There are two schools of thought when it comes to cleaning antique pottery like the piece above, those who prefer to leave it as found and those who prefer to have a piece that looks almost new.
Many collectors like the look of stained majolica. For them, the staining is part of the piece's history and shouldn't be altered. On the other hand, those who favor a piece that has been cleaned are rewarded with richer, more vibrant color and a closer approximation of the maker's actual intention in designing the piece. As a dealer, I found it was much easier to sell cleaned majolica than stained majolica so I've always been in the "clean majolica" camp.
Majolica is made from a soft, porous, earthenware body that is easily damaged and easily stained. It's very difficult to find a piece new to the market that doesn't have some degree of staining in it. Fat and sugar seem to be the biggest culprits when it comes to ware that was originally meant for use in dining.
I can't tell you how many hundreds of times I've come across sugar bowls that are virtually black in color while their lids are pristine in color. The same condition can be frequently seen in teapots. In both cases the stain has absorbed into the base container while the lid, since it has no real contact with the contents are usually free of discoloration.
Butter pats are notorious for the bad condition in which they are often found. Creamers and platters are also frequently discolored by staining from fats.
The darker the glaze color, the less of an effect the staining has on the majolica but I have found that all of them can benefit from cleaning.
Cleaning majolica is actually a very simple process that doesn't hurt the piece at all if it is done correctly. This is the process I use. Proceed at your own risk! (disclaimer over)
Before deciding if you want to clean a piece of stained majolica, you need to be certain that you are dealing with a piece that has no repair. Cleaning will remove any repair so if you suspect repair you're best to leave the piece alone or take it to a professional who will fix the repair after the cleaning.
If you are certain you want to proceed with the cleaning you need only six things: an oven, a cookie sheet, a covered, non-metallic container for the vessel or plate to be cleaned, surgical gloves to protect your hands, goggles to protect your eyes and hydrogen peroxide.
Over the years I have used many strengths of hydrogen peroxide to clean majolica. My favorite was a 115 volume clear peroxide I bought in bulk from a beauty supply house, but I have also used strengths of 100 volume, 80 volume and 20 volume that I have bought over the counter at beauty suppliers. The higher the volume, the quicker the cleaner will work but they all work basically the same way. You just need to be sure you are buying the clear peroxide and not the creme peroxide.
Peroxide is a dangerous chemical bleach so when you're using it be sure to use the greatest caution. Avoid contact with your skin because it BURNS, and always wear gloves when you are handling peroxide. It is best practice to also wear goggles when handling it, in case it should splash.
With gloves and goggles on, place the piece to be cleaned inside a container in which the piece can be completely submerged. For plates and platters I find plastic cake carriers turned upside down work very well. For pitchers, plastic beverage containers work well. What ever you use make sure that it has no leaks and a tight cover.
Pour enough of the peroxide into the container to cover the piece then close the container.
Now you leave the peroxide to do its work. In time the peroxide will seep into the pottery and dissolve the stain in the piece. It will also force the fat out of the pottery, but all this takes time. The longer you leave the piece to soak, the cleaner it gets. I usually leave it to soak one or two weeks for a mildly stained piece and a month or more for a more heavily stained piece.
After the soaking period has concluded, remove the piece from the peroxide (gloves on) and dry it lightly with a paper towel. Place the piece onto a cookie sheet (I find jelly roll pans work best) and put it into the center rack of your oven. Then you bake the piece at 175 degrees for 30 minutes. The baking forces the remaining stain out of the piece. I would suggest running your exhaust fan during the baking as the smell that results is most unpleasant.
Remove the piece from the oven and allow it to cool to room temperature.
When you remove the piece from the oven you will find that a lot of brown crud has formed on the surface of the piece. If the discharge is oily, wipe as much off as you can right away or it will partially be reabsorbed by the pottery.. Once the piece has cooled to room temperature, carefully wash it under running water. The brown crud will come right off.
That's it! If there is remaining stain you can repeat the process until it is completely clean. The peroxide can be used a number of times before it becomes depleted.
Many people are reluctant to bake the piece in the oven after soaking. You don't really need to do it to get a clean piece but I find that if you do not bake it a certain amount of the stain will slowly return over time.