News - Press Releases

Antiquing Of Brass, Copper & Bronze

By Jesse Vouk

Introduction

It's been said that "Beauty is in the eye of the beholder", and nowhere is this more true than in the antique hardware industry. Antique finishes have long been a popular option offered by manufacturers of decorative hardware of all types, including lighting fixtures, locks, cabinet hardware, fasteners and many other decorative items. Market demand for these finishes soared in the 1970's, leading to the development of efficient production scale processing techniques. Since that time, the demand has waxed and waned with the vagaries of the market. Regardless of the popular choices of the marketplace, there will always be a demand for high quality antique finished hardware.

This discussion will provide an overview of the various processes involved in the production of antique finishes. There are several different options available to the finisher at each step of the process and choices to be made, depending on relative efficiencies, operating cost, pollution implications, subsequent operations, etc. Consequently, the preferred process steps can vary widely from one plant to another. The process begins with the substrate.

Substrate Metals in Common Use

There are three types of materials in common use:

1. Solid brass, bronze or copper – Available in several forms, these materials are commonly employed in outdoor applications or high quality indoor applications. Since these alloys tend to form protective oxide films, they have come to be the materials of choice for items such as outdoor lighting fixtures, locks, marine hardware, building trim, statuary items, certain fasteners and other decorative items. This protective oxide film presents the finisher or designer with two options with respect to the finishing of the article: the piece can be given an "artificial" antique finish which is preserved by outdoor grade lacquers; or the article can be given an initial antique finish with only a temporary topcoat which allows the surface to age naturally in service. This naturally developed oxide, or patina, is often more attractive than those artificially produced and is preferable for certain items such as builders' hardware, statuary and some light fixtures.

There are several grades of these base metals employed in manufacturing the items described above. Solid grades of brass and copper sheet stock in varying alloys are used to manufacture light fixtures, fascia panels and roofing for building construction, stamped or drawn fasteners and other items which require malleability for ease of forming. Cast grades of brass and bronze are widely used for statuary, plaques or high quality hardware. Many of these items are further machined, polished or belt sanded to form the final outer surface which is then given an antique finish.

2. Steel stampings, spinnings or machined items – Steel is often used in applications requiring higher load-bearing capacity than the pure copper alloys can offer, such as fasteners or other structural members which must be able to support some weight in service. In addition, steel is usually less costly than solid copper, brass or bronze and is often preferred because of these two factors. Unlike the copper alloys, steel does not form a protective oxide layer on exposure to weathering elements. Consequently, a steel article is usually electroplated with copper or brass in order to protect it from corrosion and present a viable brass or copper outer surface for subsequent antiquing operations.

Steel is commonly used to make structural hardware such as hinges, fasteners, casket hardware and other functional items. Additionally, because of its relatively low cost, steel is used to make many indoor decorative items such as light fixtures and cabinet hardware. Many of these items are stamped out of sheet stock. Round items can often be formed in a metal spinning operation. This process begins with a flat disc of steel sheet, which is fastened to a shaped mandrel and rotated. The part is formed when a steel roller is pressed down onto the spinning surface, slowly forcing it to conform to the shape of the mandrel beneath it. Common items such as lamp bases or bezels are often formed this way, then brass or copper electroplated for antique finishing.

3. Zinc Die-castings – This third group is often used to make decorative items which have a detailed shape and which have low load-bearing requirements. Many items have a design which is too detailed to make easily out of machined or stamped steel or the value of the item is not high enough to justify the cost of cast bronze. For these lower cost articles, a zinc die-casting is the preferred base metal because it is easy to cast into very intricate shapes at relatively low cost, making it ideal for items such as cabinet hardware, light fixture components and many other decorative articles. As with steel, the zinc tends to corrode quite rapidly and must be electroplated with copper or brass for corrosion resistance and for antique finishing. Unlike steel, zinc die-castings can often have a porous surface, requiring the use of a copper strike in order to seal off this porosity prior to subsequent antiquing.

