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Past Articles from Glass on Metal

Overview of Metal Preparation
by Woodrow W. Carpenter
Volume 1, Number 1, January 1982

     Metal preparation, at least for this article, is defined as the treatment a metal surface receives just prior to the application of enamel.  Metal preparation can vary from practically none to quite elaborate procedures.  A review of literature reveals a wide variety of methods, all of which apparently worked satisfactorily with the particular set of conditions described.  Methods of preparation can vary because of many factors including, but not limited to materials used, desired final effect, or scale of operation.

     During the spinning or stamping of copper shapes, light oil is applied, making the operation easier and extending the life of the tooling.  Normally, this oil is removed prior to shipping.  If some remains on a relatively simple, flat or shallow shape, spread the oil as evenly as possible with you finger and dust some dry 80 mesh enamel over the oil.  Pop it into the furnace and watch it flame, smoke and perhaps a little soot fly around.  When the enamel is glossy, take it out.  Results may not be fantastic, but in most cases, will be quite respectable.  Transparent enamels may have a few bubbles, but many times opaque enamels look as good as when elaborate metal preparation is used.

     If the copper shape came without oil, everything is not lost - some can be applied.  Pine oil, lavender oil, fuel oil, 3&1 oil, turpentine, Vaseline, glycerin, baby oil - they all work surprisingly well.  Spread them on with your finger, or use a brush.  Sometimes better results are obtained if the dusted piece is inserted into the furnace and removed after the oil flashes.  When smoking ceases, return piece to furnace.  Repeat two or three times then complete the firing.  Admittedly, this untraditional approach will never become the most popular technique - there are too many shortcomings.  However, for classes of children, brushing some lavender or pine oil onto the copper shape then sifting on enamel may be quite adequate.

     Once water is introduced into the system, things change, because water and oil do not mix.  Also, to obtain the quality most enamelers rightfully desire, refinements are necessary.  Metal preparation may, but not necessarily, consist of four steps:

  1. Removal of oil and grease.

  2. Removal of tarnish and scale or oxide.

  3. Neutralizing.

  4. Drying.

     REMOVAL OF OIL AND GREASE.  Usually called cleaning.  There are three general methods:

  1. Organic solvents.

  2. Burning off.

  3. Chemical cleaning.

     Organic solvents are seldom used other than to dissolve any asphalt type resistant that may be present.  Many of the solvents are a fire hazard and also five off toxic fumes.  The thin film usually left on the surface of the metal is very detrimental.  Water will not wet this film, thus when enamel containing moisture is applied, it will crawl leaving bare areas.

     Burning off seems the ideal method for the art enameler.  Temperature of the furnace is not critical, because the flash point of oils and greases is well below enameling temperatures.  Length of time also is not critical.  However, if you plan to remove the fire scale later from copper, it is best not to prolong this time.  For convenience, a normal enameling temperature at 1450-1500F can be used.  When copper is heated, it turns dark within a few seconds, then almost immediately, if much oil is present, a small flash appears and an iridescence moves across the copper, indicating all organic matter has vaporized.  Remove the copper and allow to cool.  A very thin film of black oxide will be on the surface.

     In some situations, metal preparation can end at this point.  If loose scale is present, it can be brushed off with a clean paper towel.  Opaque and even transparent enamels can be successfully applied with most techniques.  Again, most enamelers want more control of background pattern when working with transparents, necessitating further metal preparation.

     If the burn off is extended for a longer time, a heavy layer of tightly adhering oxide forms.  The outer portion being black copper oxide (CuO) and the inner portion, at the metal-oxide interface, being red oxide (Cu2O).  As the layer of black oxide builds up, it prevents sufficient oxygen reaching the interface to form the higher oxide.

     Silver and gold do not combine directly with oxygen.  Therefore, no oxide is formed during burn off.  In fact, if an oxide was present, it decomposes when gold reaches 400F, or at 570F for silver.

     Aluminum forms a colorless protective film of oxide which remains colorless even up to its melting point.  Since aluminum melts at about 1220F, burn off temperature should be no higher than 1100F.  Heating at 1000-1100F for five minutes is definitely all that is required to prepare 3002 aluminum for enameling.

     A simple way to determine if burn off has been sufficient, is to flood the metal surface with water.  If water drains off in a continuous layer, no grease or oil is present.  If the film breaks, beads up, or pulls to one side, the surface is greasy and burn off should be repeated.

