SUCCESS IN THE USE OF CEMENT depends largely upon cement that is fresh. Cement is very sensitive to moisture and if kept where fogs, dew or moisture of any nature is absorbed into it, the first set or “hydration” takes place and destroys its use for fine handicraft.
TO TEST FRESH CEMENT when the cement sack is open, thrust the hand into it and see that no hard lumps are in it. Fresh cement will feel slippery and soapy when rubbed between the finger tips. If it feels gritty and sandy it may do for the rough parts or body of vases and tiles, but only fresh cement should be used to mix with color and for surfacing purposes.
THE PROPER CARE OF CEMENT requires that it be kept in a covered receptacle and kept in a dry place, preferably up from the floor if the floor is near the ground. It should be kept in a dry, tight work-shop and the doors should be kept closed at night to avoid any moisture from the night air reaching it. Nothing can restore spoiled cement and it should not be used as the results will be discouraging.
GOOD GRADES OF AGGREGATES SHOULD BE USED in cement work. Clean sand should be used and a sand that is not too fine is preferable. Gravel and crushed rock used in large work such as garden seats, large bowls and outdoor problems should be of a good grade to form a good mixture.
THE WATER USED IN CEMENT WORK should be free from all impurities. Moderately warmed water will hasten the setting or hardening of cement while very cold water retards the hardening.
THERE ARE TWO COLORS OF CEMENT, gray and white. Portland cement is gray in color and a white cement is also made that is a refined form of cement. White cement is not as hard or durable as gray cement, but gives a smooth surface and sets as satisfactorily as the gray cement. It is more expensive than the gray cement and should not be used later than six months after the sack is opened.
WHEN CEMENT IS USED ALONE IT IS TERMED “NEAT.” When it is mixed with rock, gravel or sand it is termed “concrete.” Concrete produces strength and the neat cement produces a smooth texture and surface. When concrete is used the cement and water will rise to the top and if the surface is worked and pressed with a trowel the cement is “flowed” to the top, producing a smooth surface.
MOLDS ARE USED FOR FORMING CEMENT AND CONCRETE, and are made from wood, metal or plaster-of-Paris. The forms in all instances should be tied or braced together to prevent the moisture of the mixture from running out, as the water is essential to the successful hardening of the cement. In the making of cement pottery and tiles, plaster-of-Paris molds or forms are generally used and plaster-of-Paris therefore forms an important material in the making of color cement handicraft.
Plaster-of-Paris is made in different degrees of setting periods. These are quick-setting, medium-setting, and slow-setting. Casting plaster or sculptor’s plaster should be asked for and a medium-or slow-setting plaster is preferable for the beginner.
CEMENT, AGGREGATES, WATER AND PLASTER FORM THE MAIN PARTS of our working materials excepting the color, which is especially described in the chapter on Color.