Beginning the Antiquing Process: Cleaning

Before any chemical antiquing can begin on any substrate, the surface must be free of oil, oxides, buffing compounds, mold release compounds, soldering flux, fingerprints or other foreign materials left over from the fabrication of the article. Once these materials are removed, the surface is in a chemically active state and is ready for coloring, electroplating or other operations. There are many cleaning options which could be considered. In selecting a metal cleaning process, many factors must be taken into account, including: (a) the identification of the substrate and the importance of the condition of the surface or structure to the ultimate use of the part; (b) the identification of the soil to be removed; (c) the degree of cleanliness required; (d) the capabilities of the available facilities; (e) the environmental impact of the cleaning method; (f) the cost of the operation; and (g) the nature of the subsequent chemical operations to follow the cleaning step. Because of the variety of cleaning options available, each option deserves careful consideration.

Antiqued parts from eye glass frames to door hinges.

In general, one may rank the different cleaning options in the order of increasing degree of cleanliness, as follows: abrasive blasting, cold solvent cleaning, vapor degreasing, emulsion soak cleaning, alkaline electrocleaning, alkaline soak cleaning followed by acid cleaning and finally, ultrasonic cleaning. Each of these methods has its own advantages and disadvantages and is suited to particular types of soils. There is no universal cleaning method which works well on all types of soils. For example, solid brass or copper items which are soldered together will have light oils and soldering flux on the surface, along with light tarnishing. These soils respond well to mild alkaline soak cleaners and may require mechanical agitation or scrubbing to remove all the flux. Cast bronze or brass items generally carry heavier oxides from the casting operation, but very little oil. Parts that can tolerate the surface roughening can be bead blasted with good success. Other cast parts which ultimately require a bright, shiny finish will be coated with buffing or polishing compounds which can be difficult to remove. In this case, electrocleaning or ultrasonic cleaning work well. These methods provide a combination of alkaline emulsification of oils along with a mechanical action of the ultrasonic energy or current flow to help to mechanically lift these soils from the surface. Stamped or spun steel parts usually have a layer of oil-based stamping lubricants on the surface. Because the steel can tolerate exposure to strongly caustic cleaners, the preferred method is often a hot, caustic soak cleaner or electrocleaner, often followed by a milder alkaline cleaner to ensure free-rinsing of the cleaning solutions. On the other hand, zinc die-castings are usually produced using a waxy mold release compound which can be difficult to remove. In addition, the zinc is a reactive metal which cannot tolerate a strongly caustic cleaner without being etched. Consequently, the best method here is a mild alkaline soak cleaner, electrocleaner or, perhaps, ultrasonic cleaning at a moderate pH which will not attack the zinc. Vapor degreasing can also be used with good success on machining or stamping oils or buffing compounds.

In general, it is safe to say that cleaning is the most important part of the entire finishing process and is a prerequisite to uniform and adherent electroplating, antique finishing and lacquer topcoats. Not only is cleaning the most important – it is also one of the least costly operations of the process line. Consequently, it pays to design the cleaning operation to do a thorough job on the metal surface and to perform all the recommended maintenance to the tanks. This practice represents an inexpensive insurance policy against poor quality finishes in subsequent steps.

As part of the cleaning operation, many parts benefit greatly from an acid cleaning to remove light oxides and to lower the pH of the surface. Here, several different materials can be used, including sulfuric, hydrochloric, fluoboric acids or sulfuric acid salts, depending on the base metal and the desired activity of the acid.

Testing for Cleanliness

The final evaluation of the effectiveness of a cleaning process should come from a performance test. The simplest and most widely used is the water-break test. It consists of processing the article or a standard test panel through the cleaning sequence in the normal manner, then dipping the part into clean water and observing how the water runs off the surface. A part which still carries residual oils will cause the water to bead up on the surface and form water breaks; whereas a part which is uniformly free of oil will allow the water to drain off uniformly with no water breaks. An oil-free surface will stay uniformly wet and the water will "sheet" off the surface rather than bead up.

