This investigation began with the discovery & purchase of the above covered dish (it might be a soup tureen?). When I saw it at the Salvation Army I couldn't tell how old it was... it has a 'modern' 1950s shape but it doesn't have any crazing or a worn bottom. The confetti glaze was what excited me-- it reminded me of the 'funfetti' cake mix of my childhood.
It's marked Midwinter on the bottom so after some research I discovered that this piece is newer, from 1982 and is one of the last pieces that the company produced before closing in the '80s. The shape of the piece is referred to as 'Style' and I believe the pattern is 'Carnival.'
The Midwinter company was in business from 1910-1987 and produced its most innovative pieces in the 1950s through the 1970s. Prior to WWII they manufactured rather fussy traditional chinaware but after looking to California pottery designers like Eva Zeisel and Russell Wright, Midwinter streamlined their shapes and produced colourful and abstract patterns that better reflected the new home interiors of the 1950s.
While looking through some of the Midwinter pottery pages I realized that two cups & saucers that I'd purchased a year ago were also from the company-- they're older, the pattern is "Broadway" and they're from the 1960s.
Midwinter coffee & tea pots via flickr user A30yoyo.
1950s Midwinter china, photo via A30yoyo.
Tea set in the Mexicana pattern, from the 1960s. Image via wikipedia.
An extensive set in the English Garden pattern... I think this is from the early 1970s. Image via wikipedia.
An interesting article about the history of the company: "Contemporary Midwinter Pottery"
A fairly comprehensive guide to the Midwinter patterns spanning several decades: Retroselect