Techniques: Plaster and Masonry

Fresco a Secco

A modern equivalent of Fresco a Secco for interior plaster
Fresco a Secco by Dr. Blake Ketchum

Background
Fresco a secco isn't true fresco. In true fresco, the pigment is worked into the wet plaster. In traditional fresco a secco, the pigment is mixed with a binder, usually egg, and applied to the set plaster.

My Modern Version
I developed this technique to assist in two instances, one is the finishing of old plaster in architectural setting, and the other is to finish plaster casts of sculptures. It creates a wash-like surface that is very similar to fresco and fresco a secco on plaster, using  modern and relatively durable supplies.

Supplies

  • Clear or Blond Shellac
  • Denatured alcohol
  • Ammonia to clean up shellac on brushes and sponges
  • Glass jar and lid for diluting shellac
  • Natural sponge, and brushes of suitable size for the scale of your project
  • Several large-mouthed paint containers
  • Latex or acrylic paint
  • Clear acrylic or latex medium that has a matte finish
  • Bucket of of water and clean rags
Important: Always test in an unimportant spot!


Surface Preparation
  1. Clean the surface by dusting or washing it off. Often dusting or brushing is adequate. Some authentic grime can add character in old architectural settings.
  2. The plaster must be dry. This is different than set. Plaster goes through two processes: first the chemical bonds form, this is setting. A porous crystalline structure is formed in the setting process. Extra water fills the pores. The set plaster is like a wet sponge until it dries. Drying takes a long time, sometimes months for a  thick cast. The plaster is dry enough when the surface is not cool to the touch, and a light sanding does not fill the sandpaper with dust. The dust falls away. 

Sealing
  1. Prepare a mixture of shellac and denatured alcohol in a lidded jar. 4 parts alcohol to 1 part shellac. Gently mix the solution and let it sit for an hour to properly disperse and equilibrate.
  2. Throw open the windows and put a wash of the shellac solution on the surface. Ventilation is important when working large surfaces like walls. Don't let the mixture drip. Spread it thinly. Drips will influence how the pigment is received by the surface. I prefer to work with a sponge or stippling strokes to avoid introducing pattern at this stage. Clean up your sponges and brushes in an ammonia solution.
  3. Let the surface dry for a full 24 hours.

Mixing the Pigment Washes
  1. Have some test areas prepared. Areas that you know will be dark can work as test areas for lighter regions.
  2. Mix clear medium thinly with water in a 10:1 ratio. It should be very watery.
  3. Pour off some of the medium mixture into another container and add a very small amount of acrylic or latex paint. You will be adding thin layers of the emulsion to build up color in glazes. Stir and let emulsify for an hour.
Application
  1. Use a sponge or brush and apply the glaze thinly to the plaster. If the pigment is too strong, you can quickly wash off with a damp sponge and dilute the emulsion with more of the medium and water mix. You can build up layers to achieve more saturated pigmentation.
  2. Build up your design by adding layers of colored glazes rather than mixing pigments within the glazes. Allow each application to dry before adding a new glaze.

Did you try this? I'd love to see your work. Please email a photo and description to me.

Posted on 03/26/11 _________________________________________________________________________________________________