Another method (useful on steel parts only) involves the use of an acid copper autoplating solution. Here, the cleaned surface is immersed in a dilute acid copper solution. A uniformly oil-free surface will allow metallic copper to be autoplated onto the surface in a uniform manner, with no skips or bare spots. Any uncoated areas would indicate the presence of residual oils on the surface.

Once the part has been properly and completely cleaned of all foreign materials, it is ready to proceed to the next step in the antiquing process.

Electroplating

As mentioned earlier, many parts do not require electroplating. Obviously, any solid brass, bronze or copper substrate would not necessarily be brass or copper plated as well. Once the surface is clean, it would be ready for coloring in the appropriate solution. Other parts, however, such as steel or zinc die-cast surfaces, do require an electroplated layer on the surface prior to being colored. Here, conventional plating techniques are used. The best quality plated finishes usually begin with a copper strike, followed by a generous brass or bronze deposit of approximately 0.0002-.0003 inch thickness. The copper strike is an excellent way to seal off any porosity present in the base metal and make the surface more receptive to an adherent brass deposit of low porosity.

Most commercial brass plating baths contain cyanide. Non-cyanide baths have enjoyed limited utilization because they often lack solution stability and produce deposits which are darker in color and rougher than those of conventional cyanide baths. In addition, because they contain organic chelating agents, they can be more difficult to work with in waste treating the rinse waters. A conventional cyanide bath forces the finisher to treat and decompose the cyanide residues in the rinsewaters, but the zinc and copper are often more easily precipitated. In this area, the suppliers of the chemicals normally offer technical assistance in the correct operation and maintenance of the brass plating tanks. It is important to perform routine maintenance in order to keep these baths operating efficiently.

Coloring of the Surface

Once the surface has been plated with brass, it is ready to be colored by using one of several types of antiquing solutions available:

1. Sulfur and Arsenic based solutions – The traditional way to color a brass surface is to oxidize it with one of these solutions. The sulfur-based method is often called "liver of sulfur" and utilizes a mixture of polysulfide salts to form a black or brown copper sulfide deposit on the surface. It works better on copper than it does on brass and has the inherent disadvantage of having a strong sulfur or "rotten egg" odor. In addition, sulfur has several oxidation states and can form a variety of non-reactive polysulfide compounds which greatly reduced its efficiency and tank life in a production operation. In actual practice, the bath can be somewhat erratic in its oxidizing power from one batch to the next and requires frequent dumping as it is not considered a replenishable product. It generally is considered unsuitable for production scale use.

Arsenic based solutions form a black arsenic oxide on the surface and operate at room temperature. However, they carry a significant toxicity risk for the user and must be handled with extreme caution.

In addition to the two methods above, there are other solutions which can be used on a small scale to color brass and bronze. (see Blackening and Antiquing; Nathaniel Hall; Metal Finishing Guidebook) These methods utilize a variety of chemicals to form colors on several metallic substrates and are designed to be used by individual artisans rather than in production scale antiquing operations.

2. Copper/Selenium Room Temperature Oxidizers – Since selenium is directly related to sulfur on the Periodic Table of the Elements, it undergoes many of the same reactions as sulfur. Consequently, these selenium-based solutions can be used to deposit a black or brown deposit on brass, bronze or copper at room temperature, and offer several advantages over the sulfur method. Most importantly, the selenium has fewer oxidation states than sulfur. This means that the solution is easier to control, with all the selenium going into reacting with the brass surface rather than forming non-reactive side compounds. The result is a bath that can be titrated and replenished, and operated as a permanent bath in the line, with no dumping necessary. This feature gives the finisher greater control over the operation of the bath in terms of reaction speed, the color of the finish produced and the operating cost of the antiquing step.

The only caveat that must be observed is the fact that brass plated parts will carry a cyanide residue on the surface which must be neutralized prior to immersion in the antiquing solution. This is accomplished by momentary immersion in a weak (2-5%) sulfuric acid solution to neutralize and remove the cyanide from the surface. Skipping this step will result in low-level contamination of the antiquing solution by cyanide, which will tend to chelate or complex the copper content and reduce the effectiveness of the bath or disrupt the normal chemical balance.

