Real "Museum Mats" are the best and, highest quality mat board there is, period! The Rising Co. makes genuine Museum quality mat board. (Strathmore also made Museum quality mats, and I hope that they still do so.)
A true Museum level of quality demands a mat board that will last - if properly used - for many hundreds of years. Museum Mats are the only valid choice for owners of art work of quality. A real Museum Mat board will - ALWAYS - be made, only with 100% cotton. This cotton usually comes from cotton rags - which are cleaned, and then processed - for strength. If, unspun cotton were used - coming straight from the cotton plant - then it would not be as strong, as cotton that was woven, before being converted into a Museum Mat.
Normally, Museum Mats have calcium carbonate added to the mat as a buffering agent to fight against any trace of acid. Normally, this is a good idea. However, as was mentioned, (in the Photography Section), Calcium Carbonate can react with some photographs, and related photographic materials, in a harmful way. Therefore, just to be safe, always use - only Museum Mats that are calcium carbonate free - on all photos and related materials, (such as: negatives and slides.)
How to identify a genuine Museum Mat? The "corner sample" - frame shops have corner samples, so that you can compare various mats - must state: "Museum Mat" - and - it must state: "100% Cotton (Rag)". If the store does not stock Rising Co., Museum Mat board, then walk out! They are using cheaper products - which do have a purpose - but they are not for the best. If they say that their mat board is "acid free" - what they are REALLY saying is - that it is made from wood pulp that has been chemically treated. However, those cheaper mats always become acidic again, over time - and WILL harm your artwork - over time.
Next, at the bottom of the mat board food- chain, is the ordinary "paper mat". these are generally just bleached wood pulp, and I would not use them for anything.
There is a company that is called APF Munn that makes custom mats and very, very expensive Museum quality frames. (Note - Please see section on Frames for more.)
Their "Duplex mats" are thicker and much stronger than other mat board "windows." They come custom-made to your request. Therefore, you need to think a lot about the exact style and you must give them the exact size for the "window" and exactly how wide the top sides and bottom will be - because once you buy it it's YOURS - I don't think it can be "returned" unless it were to be damages or (and I've never known them to make a mistake), some mistakes were made.
You will need to (probably) have a 3-way conversation: you, your framer, and APF Munn. (I would feel - uneasy - about dealing with a framer who had never done any business with APF Munn.)
In general the top and sides of the mat are the same width and extra size is added to the bottom. This is called "weighting" the bottom. If you don't do this then the picture looks as if it were "sinking". The amount of extra "weight" to give to the bottom is a personal choice, I tend to give the bottom of the mat more space or "weight" than most people. Remember that the frame will cover up at least ¼ of an inch and probably more. So I usually add ½ inch (for the frame) to the weight to be safe. So my "formula" is: (the width of one side + the "weight" + ½ inch (that the frame will cover up)= total "weight" for the bottom of the mat "window". However, you have to decide for yourself what looks good.
"Floating" a work on paper is very tricky. It must be attached to an acid free backing, and the best choice is probably an 8 ply Museum Mat.
"Floating" means that the edges of the art work on paper will all be seen so the picture will appear to "float" in front of its supporting backboard. If you want a decorative background, then you will need it to be as good as Museum Mat board, because it is going to touch the back of the artwork. And the backing paper - or silk - will need to either be attached to the Museum Board backing with a conservation adhesive. If it is simply pressed into place by the mat (which will need to be quite deep to prevent the art from touching the glass) - holes will have to be cut in the decorative acid free backing, so that the art may be attached by acid free hinges to the Museum Mat backing.
As you can see - floating is very hard to do correctly. (To be continued)