Pyrex is a brand, not a year. Corning Glass Works introduced it in 1915, and the Corning Museum of Glass still treats that as the start: a trademark Corning owns, used under license by the companies that have made the housewares since. The museum’s FAQ is the short version, at pyrex.cmog.org/faq. The library’s answer on letter case and formulas is the careful version, at libanswers.cmog.org. This note is how to read the bottom of the dish in your hand. It is not a second copy of how to tell vintage glassware apart. Carnival, uranium, Depression glass, milk glass, and lead crystal are that article. Pyrex can sit in a cabinet next to them and still be a different question. The question here is the stamp.
The pattern pages already exist, and they stay the objects. Butterprint is the farm print. Gooseberry is the berry. Primary colors are the solid mixing bowls. The measuring cup is the clear graduated cup. Flameware is the clear rangetop percolator. A butter dish is the covered rectangle, not the farm bowl. If your piece is one of those, open that guide after you have read the number. This page will not reprint four bowl essays in order to sneak in a fifth pattern with the adjective swapped.
What did Corning actually put on the glass?
Early clear Pyrex can carry the word PYREX and a reference to the patent. The museum holds the paperwork for patent 1,304,623, presented with the date May 27, 1919, from an application filed in 1915. The page is the patent note. A dish that says PATENTED MAY 27, 1919 is citing that patent. It is not confessing that it left the factory on that Tuesday. Treat the line as a citation. The manufacture is “after the patent, and marked this way,” which is a range, not a birthday. That is still useful. A current baking dish does not carry a 1919 patent line. An old clear loaf pan might. Read it and write it down. Do not add a year the glass does not contain.
Opal ware, the opaque white and colored kitchen glass, is later than 1915. The museum’s FAQ ties consumer opal dishes to the Charleroi plant beginning in the 1940s, the same town where Corelle Brands has gone on making Pyrex bakeware. A colored mixing bowl is not a 1915 laboratory dish. Say the form you have. A clear pie plate and a blue mixing bowl do not share a paragraph, even though both say Pyrex.
In 1998 Corning sold the consumer-products division. The library’s account, which is the one to read before you repeat a corporate legend, says that division went to Borden and then World Kitchen, that World Kitchen later used the Corelle Brands name, and that the trademark stayed Corning’s to license. A dish marked only with a later company’s language is not a 1940s bowl because the pattern looks familiar. Patterns were made for a long time, and some were brought back. The words on the bottom are the record. The color of the print is not a resume.
Does the capital letter tell you the formula?
People want a one-glance test. All capitals means the old borosilicate, lowercase means the new soda-lime, and the test fits in a caption. The museum’s library says that is not true. The change from upper case to lower case, in the research they cite, was a rebrand in the late 1970s. It is not a conclusive way to know the formula. The same answer says Corning used many glass compositions under the brand, including more than one borosilicate and tempered soda-lime, and that plants did not all switch on one Monday.
The museum’s FAQ is plainer about the bakeware you buy now in the United States: it was borosilicate in 1915, and it has been heat-strengthened soda-lime for several decades, made in Charleroi. European Pyrex, under a license to a French company, is a different product and can be a different glass. A dish that says France, or a lowercase mark from that licensee, is not a U.S. Charleroi dish. It is also not “fake Pyrex” by being lowercase. It is a different line. Read the country. Photograph it. Do not run a chemistry lesson off a font.
If you need the formula for a safety question, a caption will not give it to you. Thermal shock is the practical version: a dish that says it is not for the stovetop is telling you not to put it on the stovetop. Flameware is the clear percolator that was made for a burner, and it is marked Flameware. A blue mixing bowl is not Flameware because both are Pyrex. The kitchen note, thrift kitchenware, already separates the bowl from the pot. Believe the warning molded in the glass over a rule you saw in a comment.
Which words on the bottom are a sequence you can defend?
You can defend the words, in order of what they say, without a chart of unofficial years.
