Some adjectives with two forms often create doubt in native speakers.
Take for example these uses of the words wood and wooden:
You’ll want to budget about $4/square foot for a wood deck…
Building a wooden deck over a concrete one
68 Wooden Deck Design Ideas
Expansive wood deck with low wood railing using diagonal wood floor design
Which is correct? “Wood deck” or “wooden deck?”
Strictly speaking, wood is a noun and wooden is an adjective, as illustrated in this headline:
Wood science and how it relates to wooden baseball bats
In this example, wood functions as a noun: “the science of wood”; wooden functions as an adjective: “bats made of wood.”
But although this “rule” of distinguishing between noun and adjective can be applied to wood and wooden–and possibly to wool and woolen–most of the -en adjectives that describe substances have become relegated to the realm of poetic or figurative language.
The following examples show how the -en adjective forms were used in their older, literal senses:
“The city had been one mass of color…One great crumbling mass of ashen ruins was what we found left of all this.”
“It grew dark, and they put candles on the tables–candles set in bright, new, brazen candlesticks.”
“…the early-modern hemp industry was far more versatile than has been supposed and was capable of producing not only ropes and canvas, but also hempen cloth some of which could rival all but the finest flaxen cloth.”
“Three compartments divided the coffer. In the first, blazed piles of golden coin; in the second, were ranged bars of unpolished gold.”
“The water was formerly conveyed by wooden pipes from these basins through all the streets, and a leaden pipe, inserted in the main, supplied each house.”
“Pine lids were not put on oaken chests in England at this early period.”
“From an anecdote of Aurelian, who neither used silk himself nor would allow his wife to possess a single silken garment, we learn that silk was worth its weight in gold.”
“One of the wedding presents had been a pair of beautiful white waxen candles.”
In modern usage, the noun in each of these pairs serves also as an adjective. For example, brass for brazen and flax for flaxen:
…these tarnished brass candlesticks looked better suited to a flea-market stall than a dining table.
It was sometimes said that the flax rope was apt to break when a knot was made in the rope.
The -en forms do survive in figurative contexts. Here are just a few:
She’s a brazen woman and no mistake! (impudent)
“They’ve caught us trespassing; we’ll have to brazen it out. (face the situation impudently)
The dead man’s face had a waxen hue. (appearance of wax)
The girl had laughing blue eyes and flaxen hair. (any light shade of auburn or pale yellowish brown)
Keep an eye out for other examples of these -en adjectives in modern English.