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Clay is porous
Those ugly, greasy stains are located in the clay below the surface, not in the glaze. How do those stains get into the clay? That shouldn’t happen if the glaze remains intact. But fine cracks in the glaze can develop; this is called crazing. Crazing occurs when the glaze shrinks at a slightly different rate than the clay beneath it. To some art pottery collectors crazing may be desirable, and some glazes are purposefully to craze. But that’s not true with dinnerware, where the potter’s goal is to create a barrier to keep grease and liquids out of the clay. Crazing is commonly found in Red Wing Potteries’ earliest dinnerware (the Gypsy Trail and Provincial lines), but is less common with later lines. The pottery’s early experiences apparently lead to a better match of glaze and clay.
Some collectors use bleach to clean pottery. Some pottery reference books recommend the use of bleach. I’ve never used bleach to clean pottery, but have seen the damage bleach can do. While visiting an antique store recently, a very nice Brittany teapot caught my eye. As I approached the teapot, fine white powdery lines across the entire surface became evident. Closer inspection showed the powder was oozing from crazing in the teapot’s glaze. A quick sniff revealed the telltale odor of bleach. While the teapot was still lovely at that point, without action it would eventually become a pile of dust. It may have already been too late.
Other collectors use hydrogen peroxide, and I’ve successfully used this method on pieces in our personal collection. I’ll describe my method, along with some cautions, a bit later. I am not aware of any piece damaged by cleaning with hydrogen peroxide, thus it’s my opinion that hydrogen peroxide is significantly safer than bleach for this purpose.
Whitening vs. Cleaning
Logic would say bleach and peroxide make pottery appear cleaner because they are whitening agents. While whitening no doubt helps, my experience shows that peroxide also forces grease out of the clay and thus truly cleans the pottery. How do I know? After soaking a grease-stained piece in my peroxide bath, greasy scum floats to the surface. Because I have no experience with bleach I cannot vouch for it as a cleaning agent.
Basic science: Get the bleach out
All matter is composed of molecules, and individual molecules want as much elbow room as possible. Whenever possible, molecules will flow to an area of lower concentration. Think of a group of people in a small space, such as an elevator. Rather than crowd into a corner, people tend to disperse themselves evenly as much as possible within the available space. That’s exactly what molecules do too.
As a piece of pottery sets in a bath of cleaning agent (bleach or peroxide), molecules of the agent will try to enter the clay because density of the agent is lower there than in the bath. If the glaze is not intact, the cleaning agent will successfully penetrate into the clay. The clay will absorb more and more of the agent, until the concentration of agent is the same both inside and outside the glaze. At this point the clay is saturated with the cleaning agent.
If the pottery is removed from the bath and dried without first extracting the absorbed cleaning agent, the agent will remain in the clay. And if that agent chemically reacts with the clay (and apparently bleach does), damage will occur over time. Thus it is vital that the cleaning agent, especially if it is bleach, be extracted before drying.
How to extract the agent? Simply soak the pottery in clean water. The same chemical process that allows clay to absorb the agent also works in reverse. Agent-saturated clay will release the agent and absorb water in an attempt to maintain equal concentrations on both sides of the glaze. Eventually water will replace nearly all the agent in the clay. The piece can then be safely dried because water does not react harmfully with clay.
How long should the piece soak in clean water? One rule of thumb is to soak the piece in water at last one day for every day the piece was soaked in bleach or peroxide. Thus pottery soaked for two weeks in agent should be soaked at least two weeks in water. You may want to change the water occasionally. Doing so will eliminate any agent already expelled by the clay, and since the fresh water contains no agent it enhances the ability of the clay to further expel the cleaning agent.
It’s my belief that most damage to pottery from bleach occurs because the owner skipped or shortened the water soak step that removes the bleach from the piece. Can an item be saved if it has evidence (odor, powder) that it was bleached? Perhaps. Soaking the piece in clean water would draw bleach out from the clay, as described above. But if deterioration has progressed too far, soaking in water may cause the piece to break apart. But that would likely happen anyway with time, so it may be worth the risk.
Here is the process I follow using hydrogen peroxide. Please understand this is not a recommendation to others, I am merely sharing my experiences. While I’ve encountered no damage to pottery with this method, any use of hydrogen peroxide is at your own risk.
Hydrogen peroxide is available at beauty supply stores labeled as 20, 30 or 40 volume as either a clear solution or an opaque liquid. Volume is a term that means “percent”, so these products are 20, 30 or 40% hydrogen peroxide. Strong stuff! The peroxide used to treat cuts and wounds is only 3%; it will not work well for cleaning pottery. I use the 40 volume clear solution because it works quicker and lasts longer, but 20 or 30 should also work. I’ve never used the opaque liquid form and can’t see any advantage to it.
Hydrogen peroxide in these high strengths must be used with extreme caution. From my experience it stings badly on skin, eats a hole in clothing if not promptly washed out, and causes blistering on Formica tabletops if not wiped up. Use rubber gloves when handling the peroxide, unless you don’t mind the sting. I store my peroxide bath and supplies in an unused plastic shower, so any messes can be quickly and easily cleaned up. I use a covered plastic Rubbermaid tub to hold my peroxide bath. The container size needed depends on the size of the item to be cleaned. Of course a bigger container will require more peroxide. The peroxide can be reused numerous times. I’ve used my current batch of peroxide for nearly a year. Over time the peroxide weakens and the extracted soil and grease becomes a problem. I’ve tried to filter off the grease but with limited success. Eventually it is necessary to discard to weakened, dirty peroxide and start over with fresh solution.
How long should an item be soaked in peroxide? It depends on the piece. Some pieces clean up in just a few days; others can soak for a couple of months and still not be clean. Generally a couple of weeks are sufficient, but I use my judgment to decide if a longer soak is needed. Another plastic tub is used for a water bath soak after the peroxide cleaning. As previously stated, I soak the piece in clean water for at least as long as the item soaked in peroxide.
I have used peroxide to clean stains from dinnerware, but not art pottery or stoneware. Gypsy Trail and Provincial (Orleans, Brittany, Ardennes and Normandy) clean up quite well, but I’ve had little success with later dinnerware lines. This is likely due to the crazing issue described earlier.
Opinions vary on whether or not cleaning pottery in this manner is ethical. Some believe this process merely deep cleans the pottery. Others believe it is a form of restoration similar to repairing cracks or chips. In my opinion, if the only benefit from treatment were whitening of the clay then I’d feel this is a form of restoration. But since the treatment (at least with peroxide) actually removes grease from the piece, I believe this is a cleaning process rather than restoration. In any event, collectors would be well advised to clean only pieces that will be retained in their collection. Pottery to be resold should be left “as is”; let the buyer make the decision. Your ethics and reputation may be called into question if the buyer learns the piece has been cleaned with bleach or peroxide.
As stated earlier, I would like to gather input from other collectors on this issue. I plan to compile that input along with my research in a future newsletter article. Your name will not be mentioned in the article unless you specifically ask me to do so. Please send your thoughts and experience on this subject to the RWCS executive director at firstname.lastname@example.org with subject: Larry Roschen, cleaning dinnerware.
Dinnerware, Ask the Expert