Supports

Historically, frescoes have been incorporated into architecture as lime plaster was a common finish to walls. However, the process of carbonation, which is the essence of buon fresco can take place on any surface that will accept lime plaster so long as it provides sufficient adhesion and rigidity.

The thickness and number of layers needed for fresco plaster can vary depending on the support, unique environmental conditions, or artistic vision. Many Italian frescoes contain only two layers commonly referred to as the "arriccio", which is a brown coat and the "intonaco", which is the final painting layer. Many contemporary plasterers use three coats or more as industry standards suggest additional layers increase the durability and strength of the wall. In addition to strength, the thickness of the wall impacts the painting window, as thicker walls extend the amount of time an artist can paint. Many frescoes in North America such as those created by Diego Rivera were created using five coats that consist of a scratch coat, two brown coats, a sand coat, and finally the intonaco, known as the painting layer.

Here are brief descriptions of some common fresco supports that include both permanent and portable structures. Those ambitious enough to attempt a large-scale fresco on walls made of brick, stone, or  wooden/metal lath should research the topic more thoroughly prior to construction. 

Permanent

Brick or stone walls: An advantage of sturdy masonry supports is rigidity and absorption. Because of this, cement is not usually needed in the scratch coat formula as there is sufficient strength from the wall itself. However, plasterers must ensure proper adhesion or tooth by texturizing smooth surfaces through scratching, chipping, and/or providing a chemical bonding agent. In addition, it is extremely important that bricks containing white encrustations (salt blooms) be removed and replaced. These sulphates dissolve in water and migrate through the wall. This is concerning structurally because they interact with the calcium in the wall turning it into calcium sulphate that degrades the paint layer and supporting plaster. 

Exposure to water is problematic for other reasons. Water  (or sulphates) that enter the wall through wicking from below or behind may lead to bacteria and mold and eventually cause delamination of layers.

Fabricated frame walls: (metal or wood beams and crossbeams)
Wall joists are typically spanned by horizontal wooden lath strips or sheets of expanded metal lath. Wooden lath strips are typically 1" wide by 4' long. Each strip has a 3/8" gap between itself and its neighbor. Plaster is generously applied and pushed through the gaps to create a "key". Oftentimes it's leveled using rails and a screed. When metal lath is used, care should be taken to pull the material tight and attach it right-side-up so it will accept a plaster key (there is a top and bottom to metal lath). 

If joists are already covered with paneling, metal lath should be furred to ensure enough room for the "key". Tar paper, preferably covered in brown craft paper can be used to seal the wall from moisture. It can be stapled into place. Some brush-on liquid waterproofings may be suitable as well. 

Portable

A series of ceramic and foam panels that have
cleat hanging systems adhered to their backs.
The glossy sheen of the ceramic tile must
be removed before bonding to wood. An industrial
strength epoxy should be used as the adhesive.

Ceramic Tiles: A cheap and easy portable support for fresco works is the common ceramic tile, which can be purchased at your local hardware. Plaster is applied to the rough "mortar" side of the tile, not the decorative side as the glazed surface is not porous and too slick.

In my experience tiles made from a dark red clay work well. They can be brushed with water before the plaster is applied. The wet tile can glisten but not have standing puddles on its surface. If the surface is too wet, the plaster will not adhere and will seem to peel off when troweling. Ideally, there should be some degree of grip (suction) coming from the substrate to hold the plaster as it's being troweled. 

Two coats of plaster works well on tiles and do not need to be particularly thick (1/16" each). However, remember that the open time for painting increases with the thickness of the plaster. The mortar side of the tile has a textured pattern that can be filled with a simple sand coat. After the initial coat has set up the painting coat can be applied in the same manner.

Styrofoam Sheets:Styrofoam sheets (EPS or XPS) that are thick/or rigid enough to prevent flexing can be used as a support for plaster, although it requires a bonding cement such as QuickWall.  For smaller works (under 12") a 1" thickness may be rigid enough. For larger works, 2-3" thicknesses may be more stable. There are some great foam-to-wood glues around that can be used to adhere a cradle to the panel. Cradles can be lightweight and possibly useful for display purposes.

2" foam panel to be cut into shapes with a
"hot wire" bow cutter. 

The gray areas on these test samples have an
 application of  Quick-wall Bonding Cement

The back of this foma panel has a wooden
support. A cleat was added at a later stage as
a hanging mechanism. The frame was
recessed to give the tile the appearance of
floating off the wall. Future works will have a
wider wooden frame to offer more tensile strength
to the foam.

Foam panels should be "scratched" thoroughly to provide ample tooth for the bonding cement. Carefully follow the directions on the product being used to ensure proper adhesion and thickness. Once this layer has been applied and setting begins to occur (but not yet finished), a light scratch can be added to help adhere the next layer.  You'll find Quickwall makes the foam more rigid and ready to accept a coat (or two) of lime plaster. 

Other supports: I've mentioned a small handful of supports but there are others such as Magnesium Oxide Board, Sheetrock, Wonderboard, and various cement or ceramic forms to name a few. If it can hold plaster, it will likely hold a fresco! Beyond that, your individual needs and vision will determine which supports are most suitable for you. Experiment!