Translucent sheets, also referred to as vellum, are made from the same wood pulp as regular opaque sheets. Well, almost. In the case of translucents, the pulp is specially fibrillated, which creates fine hairs on each fiber and allows them to create a tightly interwoven mass of pulp.
Where the pulp recipe for opaque sheets asks for fillers and more before they undergo the actual papermaking process, naturally translucent sheets forgo this option. They are simply heated up and pressed several times to get rid of any extra air pockets, which makes them very compact.
An easy way to tell the difference is by looking at a translucent and an opaque sheet of the same weight. The opaque sheet is noticeably thicker than the translucent one.
Natural vs. Chemical
Translucent sheets are usually referred to as “chemically” or “off-machine transparentized.” In this case, the sheets are first manufactured as opaque papers and then treated with petroleum-based resins, heated and cured to make the paper translucent (think of what happens to the paper plate your slice of pizza had been on). This process tends to make the fibers more brittle and due to the added resins, the paper is not recyclable. However, they are great for printing heavy coverage and for use as a fly sheet. Sheets that are chemically transparentized tend to be whiter than their natural counterparts, some examples are Neenah Paper UV Ultra II and Yupo Translucent.
Naturally transparentized sheets are made the same way as an opaque white sheet, but undergo a more rigorous process during the pulping phase to achieve their “natural” translucency. Due to the way they are manufactured they lend themselves well to folding and scoring, and are great for envelopes, belly bands, and wraps, however they tend to be a bit duller in whiteness. There’s always a tradeoff.
How will you know which is which?
Most mills identify if their paper is naturally transparent and recyclable. Another easy way to tell one from the other is by looking at the product items being offered. Chemically transparentized sheets are more brittle and do not fold without cracking. So, they are not recommended for folding (and saddle stitching) and if you're looking for envelopes, only naturally translucent sheets are recommended for that: Neenah’s Clearfold or CT by Reich.
On the greener side...
In terms of recycling, any naturally translucent sheet is recyclable; and if you're looking for FSC papers, check out Clearfold by Neenah Paper along with CT by Reich Paper. If planning to use one these papers for an upcoming project, contact your Millcraft rep for more information or samples.