Enamel placed outside gets covered with grimy deposit

25 september 2014 // Door webmaster //
Pat Johnson
Enamel placed outside gets covered with grimy deposit.

I have used a soft red enamel to cover nine inch copper squares. White cut out letters are stuck to the red enamel with silicone adhesive. These panels are mounted outside in Cornwall. A grimy deposit is now covering the red enamel, making it look blotchy. (Probably the deposit is on the white as well but the effect is only ugly on the red.)

It has been recommended to me that I fire a coat of flux on the red panels to prevent this happening. (Of course I would remove the white letters first.) I don’t think this would help because this environmental deposit seems to adhere to all kinds of enamel. I experienced this when I mounted an enamelled sculpture in the garden. Within a year it was covered with a scum. All the enamels I use are lead-bearing.

Does anyone agree that covering the red panels with flux will not help? Are their any suggestions for protecting the red so that the scum will not form?

Pat Johnson

Answer 1
Randy Anheiser

Hi! My name is Randy Anhaiser, and I will see if I can help to answer your question or point you in the correct direction… To begin, enamel is (quite simply) glass which is comprised of silica, sodium carbonate, calcium carbonate, lead oxide (since yours are leaded), and other metallic salts and oxides. In lieu of sodium carbonate and calcium carbonate, boric oxide and alumina. The silica is the main component, with sodium carbonate as a fluxing agent (it reduces the melting point from 1,700°C to 850°C) The lime (calcium carbonate) prevents the glass from being soluable in water. Lead oxide increases the refractive index of the glass (making it sparkle) and metallic salts produce colors and tin oxides are usually the opacifiers. Flux is, quite simply, glass without any metallic salts or oxides, but otherwise is basically the same as the enamel. Based on this, I would not think that the application of a layer of flux would have any effect as it would still get covered with stuff just as your red enamel does. If I may ask a couple of questions, I may be able to advise a solution that will resolve your problem, or at least one that will allow you to combat it.

  1. Do you stone your enamels to polish them, or do you leave them with the fire polish such as when they come straight out of the kiln? (stoning will produce microscopic cavities that can trap dirt, so the solution would be (stop if prior choice works) water and soft brush, detergents, soaking in bleach, other solvents, or as a last resort, further stoning to remove the contaminated layer.
  2. Did you fire your enamels to a fully molten vitreous state, or are they “underfired” such as in orange skin or sintered crust. These would produce small to microscopic cavities that would be succeptable to contamination as well, and the solution would be the same as above but without further stoning as an option.
  3. What does the surface of the enamel look like? Can the stain be scratched away with a finger nail or soft metal such as copper (which will not scratch enamel, because copper is softer than glass). Is there any cracking or crazing of the surface (minute surface cracks could allow water to invade the space between the metal and enamel, which is hopeless!

Just a further hunch. Are you in an area that has calcium deposits such as limestone or marble (does your stain look like a hard water stain)? This would certainly be a strong indication that some sort of carbonate had possibly precipitated on the surface. It is easy enough to remove if the tiles have a fire polish and are not damaged. A dilute solution of hydrochloric acid works wonders in removing this kind of stain (I have hard water, and when I get tired of scrubbing the tub, I sometimes use it. LOL, don’t tell my mom because she would worry, but it does nothing to the enameled tub. I would not use it on grout for a long time because it would remove it, fyi.

I hope this is helpful to you, and if I had more information with respect to the problem, I could probably provide some assistance or guidance. Good luck with your project, and please feel free to ask if you have additional questions.
Warmest regards,
Randy Anhaiser

Answer 2
Allan Heywood
The problem can’t be addressed until the exact nature of the “scum” is known. We must first determine if it is:
  • particulate matter of various sorts co-deposited from the atmosphere with rain, dew, fog etc.
  • etching of the enamel by this material in aqueous solution
  • a living organic deposit such as lichen or algae.


Answer 3
Lilyan Bachrach

Your message only asked -enamel placed outside…The Texaco signs are enamel..granted they are porcelain, opaque enamel on steel…if you place a plant in an enameled bowl, the inside of the bowl will become crutty.

