ret (third-person singular simple present rets, present participle retting, simple past and past participle retted)

(transitive) To prepare (flax, hemp etc.) for further processing by soaking, which facilitates separation of fibers from the woody parts of the stem.
https://en.wiktionary.org/wiki/ret

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Note added at 15 mins (2018-05-26 14:23:29 GMT)
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Roszarnia – oddział zakładu włókienniczego zajmujący się przetwarzaniem słomy lnianej lub konopnej na włókna
https://pl.wikipedia.org/wiki/Roszarnia
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roszenie słomy lnianej – druga, po odziarnianiu, faza obróbki słomy w procesie pozyskiwania włókna z roślin włóknistych (len, konopie). Włókna pozyskiwane z tych roślin określa się mianem włókien łykowych.
https://pl.wikipedia.org/wiki/Roszenie_słomy_lnianej

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rettery
roszarnia { noun feminine }
a place or establishment where flax is retted.
https://glosbe.com/en/pl/rettery

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Note added at 16 mins (2018-05-26 14:24:53 GMT)
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Definition of rettery
plural -es
: a place or establishment where flax is retted
https://www.merriam-webster.com/dictionary/rettery
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Retting is a process employing the action of micro-organisms and moisture on plants to dissolve or rot away much of the cellular tissues and pectins surrounding bast-fibre bundles, and so facilitating separation of the fibre from the stem
https://en.wikipedia.org/wiki/Retting