Contrary to popular belief, dry (aka soft) pastel is a more permanent medium than oil paint, acrylic, watercolor, or gouache. “Pastel has no liquid binder that may cause the surface to darken, fade, yellow, crack or blister with time. No other medium has the power of color or stability of pastel. It does not oxidize with the passage of time.”1
Dry pastel is pure pigment. I use a variety of soft and hard pastels. I usually start with a base of hard pastels such as NuPastel or CretaColor. Then I move on to using softer pastels paying attention more to their value than their degree of softness. The softer pastel brands I prefer are: Rembrandt, Girault, Sennelier, Unison, Blue Earth, Mount Vision, Great American, Richeson Hand-Rolled, Terry Ludwig, Schmincke, and Henri Roché. I will occasionally use Faber Castell and Bruynzeel pastel pencils for specific effects.
I use archival-grade paper specifically designed for painting with pastels—its surface feels similar to fine sandpaper. Some brands I use are Sennelier LaCarte, UArt, PastelMat, Pastel Premier, and LuxArchival. Their “toothiness” enables me to paint using multiple layers of pastel in order to achieve the effect I’m looking for. Because of this tooth, the pastel adheres densely to the paper, requiring little if any fixative. I avoid using fixative as much as possible because it has a tendency to dull the refractive, sparkly qualities that are the hallmark of dry pastels.
Prior to painting, sometimes the paper will have been bonded to a hard surface called “gatorfoam” [an archival brand of foam core] or an archival mat board. I use pre-mounted papers primarily when I am painting en plein air to make it easier to transport and paint while I am out of the studio. The pastels, paper, foamcore, mounting, matting, (and protective backing if framed) I use are acid-free, museum, archival-quality materials. They are pH neutral and will not decay if handled properly. If I have framed the painting, it is mounted beneath TruVue® anti-reflection Museum Glass®, which blocks up to 99% of harmful indoor and outdoor UVA and UVB light rays. In fact, many museums today are choosing to place their oil paintings under Museum Glass in order to protect their colors from degrading. Museum Glass's near invisibility makes it appear as though there is nothing between you and your painting.
Because of its nature, a pastel painting cannot withstand any physical contact that would brush the pastel off the paper (this includes placing your pastel face down on any surface, even glass). Pastels are water soluble, so direct contact with moisture will damage your painting. This is why your pastel painting should be mounted under glass offset using plastic spacers or mats and sealed around the edges before being placed in its frame (a method known as passe-partout). This also makes it simple for you to change the frame. Your professional framer can carefully remove your pastel painting as one entire unit and safely place it in another frame with suitable depth.
Pastel paintings are best kept in rooms with temperatures between 68 ̊ – 72 ̊F with a relative humidity below 50%. Keep your pastel paintings out of rooms such as bathrooms and kitchens and never place any painting (regardless of what its medium is) in direct sunlight.
If you frame your painting using TruVue's Museum Glass, here's how to clean it while keeping its anti-reflective and UVA/UVB properties: use a 50/50 mixture of rubbing alcohol and water, place a small amount on a microfiber cloth. Do not saturate the cloth. Do not spray any moisture directly on the glass or frame as it can seep below the frame edge and damage your painting. Gently, using light circular motions, wipe the glass clean and dry.
Here is information from Frame-Tek about how to frame soft pastel paintings that you will find helpful.