     Chemical cleaning is a washing operation to remove oil and dirt from the surface of the metal.  Chemical cleaners consist of alkali and soap, or detergents along with wetting and buffering agents.  The alkali combines with soluble oils to form soaps, a process called sponification.  Insoluble oils are emulsified by the soap.  To be effective, these cleaning solutions must be operated at a boiling temperature.  When only light oils are involved, a hot solution of trisodium phosphate is usually sufficient although it is used quite extensively by industrial enamelers.  Chemical cleaning is not very practical for the crafts person.

     It is possible to scour the metal surface with household cleaners, such as Barkeepers Friend, then rinse with water to see of the surface retains a continuous film of water.  Cleaners containing chlorine must not be used on aluminum.  It causes the enamel to chip off edges.  Care must be taken that the rag or scouring pad used does not become contaminated with oil, merely spreading it from one area to another.  A simple and sure way is to burn off all oil.

     REMOVAL OF TARNISH AND SCALE.  Copper tarnish from normal room atmosphere is basic copper carbonate.  It is soluble in acids and decomposes when heated to 400F.  Silver tarnish is normally silver sulfide.  It is soluble in nitric and concentrated sulfuric.  It decomposes when heated to about 1515F.  In general, there are two methods used to remove tarnish and scale:

  1. Pickling.

  2. Mechanical.

     Pickling is an operation in which acids remove tarnish and oxide or scale from a metal surface.  Acid is not a cleaner, it does not react with oil and grease to form soluble products.  Oils merely float to the top of an acid solution, to be picked up on the metal surface when removed.  All grease and oil must be removed from the metal surface before putting it into the acid.

     Acids most often used are nitric, sulfuric, and hydrochloric.  Each acid has its own peculiar properties and they are not always interchangeable.  Metals react differently to acids.  Those above hydrogen in the electromotive force table are dissolved by all three acids forming metal salts and giving off hydrogen.  Metals below hydrogen will not dissolve in acids unless an oxidizing agent is present.  Copper, silver, platinum, and gold all fall below hydrogen.

     Nitric and concentrated sulfuric are oxidizing acids.  They first oxidize copper or silver then react with the oxide.  Dilute sulfuric and hydrochloric must have an oxidizing agent present to react with copper or silver.  Atmospheric oxygen can function as this oxidizing agent, but etching takes place very slowly.  Hydrogen peroxide or ammonia persulphate are suitable oxidizing agents.

     It is generally known that gold and platinum dissolve only in aqua regia, one part concentrated nitric with three parts concentrated hydrochloric.

     Nitric, hydrochloric and concentrated sulfuric are normally used at room temperature, because they give off noxious fumes when heated.  Dilute sulfuric can be heated to about 160F before fumes become too noxious.

     Concentrated sulfuric acid is a powerful dehydrating agent, not only absorbing free water, but it also withdraws the elements of water from organic matter.  When concentrated sulfuric acid is diluted with water, a great amount of heat is liberated.  Great care must, therefore, be exercised in diluting all acid.  Water should be placed in a Pyrex container and acid slowly added, stirring occasionally.  WATER MUST NOT BE ADDED TO ALL ACID.  This results in the sudden absorption of a large amount of heat by a small quantity of water, turning it into steam and producing a spattering of hot liquid.  Several years ago, the author of an enameling book stated the rule, "Never pour water into an acid," and most all books on enameling since, has repeated the rule.  This applies only to concentrated sulfuric.

     Nitric acid purchased from a laboratory supply house contains 69.0 - 71.0% nitric acid.  When chemists speak of dilute nitric, they usually mean the standard given in the "Handbook of Chemistry and Physics" published by CRC Press, Cleveland, Ohio.  This is 195ml of 69% nitric acid and 805ml of water, or approximately one part to four parts by volume.  This provides a strength of about 17.6% acid by weight.  Some enamelers use a weaker solution, others use a stronger solution up to one part of each, which provides a strength of 40% water.  Consequently, when developing new enamels at Ceramic Coating Company, we always use this strength to determine acid resistance.

     Hydrochloric purchased from a laboratory supply house contains 36.5 - 38.0% acid.  Standard dilute is 258ml of acid and 742 of water.  This provides a strength of about 10.5% by weight.  Industrial enamelers frequently use a little stronger solution, up to one part of each providing a strength of 20% acid by weight, normally used at room temperature.

