Three possible answers to this question:
Any sort of "fixing" or stabilization will have a negative effect on the mellowness of the texture, and will impart an unwelcome lustre, an alteration in the color, and even a loss of the subtle tonalities in the work.
Moreover, the real strength of the pastel medium lies in the unchanging nature of the pigment. Any stabilization process or "fixative", by its very chemical nature, is going to change the nature of the pigments, in the end.
In addition, unless great care is used, particles of the pastel powder may become detached in the course of the stabilization or "fixing" process.
If you are willing to do this, you must then search for the ideal fixative, and this search can, in turn, lead you on to other adventures.
In the process, you will become more familiar with the nature of the pastel pigments themselves, and this process of discovery might prompt you to alter their nature somewhat, and, thereby, to discover even more rich effects, such as the use of techniques involving the mixing of various media.
A compromise between ensuring stronger pigment adhesion and stability (the main reason for using a fixative) without damaging the mellow, "peach skin" effect of pastel is to be found in the use of an intermediate fixative process.
In this method, the final pastel layer is not "fixed". Moreover, applying the fixative during the work process itself enables you to create textured effects in the thickness of the work.
For example, Edgar Degas often worked using successive layers of fixative between his pastel layers, sometimes using quite a few of each. It was this method which enabled him to successfully use tracing paper as a medium for pastel.
One principle must be kept in mind, above all: insure that there is no rubbing possible to the surface of the work. For example, when you wish to move a drawing, be sure that the pastel is held tight, ideally between two sheets of plywood or some similar, very stiff material.
When storing your work, use the same sort of protection. For example, you can store twenty or more pastels between two sheets of plywood or similar material.
Use a frame of glass with a specially cut mat, at least an eighth of an inch thick, to prevent the surface of the work itself from coming into direct contact with the glass.
Unfortunately, there are no ideal fixatives or stabilization methods. Even the best fixatives can change the tonalities of pastels, and thus make them lose some of their mellowness and subtlety.
If you wish to try a "natural" stabilizing agent or fixative, you can try applying a solution of diluted Gum Arabic, to which you have added a little glycerin -to prevent the drawing from cracking-, plus traces of preservative to provide antiseptic protection from bacterial and fungal deterioration.
However, spraying this "home-made" fixative won't be easy: you will have to apply it with an airbrush.
Another solution to the problem is to use fixative compounds not specifically related to the fine arts, but those intended for other uses. For example, Polyurethane may be applied to the artwork with a spray gun.
The fixing coat will then be laid down in the form of an inalterable and impermeable film. However, the disadvantage of this technique is that the pastel pigments will firmly adhere to the polyurethane resin, and they will therefore tend to detach from the ground medium.
As you experiment with various stabilization techniques, you will discover your own ideas for preserving your work.
For instance, you might try mixing pastel with gouache or pastel with acrylic paints, etc. When used in these mixed techniques, the texture of the pastel is reinforced.
Finally, you can simply use oil pastels, since they do not require stabilization or fixing.
Well, how to frame a work is the eternal problem of pastel.
Pastel work must be framed under glass. Plexiglas or its equivalent is not appropriate, simply because the pigment particles may be attracted by static electricity when the pastel is brought in contact with or set near the surface of the plexiglas.
However, once it is fixed, the pastel work can be covered with Plexiglas or any other artificial "glass."
In defense of glass, however, I would say that the reflections themselves may be used to bring some "life" to your work, in that they allow for continuing, renewed interpretations of it. The reflected environment can interact symbiotically with the work, like an additional soul. Moreover, reflections require movement on the part of the viewer, who thus must work to "deserve" the work.
Indeed, when the artwork is, as it were, "encased" under glass, the viewer loses direct physical contact with it. However, it might be said that the work benefits "contemplatively", in that this very separation encourages a more mental and even spiritual appreciation of the work.
Every museum in the world uses glass for framing, not only for works on paper but also, and increasingly, to protect paintings, frescos and even sculptures. We are thus becoming familiar with this presentation of art "through windows," a presentation which reinforces the uniqueness of the work.
Lastly, why not use glass precisely for its intrinsic qualities? As in a work painted with pastel on both sides, simply held between two plates of unbreakable glass and steadied on a base, like a piece of sculpture?
Glass is not fragile once it is framed.
I use ordinary "window pane" glass, single or double strength, being sure that it is neutral in color and does not have a green cast to it.
I never use synthetic glass or Plexiglas, as it might hold a static electric charge which would attract the pastel pigments off the surface of the work.
The question of the color of the mat used in the framing of the work is, of course, a matter of individual taste, and it is difficult to give specific advice about this without seeing the actual work itself.
If in doubt, perhaps it is best to consider using a neutral color. For example, you can use a gray, greyish-brown or even a dark, quiet color which enhances and brings out the tonalities inherent in the work itself.
And, it must not be forgotten, the color, and even the shape, of the frame also plays a role in the final appearance, reception and understanding of the work. Here again, you can try some traditional alternatives: use a light-colored wood frame, or one consisting of a thin metal rod.
However, white or very light colors should be avoided as they may distract the eye too much to the frame itself, rather than to the pastel.