Plain Opal Pyrex
Only a small selection of Pyrex was ever offered as unpainted, undecorated plain opal ware, or "true opal" as collectors like to call it.
Many pieces seen offered in the collectibles marketplace as plain opal ware are in fact not. They are mainly the result of having been repeatedly run through automatic dishwashers, in which the detergents, harsher relative to handwashing liquid dish soap, had the effect of dulling and, over time, removing the paint. If the removal was complete enough, no noticeable traces of the original finish might remain.
It is important, therefore, that collectors be able to discern between that which is "true opal" and that which has been "stripped".
Round Bowls and Refigerator Storage Dishes
In the mid-1950s, a set of four nesting round mixing bowls was produced. Labels call them simply "Heat Resistant Opal Bowl Set" with no mention of Pyrex, but do note they are a product of Corning Glass Works. They can be distinguished from regular production, painted 400 series bowls that have been stripped of their color by their lack of a backstamp. If marked at all, there will only be a mold number, often in a lightly-embossed circle the diameter of the typical backstamp.
Also offered in the mid to late 1950s were 500 series refrigerator storage dishes. They, too, can be identified by the lack of a backstamp.
Ovenware and Bakeware
In the late 1950s, three plain opal 023 1-1/2 quart round casserole-based promotionals were offered in successive years, 1956 to 1958. Each came with a different metal cradle. The first was provided with a knob top lid; the others with utility lids.
Other Plain Opal Ware
The 1063 1-1/2 quart divided serving dish was marketed as an inexpensive loss leader piece from 1959 to the late 1960s.
A plain opal 053 open baker was the base for the rarely-seen Fish Dish promo of the late 1950s or early 1960s.
Some third party products used plain opal 024 2 quart round casseroles and 575 Space Savers as bases for decorated utility lids.
Corning Housewares first half 1980 catalogs list but do not picture a selection of plain opal bakeware shapes, including a loaf pan (213-W), a round cake pan (221-W), and a pie plate (209-W). An 022 1 quart round casserole with utility lid (catalog #682-W) was added at this time as well. By their time frame, their backstamps should all include metric sizes. These items were absent from second half 1980 and later catalogs.