     Sulfuric purchased from a laboratory supply house contains 95% acid.  Standard dilute is 168ml of acid and 832ml of water.  This provides a strength of about 25.7% acid by weight, which can be used at room temperature.  Commercial enamelers frequently use a strength of about 16% by weight, or one volume of acid to nine volumes of water, at room temperature.  Others use a hot solution, approximately 160F with a strength of 8% by weight, or one part of acid to 20 parts water, by volume.

Figure 1

  Copper Red Oxide Black Oxide Silver Sterling
Nitric Dil. .084 .454 1.44 .504 .108
Conc. 126.0 2.44 19.77 .792 .630
Hydrochloric Dil. .002 2.16 3.21 0 0
Conc. .004 9.20 13.19 0 0
Sulfuric Dil. Cold 0 .356 6.67 0 0
Hot .001 3.24 25.2 .005 0
Conc. Cold .001 .101 2.94 0 0
Hot .003 .774 2.91 0.36 .004
Sparex Cold N.T. .320 1.32 N.T. N.T.
Hot N.T. .565 2.89 N.T. N.T.
Salt & Vinegar Cold N.T. .284 .240 N.T. N.T.
Hot N.T. 2.19 1.20 N.T. N.T.
Bright Dip - .130 .733 .275 N.T. N.T.

Immersion Time - 5 minutes
Cold Solutions - 70F
Hot Solutions - 160F
N.T. - Not Tested

     Figure 1 shows the solubility of several materials in all three acids.  This table can be helpful in selecting pickle procedures.  For ordinary trays, plaques, etc., dilute nitric is suitable; it readily dissolves black copper oxide, slowly dissolves red copper oxide and copper to produce a fresh bright surface.  In situations where a heavy layer of red oxide must be removed, it would be more efficient to first dissolve the black oxide in dilute nitric, then into dilute hydrochloric to remove the red oxide.  Finally, a quick dip in dilute nitric to remove a little copper, thus exposing a bright new surface.

     Champleve and die stamped ware with delicate designs pickled in sizeable lots might lose some detail if pickled very long in nitric.  Thus, commercial enamelers normally use dilute hydrochloric or hot sulfuric which dissolves the oxides, followed by a quick dip in nitric.

     Copper can be removed from the surface of sterling silver by first heating to oxidize the copper, then pickle in hydrochloric or hot dilute sulfuric to dissolve the copper oxide without attack to the silver (depletion gilding).  To obtain the surface purity desired, this procedure may have to be repeated 8 to 10 times.

     Bright dip is a mixture of one part concentrated sulfuric and one part concentrated nitric with a trace of hydrochloric.  Many enamelers use this to improve the brilliancy of transparents.  Copper is first pickled in nitric, then given a quick dip of a few seconds in bright dip.  The copper surface will be brighter, a little more yellow in color, and appear to be smoother.

     Figures 2, 3 and 4 show a copper surface magnified 150 times.  Figure 2 was annealed at 1500F for 10 minutes to enlarge the grain size.  It was then pickled with dilute nitric and rinsed with water.  Some grains show up as white, others show as gray, because they are tilted slightly and reflect a different amount of light.  The dark areas are either grains tilted enough that the light is reflected outside the lens or trenches eaten by the acid causing a shadow.

     Nitric acid produces more distinct grain boundaries than either sulfuric or hydrochloric.  However, we see no indication that this has any effect on the brilliancy or color of transparent enamels.

     Figure 3 shows the same piece of copper, as shown in figure 2, after two seconds in a bright dip solution.  The grains have been evenly attacked and the grain boundaries are not very evident.  This is a much smoother surface than the one in figure 2.  It is surprising that bright dip initially attacks copper very fast and then immediately slows down.  Apparently, the metal surface becomes passive.

     Figure 4 shows a copper surface after five minutes in concentrated nitric acid.  The plate was hung by the top right hand corner.  Deep vertical grooves are quite evident, indicating over pickling.

     MISCELLANEOUS ACIDS.  Many people fear the use of acids and utilize substitutes which are less effective, but in many cases satisfactory.

     Vinegar and Salt.  A damp cloth with vinegar, daub into common salt and scour the metal surface.  Vinegar contains 4 to 8% acetic acid, which reacts with salt forming a weak hydrochloric acid.  This procedure is effective for small ware.  Much of its effectiveness is due to the mechanical action of scrubbing.  If considerable pickling is to be done in this manner, it is best to wear a rubber glove to prevent finger stains.