In practice, selenium-based oxidizers have proven to be the preferred way to blacken or brown a brass surface, due to their ease of operation, lack of fumes, dependable operation and low cost. A variety of colors can be achieved, ranging from golden brown to medium and chocolate brown and black, depending on dilution level and immersion time in the solution. Since these baths are quite safe to work with, it is usually easy for the operator to perform the necessary maintenance and operate the system without undue hazards.

3. Heated Caustic Oxidizers – These baths operate at 240 degrees F and utilize caustic soda and sodium nitrate to oxidize the copper at the surface to a black cupric oxide. Since they react exclusively with the copper at the surface, a copper-rich surface favors the formation of a black deposit in the shortest time. Consequently, many brass parts are "dezincified" prior to blackening. This is done by immersing the parts in a warm caustic bath, (180 degrees F – not hot enough to blacken the surface) to dissolve most of the zinc out of the brass surface, leaving a copper-rich surface behind. At this point, the part has a color which is quite pink and is reactive enough to be blackened by the subsequent oxidizing bath.

These heated oxidizers can produce good quality black deposits, and can be controlled by titration and/or boiling point. They do present an inherent danger to the operator because of the high operating temperature.

4. Black Nickel Coating – This process is an electrolytic blackening operation which produces a black nickel sulfide coating on the surface. The finish is very hard and durable and, in many cases, produces a true black color which the other methods cannot match. It is used most often as an extension of the brass plating operation. Since the parts are already racked for electrolytic deposition of the brass, they are ready for a second electrolytic operation - in this case, blackening, after thorough rinsing only. The bath can be operated as a permanent plating bath in the line, with periodic titration and replenishment and excellent tank life. Many experienced platers find that they can mix their own solution using commodity chemicals, rather than purchase a pre-blended proprietary product. When operated in this way, the operating costs can be quite low.

Black nickel works best on racked parts. Bulk or barrel handling methods work less well and usually result in more difficulty in achieving a uniform deposit, due to the continual interruption of electrical contact between the parts. As a result, the black nickel finish is best suited for use on high value parts which are rack-plated.

5. Verde Green Patinas – Also called "verdegris", this finish is a soft, pale green color, similar to that seen on the aged copper roofs of older buildings. Actually, the authentic green patina formed on these roofs is a mixture of many different copper compounds, including oxides, carbonates, sulfates, sulfides and more. The composition is directly related to the purity of the air in the area. For example, some copper roofs are more black than they are green, due to a higher concentration of sulfur in the air from a coal-burning power plant in the vicinity. Others are more green, owing to a concentration of nitrates in the air from automobile exhaust. Consequently, the color varies widely.

Artificial green patina solutions are, in simplest terms, mildly acidic corrosive copper solutions. They work by slowly tarnishing or corroding the surface of the brass or copper substrate, and forming some of these same green or bluish colored copper compounds. These finishes can be quite attractive when properly applied. They have two inherent disadvantages, however: the finish takes several hours to form and it is only loosely adherent to the metal surface. Consequently, the green patina solutions sold commercially tend to be workable only in small volume process lines where the finisher can afford to let the parts hang and corrode as they dry. And because, the finish is loosely adherent, it depends on the lacquer topcoat to provide the adhesion to the substrate to form a clean final finish.

Highlighting and Burnishing the Finish

Once the parts have been colored, or oxidized, to the desired finish, they are ready to be highlighted, or burnished. This operation can take several forms, depending on the final appearance requirements of the part. The essence of the operation is the removal of some or most of the colored finish to reveal portions of the underlying base metal in order to make it appear worn. In other words, the colored finish is polished off the high points, or highlights, of the parts, and allowed to remain in the recessed areas. The only way to accomplish this task is to mechanically remove the coating from these areas – there is no chemical treatment available to do this job. There are several proven methods which work well:

