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The Lost Wax Casting Process
Wax: An Overview
Ceramic Slurry Dipping Schedule
The Sandcasting Process
The Chasing Process
Methods for Joining Bronze
Patinas
Investment Casting: 1930-1950
Project #2: Sandcasting a Bas Relief Bronze Plaque
How to Build a Bronze Casting Furnace
Foundry Safety
Furnace Lighting Procedure
Project #1: Direct Wax Sculpture, Ceramic Shell Casting

Patinas

Safety

The chemicals used in patinas can be hazardous to your health. Read the MSDS (Material Safety Data Sheet) on the chemicals you are using. These can be found on the Internet, or ask your instructor to see them in class. Take the proper precautions:

1. Wear a respirator rated for chemicals.

2. Work indoors with an exhaust system, or outdoors.

3. Wear gloves to protect the skin from contact with the chemicals.

4. Wash your hands and exposed skin after use.

Surface Preparation

The surface of the sculpture will affect the patina. This is most important in transparent patinas where the surface can still be seen. Cold patinas should have a sandblasted surface and special care should be made to make sure it is very clean. Cold patinas do not bite into the surface as well as hot patinas the sandblasted surface will give it some tooth.

1. Sandblasting – produces a matte surface that takes both hot and cold patinas well. It is the primary surface used for cold and opaque patinas.

2. Polished surface – the most lustrous transparent look. Highlights can be polished and recessed areas left sandblasted to create a contrast between the two. A polished surface does not work well with cold patinas, as they do not adhere as well as hot ones. It is a waste of time if you are using an opaque patina, as it will be covered.

a. Sandblast the surface to clean it.

b. Polish to a satin finish using scotchbrite disk.

c. Polish with a cotton wheel on a die grinder using white diamond buffing compound or red rouge.

d. Clean with a toothbrush or brush with 50% soap and water, then dry and clean acetone very carefully so NO buffing compound remains.

3. Wire Brush Surface – A wire brush on a die grinder or angle grinder produces a satiny finish. Texture can be in one direction or crisscrossing. Use only with transparent patinas or if some surface will show.

4. Burnished surface – produced by rubbing the surface with a burnisher (hardened and polished steel shaped like the convex part of a spoon). Use only with transparent patinas or if some surface will show.

 

Types of Patinas

Base Coats

These are used to highlight the surface texture or create a darker undercoat. They darken the surface, and if buffed lightly after application with steel wool or a scotchbrite pad, the patina can be removed from the raised areas highlighting the grooves and indentations by leaving them dark.

Liver of Sulfur

Colors range from browns to warm black. It makes a good base for almost any patina. It can also be used alone without other patinas on top and is very durable and probably one of the easiest patinas to maintain.

For a base coat apply cold with a brush or spray bottle over the entire piece until it is an even gray. Wash with water to remove and neutralize the chemical. Highlight by rubbing with steel wool or scotchbrite pad if desired to bring out highlights. Rubbing liver of sulfur to remove it will cause it to have a leaded look that is hard to remove – avoid this unless you specifically want a gray patina.

Mixture:

Liver of sulfur 1 teaspoon

Water (warm) 1 pint

Note: Liver of Sulfur looses its potency quickly when exposed to air. Use a fresh solution each time you patina.

Birchwood Casey M24

This commercial patina produces a deep black color. It should be used full strength or mixed to a 50/50 mixture. It is durable and easily maintained.

Apply cold with brush or spray bottle to wet surface. Wash off when good even black is achieved and rub with steel wool or scotchbrite pad to bring out highlights if desired.

 

Hot Patinas

Hot patinas are one of the most common of the patinas used for sculpture today. A wide variety of colors and effects can be achieved. These patinas are applied to metal that is around 200° as opposed to cold patinas which are applied to the metal at room temperature. Heating the metal opens its pores, which allows the chemical to bond more tightly to the surface.

Procedure

1. Clean the piece with degreaser. Any speck of dust, dirt or grease (even fingerprints) could show up and ruin the patina.