Lilyan Bachrach

Answer 4
Jean Vormelker
Sounds like smog to me and that will keep coming back unless it is enclosed in a weatherproof box with a clear glass for viewing that can be cleaned regularly. I wouldn’t think that that would be a satisfactory answer. Have you tried glass cleaners? Or ammonia, detergent and water? Maybe the way to go would be to check out what commercial window cleaners use for glass. They have to deal with that kind of smugg all the time on high rises.
Cheers 🙂
Answer 5
Karen L. Cohen
I don’t see why a covering of flux would help so I agree with the enamelist that a covering of flux will not help. I would think that anything outside gets dirty, maybe scummy. When I lived in Brooklyn, NY, we were amazed how dirty the outside of our windows got – you mean we really breath that stuff! I say, just wash it every once in a while!
Answer 6
Ellen Goldman
Although I have only limited recent experience with leadbearing enamels, I doubt whether it makes much of a difference whether one covers it with a flux or not. According to the enamelist use was made of soft enamels. I suspect that the soft enamel has more to do with the problem than the lead, and the top enamel would also need to have a low melting point. Instead of applying a new flux I think that it would be better to try a new plaque with red enamel with a somewhat higher melting point. Preferably a lead-free one, of course; these are also available in the United Kingdom.
Answer 7
Lyle H. Morrow
If this situation applied to my work I would first try to determine whether the “scum” in question was organic (bacterial,algae, etc.) or inorganic (smog, acid rain, etc) in nature. If organic, there may be a coating that would prevent further growths from occuring. If inorganic and not removable, it would seem to be reactive to the enamel (highly unlikely, but not impossible. The manufacturer of the enamels would probably be the best source for advice and/or assistance. If organic and the cause can be determined, there may be chemical methods that would inhibit or prevent reformation of the “scum” via cleaning and coating. I hope these thoughts may assist you in some way. Also, contacting commercial enamelers may give you some help, since they also make panels for outside installation applications.
Answer 8
Jean F. Jenkins
I, too, have an enamel that must be out-of-doors, and this is a clean air area. I am having a waterproof box frame built for it, with an acrylic front. Although the enamel itself is as good as any glass, remember what has happened to some of the stained glass in industrial parts of Europe. Even enamel is not impervious to damage!
Answer 9
Jenny Gore

Is there a maintenance program for the panels? Does detergent remove the scum? Or has the surface actually been abraded? Soft red leaded enamel would probably be the most prone of all the colours to deterioration by the elements, and I doubt that covering it with flux at this stage will make it look much better. However I would give one piece a try. A better choice in the first instance would be an unleaded enamel that has a higher firing temperature, and a final coat of unleaded flux over that. This may darken the colour slightly so tests would need to be made.

Best of luck,

Answer 10
Cheryll Leo-Gwin
What I know first hand is this:
In my experiments with acids, acid rain, etc. on both jewelry type enamels and porcelain enamels, is that acid rain and environments will hurt porcelain enamels. Thompson enamels don’t stand up to acids as well as SOME Schauer enamels. Some Schauer enamels do very well but require buffing from time to time to remove mineral salts that build up on the surfaces over time. Even H20 won’t protect it over time. With regard to discoloration of the copper on the edges. It depends on how much of the copper is exposed to the air. It’s the oxygen that causes the discoloration. If the copper edges are submerged in water all of the time, it will not discolor… it’s the oxygen that causes the patination, not the water.
Answer 11
Coral Shaffer
Greetings. I read this question when you first put it out and also read the answers and don’t have anything to add. Red opaque enamels are often very soft and therefore more acid sensitive than some others. I would guess maybe “acid rain” or whatever is in our air these days did actually start eating into the surface. If that is the case, refiring and coating with flux might very well help. Sorry I can’t offer more. I haven’t ever had an outside installation. My experience has been with enamels in the “pickle” – reds often react very quickly making the once shiny surface dull and matt.
Answer 12
A Forton

Are you certain that the grime is specific to enamelled surfaces? I suggest to clean the surface and apply a fungicide solution. You may find that it will cure your problem.

A Forton

Answer 13
Mark Fenn
As an enamelist, I find this question interesting, but as a conservator I find I would need a lot more information before venturing a guess beyond agreeing that your correspondant is probably correct, a flux over the red will probably not solve the problem.
But in order to be more specific, I would need to know:

  • What is the nature of this “grimy deposit”- is it granular, powdery, knobby, a smooth continuous coating, a slimy layer?
    Answer: a smooth continuous coating but one that looks like it was applied with water, i.e. there are water drop sized areas of white deposit, closely packed next to each other.
    Is it in streaks? What color is it?
    Answer: Dull silvery.
    Does it dissolve in water? Can it be scraped off? If so, does it leave a smooth surface, or is it pitted?
    Answer: See above.
  • How is this “grimy deposit” like or unlike the “environmental deposit” and the “scum” mentioned further on? Is the deposit on the white as well?
    Answer: This, alas, I cannot tell as I re-enamelled the letters white and put them on new red panels.
    By the way, the metal underneath is copper. Is it on the silicone?
    Answer: Can’t tell as I no longer have the old silicon, which I scrapped off and threw away.
  • Where outdoors are the panels kept? Are they regularly exposed to rain, sunlight, snow, bird droppings, air pollution,
    Answer: Yes to all,
    runoff from a copper roof?
    Answer: Don’t know.
  • Is the red enamel “soft“ in the sense of low melting, or is this an unfired material (epoxy, paint, etc.?).
    Answer: Yes.
    Extremely soft vitreous enamel. The softest enamel I have ever used.
    Assuming it is an actual vitreous enamel, what brand is it?
    Answer: The first time round it was Thompson’s Flame Red, lead bearing. The second time Milton Bridge Post Office Red. The two enamels are the same colour and have the same melting point. I have samples of each and tested them, but did not have enamel Flame to use on the replacement panels.