1. Hand-buffing on an abrasive or polishing wheel – The buffing wheel is constructed of many discs of cotton fabric, sewn together to form a single buffing wheel, about half an inch thick. These can be stacked together on a single spindle to form a buffing wheel up to 3-4 inches wide, depending on what is needed to cover the part most effectively. Once the wheel is assembled, it can be loaded with different compounds, ranging from abrasive to fine polishing compounds, depending on the type of contrast desired on the part's surface. For example, some parts have designs which have well-defined edges to the details or have sharp corners, etc. These parts generally would be highlighted with a fairly abrasive compound in order to clean off the colored coating completely from the highlights and allow the coating to remain almost entirely in the recesses. Or, a dry, non-metallic abrasive flapwheel might be used to achieve a sharp contrast.

On the other hand, the part may have a rounder shape, with softer curves and no clear-cut, sharp edges. This part may look better with a softer contrast burnishing than with sharp contrast abrasive buffing. If so, the cotton wheel would be loaded with a less abrasive compound in order to achieve a softer shading or "feathering" of the colors on the part. Some parts go one step further, requiring no actual removal of the antique finish, but only a softening or burnishing of the coating to blend tones. This type of part might be buffed on a soft, brass wire wheel rather than a cotton wheel and a compound. This softer wire wheel would not really remove any coating, but merely smooth it out a bit, or impart a soft directional grain to the surface. An alternative method might be to use a wet burnishing wheel - a brass wire wheel wetted with a slow dribble of water to soften the abrasive action.

It is easy to see that the hand buffing operation is more art than science. Just as cleaning is important to the integrity of the deposit on the surface, so buffing is critical to the final appearance of the finish and can even determine the market value of the piece. Since the decorative hardware business is all about appealing to "the eye of the beholder", it is important to appeal to the eye of the buffer first.

2. Automated buffing machines – As in many other aspects of the finishing process, higher production volumes also produced a need for automatic buffing capabilities in order to reduce labor costs and rely less heavily on the human factor in the buffing operation. Larger volume production lines often use very little hand buffing and have come to rely on automatic machines which can be programmed to follow the shape of almost any part. These machines often take the form of a turntable, surrounded by several buffing heads, each of which is oriented to buff just one aspect of the part as it passes by. Alternatively, some machines can index the part, or rotate it so that a single buffing head does the entire job. The shape of the part will determine which type of machine will be most suitable.

3. Tumbling and Vibratory Methods – Just as hand buffing is most often suitable for high value pieces, lower value parts can often be effectively highlighted in bulk. Parts such as certain cabinet hardware, fasteners or other small parts would typically be brass plated or antiqued in bulk handling methods. If so, it is desirable to burnish or highlight in bulk as well. To do this, the parts can be burnished in a tumbler or in a vibratory mill.

A tumbler is a rotating drum which rolls the parts against each other like a cement mixer. The parts can be burnished either wet or dry, using a plastic or ceramic media, with an abrasive or a polishing compound. Selecting the desired combination of these effects will produce a variety of different burnishing possibilities. The parts can generally be taken right off the process line, without drying, and loaded directly into the tumbler.

Vibratory finishers operate in a similar manner, but use a vibrating bowl rather than a rotating drum. As mentioned, the vibratory bowls can also be charged with different types of media and compounds to achieve the type of contrast desired. Both the tumbler and vibratory mill will produce a non-directional pattern on the part surface and cannot really reproduce the effect achieved by a hand buffing operation. However, they operate at much lower cost and can be pre-programmed to produce the identical result batch after batch. Consequently, they are less dependent on the human factor for consistent quality. For certain parts, compromising on quality a bit in order to control the cost allows the manufacturer sell the finished piece at the desired price point and still make a profit.

