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Hawk-n-Trowel - Hawk & Trowel - Index

Victoria Bingham of Buon Fresco likes the eff ect of burnishing
synthetic plasters. “The fi nished walls are stunning, with depth, shine
and exquisite nuance,” she says. “When I make a marble in plaster
it looks like marble.” Photo courtesy of Buon Fresco
Venetian plaster needs to be installed by an artisan installer
working for an architect or designer.”
To get the depth and richness that typifi es the
best Venetian plaster, skilled applicators use traditional
natural lime materials or carefully formulated man-made
synthetic plasters.
Natural lime plasters
Lime plaster, sometimes called Italian plaster, Grassello
or Marmorino, is based on limestone (calcium carbonate).
Limestone is baked at high temperatures to extract carbon
dioxide and water, leaving calcium oxide in powder form,
explains Nurit Regev of TexSton Industries Inc., Canoga
Park, Calif. Next, the material is slaked — water is added and
unbaked particles fi ltered out. Th e mixture is aged for up to a
year, resulting in hydrated slaked lime. Pigments, marble fl our
14 ◆ www.Hawk-n-Trowel.com ◆ Spring 2008
or minerals are then added.
In addition, acrylic or other resins may be added to
improve adhesion to sealed surfaces, such as primed drywall.
“Very few to none of the natural decorative plasters being sold
commercially in North America are purely natural,” Sickler
says. “A plaster that has no man-made (synthetic) ingredients
in it has to be applied over an absorbent substrate like unsealed
stucco, fondo (a specialty primer), plaster or masonry. Th e
market standard for a mineral plaster to be called natural is
that it should be at least 95 percent natural ingredients.”
Scott Nelson, owner of Naturalwalls in Pasadena, Calif.,
favors hydraulic lime supplied from Europe by St. Astier
that has no resins added to the material. He emphasizes
the importance of knowing how to work with lime plaster,
especially managing the cure. “You can just put an acrylic
up and walk away,” he says. “With lime you have to manage
the rate of cure after each coat. It has to cure slowly to get
strength, so you may have to put a tarp over it so it won’t
dry out too fast and crack.” He recommends waiting 10 days
between coats. “Th is slow set time prevents the cracking that
you see with colored cement-based plaster.”
As lime plaster cures, it reverts to the same chemical
composition as limestone. Th e resulting fi nish is extremely
durable. “It lasts the life of the building, so there is no need to
recoat or repaint after just two or three years,” says Regev.
Gauthier believes this durability is an advantage of lime
plasters, which he always uses in high-traffi c areas. “Lime is
the hardest of the two,” he says. “Synthetics are softer — easier
to gouge and harder to touch up.”
He also says lime is thicker, so it hides irregularities. “You
can fl oat lime plaster over imperfections. Synthetics go on so
much thinner. Razor-thin layers give you the richest color and
depth, but imperfections will show through. Synthetics need
better wall preparation.”
Regev explains other benefi ts of lime plaster. “Th e surface
remains fl exible, which reduces cracking. And it is moisturepermeable,
so it breathes.”
Nelson has a graphic illustration of this moisturepermeable
quality of lime plasters. “We fi nished a house with
Developing skills and personal technique through training
helps contractors go beyond competing only in price.
Photo courtesy of Dundean Studios Inc.