edges and ends shall be straight and solid. Where wall boards are to be butted, the corners shall be square with both side edges. In cases where the joints are to be filled, the joints shall be square with both side edges, with a permissible variation of 1⁄2 in. in the full width of the boards. The finished product shall be dry and free from cracks and imperfections that would render such boards unfit for

use.

83c. Gypsum Tile.-Gypsum tile for non-bearing fire walls, partitions, and enclosures, and for furring the inside or outside of other walls, consists of hollow or solid tile or blocks which are manufactured at the mills and are delivered for erection on the building site.

Generally, gypsum tile consists of about 95% finely ground calcined gypsum uniformly mixed with from 2 to 5% by weight of fibrous material, this compound being mixed to a plastic state with water, and molded in the form of the desired units.

Commercial standard sizes and weights per square foot for the uses stated are about as follows:

12-in. split furring (11⁄2 × 12 × 30 in.)...
2-in. split furring (2 × 12 × 30 in.).....
12-in. solid tile or block (11⁄2 × 12 × 30 in.).

2-in. solid tile or block (2 × 12 × 30 in.).

3-in. hollow tile or block (3 × 12 × 30 in.).

4-in. hollow tile or block (4 X 12 X 30 in.).
5-in. hollow tile or block (5 × 12 × 30 in.).
5-in. solid tile or block (5 X 12 X 30 in.).
6-in. hollow tile or block (6 × 12 × 30 in.).
in. hollow tile or block (8 X 12 X 30 in.).

FIG. 5.-Showing application of Sackett soffit ceiling on reinforced concrete construction. Note how flanges of steel clips imbedded in concrete joists are bent each way holding Sackett board in place.

4.9 lb.

9.4 lb.

7.2 lb.

9.4 lb.

9.4 lb.

13.0 lb.

15.6 lb.

20.0 lb.

16.6 lb.

22.4 lb.

Special sizes of gypsum tile can be made to order in any size, thickness, or shape to meet requirements.

For ventilation ducts or special construction the tile can be manufactured with one or both faces smooth and of any desired density and strength.

Gypsum plaster tile are used as a substitute for clay tile, cement blocks, brick, concrete, metal lath and plaster, sheet metal, and wood lath and plaster in the following nonbearing constructions:

Corridor walls, partitions, and wall furring.
Fire division walls.

Elevator, stairway, and dumb waiter enclosures.
Light wells, pipe chases, and veut ducts.

Floor fillers, also roof fillers for insulating purposes.
False columns, pilasters, etc.

In buildings of fireproof construction or for any construction required to be of high fire-resisting value, gypsum tile are used for the following, in addition to the uses (as stated) for the protection of combustible construction: Non-bearing, "fire" or "fire division" walls.

Steel girder, truss beam, and lintel protection.

All partitions, walls, furring, etc., should be started upon solid flooring. In all fireproof buildings they should be started upon fireproof floor or other fireproof construction; and in all buildings "fire" or "fire division" walls should be constructed upon fireproof floors or fireproofed steel supporting beams or girders. All corners should be bonded, and connections to other walls should be suitably bonded or anchored.

Generally, gypsum furring should be laid up against the wall or partition to be furred and be securely wired or spiked to such. Free standing furring (not against a wall or partition) should be secured by metal separators spaced not further apart than 3 ft. in any direction.

Trim, grounds, chair rail and similar fixtures should be nailed directly to the tile or block. Black boards, heavy toilet fixtures, mantels, etc., should be secured to 2-in. nailing blocks which are nailed to and are full size of the end of the gypsum tile.

All tile construction, furring, girder, truss, beam, and lintel protection of gypsum should be laid up in a mortar composed of not less than one part of gypsum plaster to not more than two parts of clean sharp sand by weight. Steel construction may be protected by pouring the gypsum around the member in form work and to the required thickness. All door buck and similar framing should be anchored to the gypsum construction in a suitable manner.

In "The American Architect" for April 14, 1918, in treating of Reinforced Gypsum Roof Tile, the following mixing precautions and suggestions as to determination of strength are noted: 1 This material supplied by V. G. Marani, Chief Engineer of the Gypsum Industries Association.