A bottom that says PYREX, a trademark line, and little else is an earlier kind of mark than a bottom that adds a model number and a capacity. The number is the shape. On the clear liquid measuring cups, 508, 516, and 532 are the sizes that guide already explains. Extra small digits beside the model are mold marks. The measuring-cup page says not to promote them into a date. The same warning applies to a bowl. A model number that you cannot read is a dish you have not identified. Turn it until the digits are sharp. A thumb in the frame is not a number.
MADE IN U.S.A. is a country claim. “by Corning” or a later company name is a different claim. Write the one you see. A microwave line, or a line that says the piece is for baking and microwave and not for the broiler or the stovetop, is a warning from a kitchen that had those appliances in the instructions. It is later wording than a bottom that only says ovenware. You do not need a secret chart to see that. You need to read.
A metric capacity next to the cups or quarts is a later bottom than a piece marked only in U.S. measure. Collectors sort on that. The thing you can prove is the words. If both a pint and a milliliter are molded, say so. If only the pint is there, say that. Do not invent the year the metric line began in order to sound precise. Precision you cannot show is a false listing.
Some pieces carry a row of dots or dashes. The museum’s library, citing a Corning product-liability specialist, says Corning used a system of dots and dashes as a manufacturing code for the glass, the plant, and the time. This article will not publish a decoder. If you see the dots, photograph them in focus. If you have not looked the code up from a source you can check twice, leave the month out of the title. An unidentified code is honest. A guessed month is a return.
How do you date a pattern without guessing a catalog year?
The print is the pattern. The number is the size. A white bowl with a blue farm scene is Butterprint if that is the print, and the molded number says which bowl in the nest. A faded print is a faded print. It is condition. It does not move the bowl into a rarer pattern, and it does not make the number unreadable if the number is still in the glass. Dishwashers did a lot of the fading. You cannot unfade it.
A lid has its own number, often on the rim or the knob side, and it has to match the dish. A clear lid on an opal casserole can be the right lid. A lid from a different diameter is a lid you happen to own. Say which. The Gooseberry guide is the berry and the lid. Do not force a pink floral you do not recognize into Gooseberry because pink is the only color you remember. Pink Daisy is a different print. If you cannot name it, describe it and show it. Buyers who know the print will know. Buyers who do not should not be told a name you hope is close.
The butter dish is the worked example of a form that is not a pattern. The rectangular covered dish is marked 72-B on the base when it is that dish. The cover is often unmarked. Parts lists call the cover 72-C. A Butterprint 72-B, a clear 72-B, and a 72-B in some other opal print are three searches that share a shape. The butter-dish guide is those searches. Dating it is still the bottom: 72-B, the trademark line, the country, and the print if there is a print. A mixing bowl with a farm scene does not become a butter dish because both are Pyrex and both are “butter” in somebody’s title.
Primary-color bowls are solid color, no print to fade. A later yellow bowl with a modern backstamp is not a 1940s primary because it is yellow. The primary guide says that. Believe it. Color is the object only after the stamp agrees.
What should you refuse to put in the title?
A year you took from a blog chart you cannot reconcile with the words on this dish. A formula you took from the capital letters. A pattern name that is a neighbor of the print rather than the print. “Rare” because the shop only had one. A price, which this page does not have and will not invent. Sold listings of that model number, that pattern, and that condition are the price. How to read sold comps is the method. A complete nested set is a different object from one bowl. Chips at the rim are the usual damage. Show them before you talk about a date.
Corning Ware is not this article either. The blue cornflower and Spice of Life are ceramic, with their own model codes, on their own guides. A white square dish with a flower is not Pyrex because it came out of the same cabinet. Read the bottom. If it says Corning Ware, you are on a different page. The kitchen roundup points at both.
What can a photo settle?
A photo can settle the entire bottom, every word, the model number, the capacity, the patent line if it is there, the dots if they are there, the print, and the chip. It cannot turn a letter case into a laboratory result. It cannot apply a dot code this article refused to invent. It cannot see a crack under the handle you did not turn.
The Worth app can name a familiar pattern from a clear photo and can read a sharp backstamp. It should be shown the bottom, not only the pretty side. It will not certify borosilicate, and it will not invent a year the mold does not carry. The museum’s two pages are the history. The number in the glass is the dish.