Preservation of Animation Cels
(Not knowing your skill set and tool collection, I will answer this question by describing how I as a professional framer would frame this for a customer.  Many of my methods are basically preferences I have settled into over the years.  Where applicable I have included links to other methods.  In this scenario I am cutting the mat, spacers and the glass myself, but I am ordering a custom-made frame from a supplier.  Feel free to ask for clarification on terminology, tools etc... in the comments.)

[1] Design and Cut the Mat: Considerations include: A mat provides a clean uncluttered field surrounding artwork, which can bring the work forward. It is practical if there are edges to the artwork which you prefer to keep hidden. It can serve the purpose of keeping a vulnerable artwork away from the glass.  You can also use a mat to bring out a particular color in the artwork.

Sometimes you may not even want a mat, for instance here you might prefer that the celluloid float minimalistically within a simple frame.

For examples’ sake, here let’s assume that the celluloid’s actual dimensions are 11” x 11”, that the area of the celluloid which you want to be visible is 10” x 10”, and that you would like a mat of 2” width visible. Because it is celluloid you want the mat-artwork-mat sandwich to be “spaced” away from the glass, (“Since animation cels are painted on both sides of each layer, they need to be matted on both sides to insure sufficient air space between them.”)  , and you want to be able to see both the front and back of the artwork.  (The design I am describing here allows you to take an artwork off the wall to see the back side of the cel. If you wanted the artwork to have a finished frame from both front and back, this would be done by working closely with a framer to choose that type of specialty frame moulding, and the assembly steps would therefore differ.)

When I cut a mat, I add the frame’s rabbet depth (let’s say that is 1/4”) and 1/4” to the bottom part of the mat for the optical effect of it being weighted, which is unconsciously more pleasing to the eye than having all of the sides being the same width. [(Bottom-weighting is of course optional. Here is an explanation over at framersworkshop.com of bottom-weighting: “The optical center is higher than that of the geometric center; in other words, where your eye is drawn is actually above the actual center of the space.” This source uses the golden mean to determine the amount to weight the bottom of the mat. I typically just use 1/4” for an artwork of the example size.)][1]

So far, we have a mat design that is 14-1/2” wide by 14-3/4” high, as I have diagramed here: 

Therefore, in order to be able to “sandwich” the cel between two matboards, we will cut two matboard “blanks” to the outside dimensions of 14-1/2” W by 14-3/4” H, then cut the beveled openings using a mat cutter.

2 Cut the Glass: Two pieces (for front and back) will be cut using the same outside dimensions that we used for the matboard. Make sure you select the highest-possible-rated UV Protection glass, because, as you pointed out, celluloid is very vulnerable, and would be readily degraded if left unprotected from light.  (You will also want to display this in a room where there is no direct light.)

3 Frame Design: In order to be able to lower your glass and “mat-artwork-mat sandwich” layers into the back of the frame, the inside dimensions of the rabbet of the frame will need to account for approximately 1/8” “slosh room” in both height and width dimensions.  Some frameshops automatically add this, so when you order the frame be sure to confirm if they will be adding slosh room, or if you should include it in your measurements (which would come to 14-5/8W x 14-7/8”H.)

Additionally, some framers just want to know the dimensions of the visible area of the artwork and mat (not including the depth of the rabbet OR the slosh room), which in this example would be 14” wide by 14-1/4” high.  Again, it is very important to confirm with your framer if they would like the dimensions given to them with or without the rabbet depth.  

NEVER phone in a custom frame order.  ALWAYS give your order to a custom frame supplier IN WRITING, be it via fax, email, or a COPIED hand-written note.  This way, if you have the misfortune of receiving a frame that is all wrong, you have a paper trail to confirm the information you originally supplied.

For this celluloid, as I diagramed below, you will want to choose a moulding with adequate rabbet depth for two layers of glass (3/32” each), plus 1/4” front and back for spacers to give the artwork room to breathe, plus two layers of matboard (1/ 16” each), plus a minimum of 1/4” working room for securing the sandwich into the back of the frame.  (The thickness of the celluloid, represented by the red line, is negligable.) This total comes to 1-1/16”, which is a very deep rabbet, but not impossible to find (especially if the framer has frame moulding stock that is used for shadow boxes.)


4 Assemble the Mat-Artwork-Mat Sandwich: Wearing clean cotton gloves, and handling the celluloid only by the edges, position it to your liking on the back of one of the mats. Using archival framing tape (my favorite brand is Filmoplast P90) afix the celluloid to the mat board, carefully placing the tape so that the least amount of celluloid possible (I would shoot for 1/8”) is actually in contact with the tape, yet the artwork is secure enough on all sides to not slip within the mat sandwich.  (Because we are working with a two-sided artwork, there is no advantage to using the Hinging Method here.) Using a bone folder, rub the tape well to enhance adhesion. (The folks over at framersworkshop.com have a much more conservative approach about securing a celluloid: “They should not be hinged to their backings. Instead, we use no adhesives on the cels and recommend the use of edge strips that gently hold the layers in place.”)

Once you have the cel secured to one mat, apply some double-sided tape around the outer edges and secure the mats together back-to-back.  (I use 3M ATG tape for this.  It is not archival, but with a 2” wide mat, the ATG should be at least 1/2” away from the edge of the cel. If you are very worried about the use of ATG adhesive that close to your artwork, you could glue the mats together at the edges with acid-free glue such as Yes Glue.)

5 Assemble and Secure All Layers into the Frame: Again wearing cotton gloves, clean one piece of glass and place it into the frame. 

Cut precise 1/4” wide strips of 3/8” Foamcore for spacers. White spacers may be overly visible, in my experience black spacers are not noticeable, so either use black foamcore, or color it black with a felt pen.  Cut the Spacer parts to lengths that will fit precisely into the rabbet from corner to corner. Secure them into the frame, abutted tightly against the glass, with ATG.

Lower the Mat-Art-Mat Sandwich into the frame, making sure the side you want to the front, is to the front.

Repeat the placement of black foamcore Spacer Strips behind the Mat-Art-Mat Sandwich.

Clean and place the back piece of glass.

Secure the back glass into place.  There are many possible methods for this including brad setters and point drivers.  It would be risky to use these tools right next to the glass, since the points / brads could scratch the glass, and the tools (especially the point driver which is spring-loaded) could impact the glass, potentially breaking it.  (These are not concerning issues when framing art that only needs to be visible from the front of the frame and therefore has an acid-free matboard or foamcore backing.)  Since we don’t have a backing to protect the glass, I would cut 1/2” - 1” wide strips of matboard, and ATG them along the outside edges of the back glass, then with either a point-driver or brad setter, secure the glass in place.  

7 Finishing Touches: Apply a hanging wire to the back of the frame; apply felt bumpers to each corner on the back of the frame. Voilà!