The night sky is only a sort of carbon paper,
Blueblack, with the much-poked periods of stars
Letting in the light, peephole after peephole--
Greetings from your mid-century U.S.-business-office material-culture antiquarian. In 2025 we have photocopiers and "transfer paper," but what Sylvia Plath meant by "carbon paper" are very thin single blank 8.5 x 11-inch sheets, often midnight blue ("blueblack"), and brittle as nori. One side of each sheet is sticky with a blend of dark ink and wax. Placing the ink side face down between two blank sheets of paper and writing on the top sheet creates a duplicate called a "carbon copy," or "a carbon" for short.
To get the most from every sheet, frugal typists re-used carbon paper three, four, or even five or six times, until its ink was depleted and the sheet so fragile (like toasted nori) that using the typewriter key for "." punched little holes in it.
Carbon paper has its place in Plath studies.
The carbon paper's ink side preserved a mirror image of any typewriting. Because of this, some years ago archivist Peter K. Steinberg was able to retrieve two previously unknown Sylvia Plath poems, "Megrims" and "To a Refractory Santa Claus," from carbon paper in the Lilly Library.
As with other documents, for business and professional transactions carbon copies weren't acceptable. Sylvia learned this in 1958 when celebrated poet Marianne Moore, a business-school graduate [1], returned Sylvia's carbon-copied poems with a "tart and acidy" note about how typing was such a chore. Plath grieved her "great & stupid error -- sending carbons to the American Lady of Letters," [2] and she and Ted Hughes hated Moore for the rest of their lives.
For letters or manuscripts in triplicate or quadruplicate, the typist sandwiched sheets of carbon paper between three or four sheets of blank paper. Rolling this sandwich evenly around the typewriter's platen was a feat. The copies were to be systematically color-coded, indicating the document's urgency, importance, or routing. Carbons of business documents mostly began with white paper (often ultra-light "onionskin"), then green, yellow, pink, and "gold" (mustard): all bilious, unwearable pastels now seen only on pharmaceuticals.
[1] Moore "completed the business course at Carlisle (PA) Commercial College in 1910, [and] taught stenography and typewriting at Carlisle Indian School." Twentieth-Century American Poetry (2004), Gioia, Mason, and Schoerke, eds., p. 227.
[2] Journals, 17 July 1958.