SHREWSBURY, Pa. — Come upon someone holding a brightly colored mixing bowl up to the daylight, while browsing a yard sale or flea market, and you might have just spotted a Pyrex collector.

The iconic, colorful glass bowls and cookware, so familiar in the kitchens of our moms and grandmothers, have become “hot” in the vintage houseware markets.

“First thing I do if I find a piece of Pyrex is hold it up to the light and look for scratches,” said Patrick Stachelek, a 25-year collector of the durable, Corning Glass Works-manufactured kitchenware.

When his collection grew so large that he had more sets of nested mixing bowls, casseroles and lidded refrigerator storage sets than he needed, he began selling them.

Today, Patrick J. — the name under which he markets items through his Back Porch Antiques and Collections business — offers his finds at two York County, Pennsylvania, vintage sales sites: Serendipity Antiques in Shrewsbury and Mad Moon Marketplace in Red Lion.

“How-to” tips on hunting for the Pyrex pieces recommend checking the embossed backstamp on each piece. With a bit of research in Pyrex-related publications and the many internet sites available, the information included on the backstamps is helpful in identifying the approximate age of a piece. New Pyrex patterns and colors were introduced regularly, and can also help in determining the approximate age.

“Condition is what it’s all about,” said Stachelek.

The brightness of color on the pieces also weighs heavily in determining their value and collectibility.

“Early dishwashing detergents were very hard on the colored finishes,” Stachelek said. “Don’t put Pyrex in the dishwasher. Wash it by hand,” he advises, even today.

Naturally, the brighter and newer-appearing the color and finish of Pyrex, the more desirability — and value — a piece is likely to have.

In addition to eyeballing each piece by looking for scratches and wear, Stachelek checks for chips and nicks in the glass finish. Experience has taught him that one of the most likely places for Pyrex to be damaged is on the underside of the handles, or rims, molded into many of the pieces.

While relatively few people were stalking Pyrex pieces when Stachelek began his quest for the cookware, interest in the vintage pieces has soared in recent years. And, since millennials fondly remember Pyrex being in their grandmothers’ kitchens, their desire to acquire the cookware has risen and so have prices.

Pyrex pieces in lower demand may sell for under $10, depending on size, style, color, pattern and condition. Complete sets — like the nesting “primary color” mixing bowls in four sizes that were hugely popular in the 1950s and 1960s — may command considerably more, especially if they are in good condition. Some of the more rare and unusual Pyrex pieces can bring prices into the hundreds of dollars.

Lidded sets of oblong Pyrex “leftovers” refrigerator storage pieces, especially if the lids are still intact, are much in demand as well.

“Cinderella” bowls, with a lip, or handle, on each side for ease of pouring, are also popular with today’s vintage collectors.

As with many items offered at auctions and online bidding sites, demand by multiple potential buyers can sometimes nudge prices up higher than what might be ticketed on a piece displayed in a shop.

Pyrex, branded by Corning Glass Works when it bought the process in 1915, was first manufactured as clear, see-through material, enabling cooks to observe the cooking of the contents. Colored, vintage bowls, in high demand today, were not introduced until 1947.

Stachelek credits media-homemaker-maven Martha Stewart with having spurred considerable interest in Pyrex, while sharing with fans her favorite patterns and uses for the cookware. With more than 150 patterns introduced over the years, in a variety of bright colors, Pyrex became a midcentury branded name.

Standard patterns were produced for at least two years, while “promotional” patterns ran for limited times, creating more demand among today’s serious collectors.

“Butterprint,” a pattern rolled out in 1957, is popular with current collectors. It features a stylized plain farmer and his wife, along with depictions of farm crops, like grain sheaves. Turquois and black patterns were introduced in mid-century, when both those colors were popular kitchen décor. Pyrex with a matte finish was manufactured for a period of time, but the surfaces’ colored finish scratched quite easily, with many remaining pieces reflecting that damage.

Due to its popularity, Pyrex has become more pricey and less readily available to today’s collectors. With searching, and maybe a bit of luck, it can still be found at estate sales, flea markets and thrift stores, as well as on numerous internet sales sites. Retail shops specializing in antique, retro, vintage and midcentury modern “memory” merchandise also carry Pyrex.

Like many of the current Pyrex collectors, Stachelek’s interest in the colorful cookware had roots in his childhood memories.

“My mother had one of the nesting sets of primary color bowls,” he said. “I had a double shelf in my kitchen that mostly just held ‘junk,’ so I decided to get rid of that and collect some other Pyrex bowls to display there with her bowls.”

Searching thrift stores, flea markets, yard sales and similar second-hand marketing sites, he began amassing his extensive collection.

At that time, he recalls, there was not a lot of information available about Pyrex, and he found only one book that had been published about it. Greatly expanded collector publications and myriad internet sites have since made historical Pyrex information more readily available.

“It was fun to search for and find (Pyrex),” Stachelek said. “Most of the sets that I had were put together, piece by piece. I tried to buy only the good quality ones and put sets together. I had most of the early ones at one point, and had accumulated a lot that I didn’t need,” he said of his entry into marketing his collections.

Joyce Bupp is a freelance writer in York County, Pennsylvania.