▁half ▁for ▁mo ist . ▁Pat ents ▁issued ▁to ▁Mary ▁Ev ard ▁are ▁US 7 6 3 1 5 ▁and ▁US 7 6 3 1 4 ▁on ▁April ▁ 7 , ▁ 1 8 6 8 . ▁She ▁demonstrated ▁this ▁st ove ▁with ▁her ▁husband ▁at ▁the ▁St . ▁Louis ▁World ' s ▁Fair . ▁ ▁In ▁ 1 8 6 7 ▁Elizabeth ▁Haw ks ▁of ▁New ▁York ▁invent ed ▁and ▁received ▁a ▁pat ent ▁for ▁a ▁b aking ▁attachment ▁for ▁st ov es , ▁intended ▁to ▁spread ▁heat ▁thoroughly ▁throughout ▁lo aves ▁while ▁keeping ▁the ▁top ▁cr ust ▁tender , ▁which ▁she ▁called ▁an ▁" A ux ili ary ▁Air - ch am ber ▁for ▁Sto ves ." ▁ ▁This ▁was ▁so ▁successful ▁that ▁she ▁sold ▁two ▁thousand ▁within ▁months ▁of ▁its ▁release . ▁ ▁Sto ves ▁of ▁that ▁era ▁commonly ▁burn ed ▁char co al ▁as ▁well ▁as ▁wood . ▁These ▁st ov es ▁had ▁flat ▁to ps ▁and ▁the ▁heat ▁was ▁concentr ated ▁on ▁one ▁side ▁of ▁the ▁st ove ▁top ▁so ▁that ▁cook s ▁could ▁cook ▁things ▁at ▁different ▁temper atures ▁based ▁on ▁where ▁the ▁pot ▁or ▁pan ▁was ▁located . ▁This ▁was ▁called ▁the ▁" p iano " ▁system . ▁After ▁coal ▁was ▁replaced ▁with ▁gas , ▁French ▁che fs ▁continued ▁to ▁prefer ▁the ▁smooth ▁cook ing ▁surface ▁and ▁so ▁the ▁majority ▁of ▁French ▁gas ▁st ov es ▁had ▁flat ▁metal ▁surfaces ▁over ▁the ▁gas ▁burn ers , ▁which ▁continues ▁to ▁be ▁known ▁as ▁the ▁" F rench