Once upon a time, in the days after World War I–when it became socially acceptable for women to take up once-taboo practices like smoking and wearing makeup– the powder compact was an ornate feminine necessity made to be flaunted in public so as to inspire admiration in men and envy in other women. In fact, powder compacts were so essential that, even in the lean years of World War II, the shoulder bags of New York policewomen were built to accommodate cosmetics as well as the service revolver. 
Although cosmetics had been used since ancient times, the powder compact did not arrive on the scene until the beginning of the 20th Century. Evolving from small boxes of loose face powder, compacts satisfied the need for portability as society became literally more mobile and women began to enter the workforce in ever greater numbers. Influenced by movie stars and fashion plates, everyone wanted to “look their best” at all times, and that meant being able to quickly and easily freshen their makeup several times a day. Compacts helped to serve that purpose.
By the 1920’s and 30’s, compacts were offered in such variety that they could speak to the taste of almost any individual. Art Deco designs were particularly popular, as were exotic images based upon Egyptian motifs. Bakelite, shell, silver, and gold were just a few of the materials used in the manufacture of the cases.
Even World War II could not curtail the compact, though most metals were necessarily replaced with materials like wood and plastic. It was not until the Sixties, with its emphasis on the “natural” look, that the compact was demoted to a mostly utilitarian object. 
Collectors Items: According to Wikipedia, compacts prior to the 1960s are vintage and are very desirable as a collector’s item. Two of the top names in the compact business are Kigu’s ‘Bolero’ series and Stratton’s ‘Waterfowl’ series. Owning each piece of a particular series certainly adds to the value of your collection. In 1997, Stratton was taken over by Cork International. Although they are still making compacts with the Stratton name, they are no longer manufactured in Britain [and the quality is not the same]. If your compact has the famous “Compact in Hand” logo on the inner lid, it was made from 1950 to 1970. The “Compact in Hand” mark indicates the compact has a self-opening inner lid. This was patented in 1948 and is unique to Stratton compacts. It helped prevent damage to fingernails. 
Many collectors seek out a certain period or style, such as enameled Art Deco vanity cases with finger rings and lipsticks or World War II sweetheart sets and compacts. 
Replacing missing or broken mirrors: This is a tricky subject. If you have a very valuable antique compact with a broken mirror, I would advise leaving it alone. You could devalue your compact with a replacement mirror.
The mirrors in vintage compacts were much thinner glass than is available today. Trying to put in a modern mirror can distort your compact case or break the hinge. That leaves you with the only two options I can think of: pirate a mirror from an old unusable compact of the same size and shape or find a reputable glass dealer with a supply of vintage mirror on hand. 
There’s even a British Compact Collectors Society, which according to their site “has in excess of 200 members, several from overseas and growing all the time. Established in 1995 by Juliette Edwards as a result of interest in her privately-published book on British compacts, it is chaired by Juliette with the assistance of an enthusiastic executive team. Juliette is also the author of a book entitled “Powder Compacts – A Collectors Guide”, published by Miller’s in 2000.
Over at AlleyKatsStore.com, Amanda Lee says that you can even make your own pressed powder. “To begin, mix your loose powder with alcohol in a small dish, and form a soft paste. Then, press this paste into the bottom of your compact, taking care to fill the pan as evenly as possible. You can use a textured paper or mesh to press a design into the top, if you so desire. Allow the powder to dry thoroughly, and that’s it—you have created your own pressed powder!”
UPDATE : After I made this post, Jane Johnson over at Compact Diaries left a comment disputing the statement that Stratton compacts aren’t of the same quality today. Here is what she had to say:
“What a lovely, well-researched article! Thank you for crediting my photos My only disagreement with your post is your aside about Stratton’s current quality. Cork International no longer owns the brand. The current owners have worked hard for the last five years to return Stratton designs to their former glory. Although Stratton can’t manufacture all elements of their cases completely in Britain (there just aren’t the factories able to produce the metal work required), many of the lids are made using traditional techniques in the UK. Every Stratton compact is hand-assembled and finished in Birmingham. I don’t work for Stratton myself, but am an avid collector and stock their current ranges on my website (www.vanroe.com).” Visit Jane also on WordPress at My Powder Compact Diaries.
- Compact Collectibles: The Pow(d)er of the Puff 
- Collector’s Corner:Powder Compacts 
- What to Know About Collecting Compacts 
- Miller’s: Powder Compacts: A Collector’s Guide
- British Compact Collectors Society
- My Powder Compact Diaries (wordpress)
- Including Vintage Glamour in Everyday Life
- Vintage Compact (etsy)
- Vintage Floral Compact (auntjudy’sattic)
- Vintage Gilt Compact
- Vintage Mesh Bottom, Enameled Powder Compact
- 14 Vintage Compacts You’ve Never Seen (bellasugar.com)
- Do You Carry a Compact? (bellasugar.com)