Protective Topcoats

After coloring and highlighting are completed, the part is ready to be topcoated to protect it from corrosion. Even though the parts may look completely finished, the decorative antique finish is quite susceptible to corrosion or tarnish unless protected. The products most often used to accomplish this are clear lacquers. As in all the previous operations, there can be many options open to the finisher, depending on the durability required of the final finish, operating cost, equipment cost, environmental concerns, etc. In actual practice, there are a few options which provide the most benefits:

1. Air-dry lacquers – These products can be water-based or solvent-based and commonly utilize acrylic or urethane polymers to form a protective film. The acrylics are the lower cost option and can provide an effective topcoat for many parts used indoors only, such as light fixtures, wall sconces, etc., which do not see heavy wear. Generally, solvent based lacquers are more protective than water based products, but also present a potential solvent fume problem in terms of discharge into the atmosphere.

2. Baking or cross-linkable resins – These products are widely used on parts which require high wear resistance and/or outdoor exposure and include polyurethanes, epoxies, nitro-cellulose lacquers - all of which can cross link during drying to form a very dense and tenacious film. Very often, they are cured in an oven, at 250-350 degrees for ten to twenty minutes, to speed drying. These products are suitable for high value parts or surfaces which must be exposed to outdoor weathering elements.

It is also possible to use lacquers containing corrosion inhibitors which specifically protect copper alloys. The most widely used is benzotriazole and its related compounds. These materials can be blended into many types of lacquers in small concentrations and provide an extra measure of corrosion resistance, making them particularly well suited for use on items such as marine hardware, building components, etc.

3. Clear powder coats – Relatively new on the scene, these topcoats produce coating thicknesses of 2-4 mils, and offer extremely high protection levels. They are applied like any other powder coat - in a dry, electrostatic spray, followed by 350 degree oven bake. Powder coats are not suitable for all parts. They work best on parts which have an open shape, with few or shallow recessed areas and can be susceptible to the Faraday Cage Effect. This is commonly seen with any electrostatic or electrolytic operation (including plating) and prevents deposition in deep recesses. Consequently, it is difficult to powder coat the inside surfaces of many parts.

4. Electrophoretic liquid lacquers – These products are not new, but they are just now coming into popular use. They are liquid lacquers, to be used as an electrophoretic immersion at the end of the plating line, followed by an oven cure. Though not commonly used on parts that are highlighted after coloring, they do find use as a clear sealant over a solid black finish, like a black nickel finish. In this setting, the part is racked and taken through the plating operation, then black nickel, then electrophoretic lacquer - all requiring the use of current to do the coating.

5. Paste wax and oil finishes – Some parts do not require a permanent antique finish, but are designed to allow the surface to age naturally in service. For example, brass hand rails, building fascia panels, elevator panels, and other parts can be initially sealed with a temporary protective film such as paste wax or oil. Then, when installed, they will be handled during normal use and be constantly "burnished" by this contact. Over time, they will develop a natural, soft patina which will ultimately be permanent because it is being constantly developed.

Waste Treatment

This area is of critical importance to the Metal Finishing Industry because a chemical process line cannot operate without proper treatment of waste products, as mandated by the Federal EPA and appropriate State or Local agencies. Since these process lines utilize a variety of different chemical products, it is impossible to offer a simple overview of the waste treatment picture. A few comments are in order, however, about the types of wastes generated in these lines and the waste treatment methods commonly employed to achieve compliance with the regulations:

1. Alkaline Cleaning residues: These residues are primarily composed of non-hazardous alkaline salts such as sodium hydroxide, sodium carbonate, sodium phosphates, wetting agents and other compounds which are not specifically regulated. By virtue of their operating pH, they tend to dissolve metals from the parts being processed – in most cases, copper and zinc. Simple pH adjustment is very effective, in precipitating much of the metal content and bringing the effluent into the acceptable pH range of pH 5 - 9. Any remaining metal content can be precipitated with the help of specialized flocculants.

2. Acid residues: Acid solutions quickly dissolve metals from the parts being processed and, like the alkaline chemicals above, respond well to simple neutralization techniques to precipitate the metal content. Acid and alkaline rinse waters are typically mixed together for treatment, and help to neutralize each other.

3. Cyanide residues: The rinses following the brass plating bath will contain cyanide, copper and zinc. This rinse water is typically subjected to a cyanide destruct process which oxidizes and decomposes the cyanide to harmless chemicals, and also precipitates the copper and zinc content. The metallic sludge is then collected on filters and disposed of as hazardous solid waste.

