What is "ply" in framing supplies?
What is "ply" when it comes to mat board, mount board, and back board?
I always understood the word "ply" to mean layers - for example, two ply toilet paper can be separated to two sheets.
When it comes to framing supplies such as mat board, mount board, and back boards, the term seems to simply imply thickness. Is this the case or do those materials manufactured in layers and pressed together somehow to form one board?
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Q. What is the difference in 2 ply, 4 ply, 6 ply and 8 ply? Which ply do I need?
A. The term 'ply' refers to the thickness of the mat board. The higher the number, the thicker the board. The most common thickness used is 4 ply (around 1/16th of an inch). You can tell the difference in thickness by saying 2 ply is 1/2 the thickness (1/32") of 4 ply. 6 ply (1/8") is 4+2 ply, so it would be 1 1/2 times as thick. 8 ply (1/4") would be 4+4, or 2 times as thick. It is double the size of 4 ply (double the thickness). Which ply should you use? That depends on what you are doing! 98% of all mat board sold is 4 ply. 6 and 8 ply mat is often used in museums or galleries for special presentations of artwork or photos. The 45 degree bevel allows the extra thick core to show and gives a dramatic effect. Double matted means you will have 8 ply mat on your artwork if you use the 4 ply board. In most cases, 4 ply works great, and anything more is up to you! If you choose a thicker or thinner mat board, you must adjust the blade on your mat cutter to avoid over or under cuts. If you are just starting out in mat cutting use 4 ply only. Also, some mat cutters (both manual and computerized) will not cut 8 ply. If you're interested in a great machine that can handle an 8 ply board, check out the Logan 850 mat cutter!
(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
The first step in discussing any paper board is to define a board. A paper board is defined by its thickness. Any paper that is 0.012 inch or more, which is a little less than 1/64 inch in thickness, can accurately be called a board. Tissue paper is approximately 0.001 inch thick, bond paper ranges from 0.003 to 0.004 inch, and one-ply museum board ranges from 0.0125 to 0.015 inch. The material most commonly used to store and protect fine art is four-ply museum board and its thickness ranges from 0.050 to 0.060 inch (0.060 inch is almost 1/16 inch.
At one time there were several ways to define various boards by their thickness. A standard mat board was said to be fourteen-ply (roughly the equivalent of putting together fourteen sheets of bond paper) and double-weight mat board twenty-eight-ply. A standard museum board, which is approximately equal to a mat board in thickness, is called four-ply because each ply is much thicker than bond paper and it takes only four to reach the desired thickness. Today, most manufacturers of paper boards are shifting toward using a point system of measurement, which will make it easier to comprehend thickness and to compare various boards. In the point system, one point equals 0.001 inch. Therefore, if the average mat board or museum board is between 0.050 and 0.060 inch it would be between 50 pt. and 60 pt. in thickness.
There can often be as much as a four-point variance of thickness within a manufacturer's advertised thickness. Manufacturers of mat board have, for the most part, dropped their system of defining thickness by plys in favor of the point system. Some manufacturers who make both mat board and boards for illustration, as well as for graphic arts, refer to their 50 pt. to 60 pt. boards as single weight, and their 100 pt. to 120 pt. boards as double weight. Museum boards are still most commonly referred to in plys, although the point system is rapidly taking over.
Develop, stop, fix.... wait.... where's my film?
Having spent some time hanging out in paper mills (babysitting control systems), I suspect the 'ply' relates to adding multiple layers of the wet pulp down in sequence to build thickness. But the created plies will likely have pretty much merged and disappeared by the time the finished product goes through the many rollers. It is also likely that in modern processes they may not lay down four distinct plies to get '4-ply' thickness.
There are some mind-boggling bits of arcane terms and dimensions in paper making, partly thanks to it being rather ancient technology. Even the pounds per ream is screwy in that paper for different applications may use a different sheet size to calculate the weight. Most stuff now seems to show grams per square meter which is a lot more consistent in that regard.
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Ply is related to the layers of substrate such as cotton in the case of rag mat board used to create the mat. 4-Ply is the most common and is approximately 1/16 inch thick. 8-ply will be twice as thick or about 1/8 of an inch. 6-Ply of course will be in between. 2-ply board is only 1/32 of an inch thick and is not really used like a traditional mat board. It is too thin for a meaningful bevel cut in the opening window. 2-Ply is primarily used for archival mounting purposes when you want to mount on cotton rag board. Since the 2-ply is thinner it less expensive, however it is not thick enough to serve as a frame backing so it will need to be backed with something like foamcore.
A lot of mat board is made from other substrates like wood pulp or alpha cellulose and does not appear to actually be composed of individual layers or ply's and the manufactures do not seem to refer to them as 4-ply or 8-ply. They will be standard or double thick where standard is about 1/16 of an inch. For clarification and lack of more common term I choose to still refer to these as 4-ply on my frame companies website.
Different companies 4-ply or 8-ply or standard mat board will be different thicknesses. I have noticed that Bainbridge 8-ply alpharag will be a full 8th of an inch thick and many other companies will be just a little thinner. Also, although the Bainbridge 4-ply alpharag matboard is a full 1/16 inch thick, the Bainbridge alphamat is a little thinner, and the paper matboard that we sell is thinner still.
One of the reasons I really like the Bainbridge 8-ply rag mat board is that because it is a full 1/8 of an inch thick and we cut the bevel at 45 degrees it results in a bevel that is nearly 1/4 of an inch and is therefore visible at typical viewing distances and thereby adds signficant presence to the framing package.
I use Crescent Cotton Rag Mat 6 or 9-ply w/ 45° bevel for all of my conservation framing, and have maintained that since 1996. This stuff has never been and never will be cheap.
Canon EOS1N ('Brutus', 1993—), TS-E 24mm f3.5L, 20mm f2.8, 17-40 f4L, 70-200 f2.8L
Pentax 67 ('Pentaximus', 2010—) + SMCP 45mm f4, 55mm f4 & 165mm f4LS;
Zero Image 6x9 multi-format pinhole (2008—); Sekonic L758D;
Olympus XA, Nikon Coolpix P7700
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In addition to the excellent responses previously, ply is also a function of how much a board is calendared. A calender is a device with two or more rollers through which the paper is run. The compression of the rollers and the application of heat give the paper the degree of its smooth and glossy properties.
Strathmore Bristol Board is the smoothest board that is made. It is calendared so much that 4-ply Bristol Board is thinner than 2-ply mat board. We use this Bristol Board in the dry mount press between the print and the platen. It is very expensive, but will last for thousands of dry mounting operations. We use two boards between the print and the platen and one board underneath. We cut the Bristol Board the same size as our mounting board.
Michael A. Smith