2. Heat the metal with a torch until around 200-250°F. To test this spray a few drops of distilled water on the metal. If the water sizzles or steams but does not ball up, it is the right temperature. If the water balls up and bounces off, it is too hot. If the water runs off without steaming, it is too cold.

3. Apply the patina to the hot metal. Continue heating with the torch as often as needed to keep the metal hot while applying the patina. There are a number of ways to apply it, each creating a different affect.

Spray Bottle – Even coloring

Long Haired brush – tortoise shell, marbling, coloring specific areas.

Stippling with shorthaired brush – mottled coloring, marbling.

Hot Patina Chemicals

In general, all chemicals should be mixed as a weak solution: a teaspoon dissolved in a pint of water. Add more chemical if necessary, but too much can cause the patina to build up too fast making thick chalky areas that will flake off. Chemicals that will not keep well, such as liver of sulfur can be made in 1/4 pint batches in the same proportions.

Liver of Sulfur light gold to black
Ammonium Sulfide brown to black
Cupric Nitrate green to blue-green
Ferric Nitrate gold to red-brown
Ferric Chloride rust brown
Bismuth Nitrate white
Potassium Dichromate orange
Ammonia blue
Bichromate of ammonia light green
Sodium Thiosulphate gives colors a metallic look
Potassium ferrocyanide red-brown to purple

Transparent patinas

Some hot patinas are transparent. These can be built up in layers just as glazes are in oil paints. Highlights can be taken down with steel wool after several applications of color, and additional colors can be added which result in highlights a different hue. Be aware that these patinas will show the surface of the metal, so surface preparation is more crucial. Transparent patinas are:

Cupric Nitrate
Ferric Nitrate
Potassium Dichromate
Sodium Thiosulphate

Cold Patinas

There are varieties of methods of applying cold patinas. What they have in common are that the chemicals are applied cold to the surface of the metal. The most common are:

Spraying/brushing/sponging – Applying the chemicals to the surface of the metal.

Buried – The sculpture is buried under sand, sawdust, kitty litter or another absorbent material that is moist with chemicals.

Wrapped – The sculpture is wrapped with cloth that is soaked in chemicals.

Fumed – The sculpture is placed in a plastic tent with bowls of liquid chemicals. The fumes of the chemical react with the surface creating a patina.

Vat – The sculpture is dipped or allowed to soak in a vat of chemicals and water. The chemicals may be cold or heated to boiling.

In this class, we will primarily be using hot patinas and buried patinas. You are welcome to use any of the other techniques. Be aware that cold patinas have the following in common:

All cold patinas take much longer then hot patinas. Expect times from several hours to days or even weeks.

They are usually not as durable as hot patinas that bond to the metal better.

Initial cleaning of the surface is of greater importance since cold patinas do not adhere as well as hot patinas.

Cold Patina Chemicals

Ammonium Chloride light blue
Cupric Chloride green
Ferric Chloride orange or brown
Sodium Chloride (table salt) reactant
Ammonium Hydroxide reactant, blue
Vinegar (acetic acid) greens
Copper Sulfate reactant

Other reactive substances

Urine
Sour Milk
Used Kitty Litter

Applying Cold Patinas

Brushing/Sponging/Spraying

Use only natural bristle brushes with plastic or non-ferrous handles for chemical applications to prevent contamination with steel or tin. Except for liver of sulfur and Birchwood Casey M20, which react immediately, all these patinas may take from minutes to days or weeks to work. Most cold patinas take time to cure and bite into the surface once color has been achieved. Once cured, the sculpture should be rinsed with water to neutralize any chemicals, and the surface must be allowed to dry fully. Once the chemicals have been applied, the piece should be tented in some way unless the weather is humid. This process of curing and drying can take up to a week.

Wrapping Techniques

Though messy, some interesting effects can be achieved with this technique. Strips of cotton (or some other natural fibered cloth) are soaked in chemicals and then wrapped around or laid across the sculpture. This gives the surface both a damp atmosphere and holds the chemicals against it. Allow plenty of time for the patina to develop, once disturbed the surface is difficult to repair. Two to three days is recommended.