4. Solid Waste: The waste treatment methods above generate hazardous solid waste in the form of metal-bearing precipitate which is commonly collected on a particle filter cartridge or plate filter element. This solid waste can be sent out to a licensed waste treater for proper stabilization and landfilling.

5. Dragout Rinses: These are often used as preliminary rinses following a heated process tank, such as a heated cleaning tank or plating tank. Dragout rinses are perhaps the single most effective and least costly way to minimize chemicals in the drain. They are typically followed by a treated rinse which is fed to Ion Exchange or other treatment. (See Typical Process Cycles, example "A".)

Ion-Exchange System

For process solutions carrying only a moderate level of metals, a single dragout rinse is sufficient. A brass plating tank, on the other hand, will contain fairly high concentrations of cyanide, which is costly to treat. Consequently, it is common to see two or three dragout rinses used to minimize the level of cyanide sent to waste treatment.

6. Copper and Selenium-bearing effluent from Room Temperature Oxidizing lines: Room Temperature oxidizers are perhaps the simplest to operate because they respond so well to treatment by Ion Exchange techniques. Some lines are set up with the rinsewaters going in two different directions, so to speak: the rinsewaters from the alkaline clean and acid tarnish removers tend to neutralize each other in the drain, and are sent to a pH adjustment to complete the precipitation process; meanwhile, the rinsewaters following the copper/selenium based oxidizers can be treated by Ion Exchange to purify the water and re-use it, with none of this water entering the drain. Or, another option in many cases, is to treat all the rinsewater in the line with Ion Exchange. Since all the rinses can contain metals, none can be considered sewerable. But, since the total dissolved solids content of these rinses is usually quite low, Ion Exchange is able to purify all the rinsewaters, in many lines, and return them to the rinse tanks to be re-used over and over again. (See Typical Process Cycles, example "A".)

In general, Ion Exchange works well when the total dissolved solids content of the water is 1000 ppm or less. For higher concentrations, pH adjustment and neutralization techniques are more efficient; however, concentrations under 1000 ppm are good candidates for Ion Exchange.

Most Ion Exchange systems are equipped with a conductivity light which signals the operator that the resin tanks are saturated and ready for regeneration. The regeneration can be performed on site, or the resin tanks can be shipped to a licensed waste treater for regeneration. The waste treatment company then takes ownership of the metal wastes and disposes of them in an approved manner.

Responsible chemical suppliers offer advice on proper waste treatment techniques for their products. There is a great deal of additional information available in the Metal Finishing Guidebook or in other industry publications.

In summary, there are many different aspects to antique finishing, which take some time and experience to learn. As long as the decorative hardware industry is in existence, however, these finishes will be in demand, and will evolve to meet the needs of the marketplace. The trend is toward safer processes, less polluting chemicals, easier and shorter processes. As always, cost is of prime concern. The industry is working to eliminate the hazardous and costly elements of traditional processes and replace them with materials and techniques which are more compatible with modern day priorities.

Attribution: Metals Handbook; c.1982; American Society for Metals. Vol S: Surface Cleaning, Finishing and Coating.

Typical Process Cycles

A. Solid Copper, Brass - (for example, a soldered light fixture assembly)
  1. Mild Alkaline Soak Clean: 8-10 oz./gallon mix; l50 degrees F; 4-6 minute soak with air agitation.
  2. Dragout Rinse: Non-flowing rinse to remove most of the cleaner residues.
  3. Overflow Rinse: Treated by Ion Exchange.
  4. Mild Acid Tarnish Remover: 10% sulfuric acid; Room Temp; 1-3 minutes.
  5. Overflow Rinse: Treated by Ion Exchange.
  6. Oxidize: Blacken or brown in Room Temperature oxidizing solution; 1-3 minutes.
  7. Overflow Rinse: Treated by Ion Exchange.
  8. Final Rinse: Deionized Water to minimize water staining during drying.