Dear Mr. Fenn,

I have found your response most helpful. In fact, the problem may be solved. As I was writing an answer to you, I suddenly remembered that I had kept two of the original red tiles and immediately I found them and performed some tests.

Before I tell you about the tests, it would be helpful if I described the panels more fully. The National Trust in the United Kingdom maintains a number of historic houses around the country. One of these, built in the early 20th century, is in Cornwall. The style is Arts and Crafts. Originally there was a wrought iron arch over the entrance to the grounds, on which were mounted nine individual panels. the panels were 8″ square each, with a red enamel background and white enamelled cut out letter, about 7″ high, mounted on each. After many decades, during which the house came into the hands of the National Trust, it was decided to replace the enamelled letters and their backgrounds. That was 20 years ago and I made the new red background/white letters panels at that time. I don’t recall if I saw the original pieces. Over the ensuring period, a grey deposit settled down on the red enamel of the new panels.

Now, looking at the 20 years old red panels, you can see where the white letters (now removed by me) were placed. The enamel under the letter is bright and shiny. There are remains of silicone adhesive in these bright areas, enough to show me that I had not applied the adhesive over the entire back of the letter. In other words, the physical presence of the letter masked the enamel and kept it pristine.

All around the letter shape is a nearly opaque, almost opalescent coating (except near the edges, which were probably protected by the mounting devices). My tests involved trying to remove this grey coating firstly by scrubbing with a gritty kitchen cleanser, then by using paper and a dilute nitric acid solution, then paper and malt vinegar, then paper and fairly dilute ammonia solution. Paper and water alone didn’t seem to shift it. In all of the other four texts, the scum departed with a bit of effort, leaving a dull matt enamel surface exposed.

I then tried, in an area where the shiny enamel was next to the grey scum, to use just a plastic scouring pad and water. I scrubbed both the shiny enamel and the scum in the same sweeping motion. The scum departed and left the shiny enamel undamaged but exposed the same dull matt red that I had found revealed under the scum removed by the acids and the ammonia. I then repeated this test using the scratchy kitchen cleaner. This cleaner did damage the shiny red enamel a little bit, but nothing like the matt surface that appeared from under the scum.

It therefore seems to me, as a result of these last two tests in particular, that the scum is actually degrading the enamel surface and that a layer of flux might help preserve it, at least for a period of time. When I put my enamels in my dilute nitric acid solution, the soft colours immediately become matt, but the flux I use stays shiny longer, although it too will become matt if I leave the piece in the acid for, say, half an hour.

I have thought further about the deposit on the red enamel and wonder if it isn’t indeed a coating of lead. It looks metallic. Could acid rain have leached the lead out and then deposited it on the surface of the enamel?

Pat Johnson

Answer 14
Cullen Hackler (Porcelain Enamel Institute)
In reading the “question,” it seems as though there is some external environmental condition that is causing a “scum” to form on all porcelain/enamel (P/E) surfaces. Since the coating is glass, a glass cleaner may remove the stain. On the other hand, red P/E coatings, made with cadmium sulfo-selenide pigments, can weather and are often covered with a clear glass to “prevent” this weathering. There is an ASTM test C-538-83 to test the color retention of red, orange and yellow porcelain enamels. If the red is covered with a clear glass and the scum still forms then I would expect that the scum has nothing to do with the P/E and would form on any surface in the same environment.
Answer 15
Josy Trageser
Although I live in the US, it is in an area, the Ohio River Valley which is known for it’s pollution. I design enamel garden sculptures and can no longer tell my customers that enamels are impervious to the elements as I did fifteen years ago. After two years outside, the sculptures become etched by the acid in the air. Think about the dirt that collects on your windows and your car.
If you think that the problem comes from “without” the work (that is in the atmosphere) you might want to consider a good coat of car wax twice a year.
Pat Johnson

Today I did a series of tests of various fluxes to put over the Post Office Red. I tried a soft flux, (Latham 232), a hard flux (Soyer 2) and a lead free flux (Thompsons LF300). All three covered the red well, all in the same firing conditions – 910 degrees C and 1 min. All test tiles were 1″ x 2″.

I am going to propose to the client to mount these tests, plus one unprotected sample of PO Red, outside for a number of months and see which one does the best. Meanwhile I will suggest to them that they cover their PO Red tiles with a clear varnish. That should keep the acid rain away until the test tiles have had time to react to the atmosphere.

I can’t tell how much I appreciate everyone’s help.
Pat Johnson


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