Most decorative contractors buy stamps because they want a new pattern—perhaps something realistic or aesthetic—to add to their offering. Few contractors consider the stamps' physical properties, but they should, because there are differences between products that can affect your work.
In the 1920s, Albert Shore developed a scale for measuring the hardness or rigidity of materials. Scales for the construction of stamp materials rated Shore A and Shore D are the most frequently used. Shore A is used for softer materials and Shore D for harder ones. Most platform stamps are cast with Shore A-rated urethanes. Shore also invented the Durometer that measures each Shore hardness on a scale of 0 to 100. Most manufacturers' stamps are Shore A with a hardness range of 65 to 90.
What contractors look for
Very few contractors order stamps by the Shore hardness number and don't know the hardness number of the stamps they own. They expect their purchase to perform the way they want. Yet stamp hardness is important in relation to concrete hardness.
When concrete is soft, stamps made with stiffer urethanes are best. But when concrete is on the verge of being too hard, it is better to use stamps with lower Durometer numbers. A complete set of stamps usually has one soft-platform “flex” tool for patterning difficult areas that also works better on stiffer concrete.
Joe and Jerry Garceau, owners of Butterfield Color, Aurora, Ill., were decorative concrete contractors before they were manufacturers. They learned that the window of opportunity to stamp impressions on an 8-cubic-yard, 4-inch-thick slab is approximately 45 minutes. So they designed stamps to be hard enough to carry a person's weight during stamping without causing a depression in the concrete under their feet, allowing the work to be completed within the desired time frame. Their tools' hardness also allows workers to feel the void between the stamp and the concrete so they will know when the texture and pattern of the stamp makes good contact with the concrete.
When workers are stamping impressions on softer concrete, stamps must be soft enough to roll a corner upward to release the vacuum and enable stamps to be lifted without damaging the impressions left in the concrete.
Scott Thome, director of product services for L.M. Scofield, Douglasville, Ga., says there are regional differences. For instance, L.M. Scofield discovered that contractors in California prefer stamps with lower Shore hardness ratings, while Midwestern contractors choose stamps with higher hardness numbers. Thome says that hardness changes depending on the size of a stamp, with larger stamps typically made of stiffer materials.
Resistance to temperature, solvents
Current urethane resins are much more resistant to changes in ambient temperatures than previous versions, although it's best to keep all stamps away from temperature extremes. However, the type of urethane resin and ambient temperature conditions during casting remain important considerations. Dimensional changes in a tool and curling can result.
Several years ago, contractors wanted to avoid using powdered release agents for some jobsite conditions. They discovered that spraying mineral spirits on fresh concrete enables a smooth release. But they found that the urethane resins at the time were sensitive to solvents such as mineral spirits, resulting in swelling and curling that destroyed the stamps. Most resins today are resistant to these solvents.
Be informed when you buy
Experienced decorative contractors who install impressions in concrete know what they want, though they may not be able to vocalize it. This information should help you define what you want for future projects.