     Metal shapes may be submerged in a solution of vinegar and salt.  Use about 45 grams (1.6 oz.) of salt to one cup (8 fl. oz.) of vinegar.  For hydrochloric to be effective, strength should be about 12% and this strength cannot be obtained using 4 to 8% acetic acid.  Figure 1 shows the solubility of copper oxides in this solution.

     Sulfuric Acid Salts.  Sodium bisulfate, also known as sodium acid sulfate, or niter cake, a by-product from the manufacture of hydrochloric and nitric acids, is sometimes used.  It is a white crystalline powder.  Dissolve approximately 10 ounces in a quart of water.  This can be used cold, but it is much more effective if heated.  Figure 1 shows the solubility of copper oxides in this solution.

     Mechanical Methods.  Many things can be used to mechanically remove tarnish or oxide, i.e. emery cloth, abrasive stones, wire brush, slurry of water and scrap enamel, Bright Boy, and finally steel wool.  Great care must be taken when using steel wool to prevent small pieces from getting into the enamel and producing black wiggly designs where they are most unwelcome.

     Lea compound, a greaseless polishing agent, can be applied to a buffing wheel and used for both cleaning and removal of tarnish and scale from larger pieces.

     NEUTRALIZING & DRYING, should accomplish two things:

  1. Neutralize any acid remaining on the metal surface.

  2. Retard discoloration of the metal surface.

     Should acid be left on the metal surface, the enamel will blister and become discolored.  If the metal surface dries slowly, it will usually have a mottled pattern of discoloration.

     For simple shapes, the easiest way to neutralize the surface is to rinse it well with water.  The easiest way to prevent a mottled pattern of discoloration is to wipe it dry with a clean cloth or paper towel.  Enamel can be sifted directly after rinsing, thus eliminating the need to dry.  If some time is to elapse prior to coating all or any part of the surface, it is desirable to dry the metal surface.

     Many methods have been suggested in literature.  An early one was to apply saliva to the surface and distribute it with a clean finger, followed by rinsing again with water.  This would neutralize a small amount of acid.  This method appears to be limited to small pieces rather than for mass production.

     Moistening a clean cloth with soapless detergent and wiping the surface, followed by rinsing in water is one recommendation.  This should neutralize acids.  Any metal salts present should be picked up by the cloth.  Subsequent rinsing might be more thorough as the detergent would reduce the surface tension of the water.

     Some authors, starting with Cunynghame, recommend a weak solution of ammonia, followed by rinsing in water.  Again, this should be effective as a neutralizer.

     Scrubbing with pumice or plaster of paris has been suggested.  This would mechanically remove any metal salts left on the surface.  We suggest using scrap enamel, since it is usually on hand and works well.

     Some commercial enamelers use a weak, hot solution of trisodium phosphate.  As well as neutralizing, the warm ware dries more rapidly, reducing mottled discoloration.

     Others use a hot soda ash and borax solution.  Three pounds of soda ash and one pound of 10 mol. borax to one hundred gallons of water, usually at a temperature of 150-160F.

     Commercial enamelers who process several pieces as a batch, might wish to take into consideration that copper nitrate is more soluble in water than copper chloride.  Copper chloride is more soluble in water than copper sulfate.

     The mottled discoloration has no effect on opaque enamels.  However, it is usually evident after the first firing of transparents and perhaps even after several firings.  As mentioned earlier, simple pieces can be dried with a cloth, but some batch or complex pieces cannot be easily dried with this method.

     Drying can be accomplished by placing the piece into an oven at about 300F.  Blowing with hot air has been suggested, but care must be taken that the air contains no oil.

     Early literature recommends placing the ware immediately after rinsing into clean saw dust.  Some specified boxwood sawdust, others specified oak sawdust.  This works quite well.

     Some enamelers simply leave the ware submerged in water until time to enamel.  Then remove and dry it.

     Cunynghame and one of our European competitors, recommend that the metal be placed into the furnace for a short time until the metal begins to change color prior to applying transparents.  We doubt the logic of burning off the grease, pickling in sulfuric, going into bright dip, drying with a clean cloth and then putting it into the furnace to obtain a thin coat of oxide prior to applying transparents.

     In conclusion, there are many ways to prepare metal for enameling.  Simple methods are not necessarily bad, and complicated ones are not automatically good.  Select the one that fulfills your needs at the moment, even though it may differ from previous recommendations.   


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