Birchwood Technologies - BC100 Tank System



(See notes on Ion Exchange treatment under Waste Treatment section.)

B. Steel Stamping - (for example, a stamped lamp base). Rack processed.
  1. Heavy-duty alkaline soak clean: 10-12 oz./gallon mix; 170-180°F; 4-6 minute soak.
  2. Alkaline electroclean: 12 oz./gallon of high caustic formula, 160°F; 6-12 volts anodic current; 100-150 amps/ft_ current density; 2-4 minutes.
  3. Rinse: clean tap water; 20 seconds.
  4. Rinse: clean tap water; 20 seconds.
  5. Acid pickle: Hydrochloric acid; 30-40% by volume; room temperature; 2 minutes.
  6. Rinse: clean tap water; 20 seconds.
  7. Rinse: clean tap water; 20 seconds.
  8. Copper strike: 75-120°F; 6 volts; 15-20 amps/ft2; 2 minutes.
  9. Brass plate: 90°F; 6-10 volts; 15-20 amps/ft2; 15-30 minutes.
  10. Rinse: clean tap water; 20 seconds.
  11. Sour rinse (to neutralize cyanide): 2% sulfuric acid; room temperature; 30 seconds.
  12. Rinse: clean tap water; 20 seconds.
  13. Oxidize in black nickel or room temperature solution.
  14. Rinse: clean tap water; 20 seconds.
  15. Rinse: Deionized water (to minimize staining during drying).
  16. Warm dry: 130°F.
  17. Highlight on cotton buff with abrasive compound.
  18. Lacquer with nitrocellulose lacquer.
  19. Oven cure at 250°F; 15-20 minutes.
C. Zinc Die-Casting - for example, cabinet hardware). Rack processed.
  1. Deburr in vibratory finishing machine, using ceramic media and deburring compound.
  2. Mild alkaline soak clean: 120°F; 5 minutes.
  3. Mild electroclean: 120°F; 3 minutes.
  4. Rinse: 20 seconds.
  5. Rinse: 20 seconds.
  6. Acid pickle: sulfuric acid salt; 8 oz./gallon, 75°F; 2 minutes.
  7. Rinse: 20 seconds.
  8. Copper strike: 2 minutes; 75-120°F.
  9. Brass plate: 30 minutes; 90°F.
  10. Rinse: 20 seconds.
  11. Dezincify: 180°F; 5 minutes.
  12. Rinse: 20 seconds.
  13. Blacken in hot caustic oxidizer; 240°F; 15 minutes.
  14. Rinse: 20 seconds.
  15. Deionized water rinse: 20 seconds.
  16. Dry with warm air.
  17. Highlight on an automatic buffing machine.
  18. Lacquer.
  19. Bake cure.
Selection of Cleaning Methods
  Advantages Disadvantages
Abrasive Blast Cleaning
  • Removes heavy oxides
  • Can be used on any metal
  • May eliminate other cleaning
  • Simple equipment
  • Low skill levels required
  • Dusty
  • Time-consuming
  • Imparts coarse texture to surface
  • Costly
  • Oil films only partially removed
Alkaline Soak Cleaning
  • Effectively removes oils
  • Inexpensive to operate
  • Simple equipment
  • Requires heat to work well
  • Safety hazards-heat; chemicals
Electrocleaning
  • Effectively removes oils and other stubborn soils
  • Relatively low cost operation
  • Can remove insoluble particles
  • Requires current flow; rectifier
  • Safety hazards-heat; chemicals
Vapor Degreasing
  • Very effective at heavy oil removal
  • Solvent can be re-distilled and re-used
  • Excellent as pre-cleaner to eliminate other cleaners
  • Will not remove some water-based soils
  • Solvent emissions-highly regulated
  • Requires extensive equipment
  • Requires maintenance of solvents
  • Must be followed by secondary cleaning
Ultrasonic Cleaning
  • Very effective oil remover
  • Equipment very expensive
  • Can remove insoluble particles
  • Works on any type of solvent or aqueous cleaning


Learn More