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A workable mortar has a smooth, plastic consistency. It's easy to spread with a trowel and readily adheres to vertical surfaces. Well-graded, smooth aggregates enhance workability. So do air entrainment, lime, and other proprietary plasticizers that are now available. THE OLDEST PLASTICIZER In addition to improving workability, lime improves bond, increases water retention, and helps heal hairline cracks. But lime can burn a person's skin. And, because lime is white, color pigments must be added in greater quantity than required by mortar made with proprietary plasticizers to produce rich colors. PROPRIETARY PLASTICIZERS Several proprietary materials are now sold to partially or wholly replace lime. One plasticizer used as a complete lime replacement contains, among other ingredients, natural bentonite clay as a lubricant. Another proprietary mortar plasticizer is described by its manufacturer as a "liquid resin-based chemical concentrate. A third proprietary plasticizer can completely or partially replace lime in a mix. This thick, clear liquid plasticizer increases water retention, air content, tensile strength, and bond strength.