Opus incertum (irregular masterwork) was an ancient Roman construction technique, using irregular shaped and random placed uncut stones or fist-sized tuff blocks inserted in a core of Opus caementicium.
Initially it consisted of more careful placement of the coementa (rock fragments and small stones mixed with concrete), making the external surface as plain as possible. Later the external surface became further plain by reducing usage of concrete and choosing more regular small stones. When the use of concrete between stones is particularly reduced, it is defined opus (quasi) reticulatum.
Used from the beginning of the 2nd century BC until the mid-1st century BC, it was later largely superseded by Opus reticulatum.
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