Buried

This is one of the oldest methods of patination. Rich colors and textures can result from buried patinas. The sculpture is buried in a container filled with some absorbent material such as sand, sawdust or kitty liter that has been dampened with chemicals. The proper moisture is achieved when the material “balls” up when squeezed. Too much will cause pooling of the chemicals in cavities of the sculpture, and too little will cause spotty results. The coarser the material the more open the texture of the patina.

 

Check the results by digging down and exposing the top of the piece. Once the proper color is achieved, the sculpture must be thoroughly washed to neutralize the reactions and remove particulates. Sufficient time must be given for curing, and the piece should be kept out of direct sunlight as this can change the color. After one week, seal it with a thin lacquer coating and one or two coats of Trewax.

 

Protecting and Sealing the Patina

Left unprotected the moisture and chemicals in the air will alter the color of the patina, just as it would the bare bronze. Depending on the location the colors could range from browns, blacks or greens. A textured surface will oxidize quicker then a smooth one. To protect the patina, as well as the bronze itself, it must be coated with some sort of protective surface. What you use depends on the type of patina you have used and the finish you desire.

Non-Wax sealers

Shellac, Incralac, lacquer or acrylic can be used on both hot and cold patinas by themselves or with a wax coat over it. Waxes are often applied over the top of these finishes to soften the glossy appearance. Cold patinas are more fragile and require a layer of lacquer or acrylic to strengthen it.

These sealers should be applied in thin coats. If painted rather then sprayed on, they should be diluted to a 50/50 mixture with an appropriate solvent. European patineurs have been using varnishes and lacquers for over 200 years. Make sure the sealer you purchase is made for use on metal, not wood.

Spray lacquers give a glossier look then varnish. Apply to bronze that is dry and slightly warm in two or three thin coats.

Incralac is one of the best coatings for outdoor use. It is produced by Stan Chem. It is a clear acrylic lacquer suspended in three solvents: toluol (toluene), xylol (xylene) and methyl ethyl ketone. The plus is that Incralac's acrylic nature gives with the expansion and contraction of bronze in an outdoor setting for many years. It dries very quickly and does not change the color of most patinas. If it is to be brushed on, dilute 50/50 with xylol or toluol. Indoor bronzes need only one thin coat. Outdoor bronzes require three coats. The downside is the health hazard. The fumes given of by spraying Incralac are both explosive and toxic. Wear a respirator rated for organic vapors/acid gas as well as working outdoors (if indoors use with a ventilation system that has an explosion proof motor).

Waxes

Waxes have been used for over 2000 years and though they do not last as well as the sprays, they will provide protection for indoor sculptures for some time. In the case of outdoor sculptures, they should be reapplied yearly. Well buffed wax imparts a nice luster the finished surface. Most waxes will darken the patina’s color. If this is not desired, use a clear spray lacquer. Be careful with cold patinas that are fragile and may be damaged by rubbing. These should be lacquered first to fix and seal the patina.

Waxes may be applied in two ways, warm or cold. The advantages of warm application, is that the wax goes on smoothly and penetrates the patina. This is not always possible, and in these cases, the wax may be applied cold.

Warm Application

For warm application, Johnson’s Wax, Treewax™, and Renaissance Wax are some of the waxes that can be used. Ronald Young recommends Renaissance Wax or Constantine Wax, while Patrick Kipper recommends Johnson’s Wax. I have used Johnson’s, Carnauba and Renaissance wax to good effect. Do not over apply any of these waxes. Thinner coats actually work better because they are smoother and shed water better. In addition, thin coats can be buffed to a smooth glossy finish while if they are applied too thickly the waxes will create white deposits that are difficult to buff out.

Be sure the sculpture is completely dry; otherwise, the wax will trap the water underneath causing problems later. Warm the piece (warm not hot) to aid in penetration. Apply a thin coat of wax by dabbing with a brush moist with the wax, and allow it to sit overnight. Rub the piece to a final polish with a soft, clean cloth.

Cold Application

Wax that is applied cold can be diluted with benzene, naphtha or mineral spirits to make application easier and smoother. Apply one to three thin coats depending on whether the sculpture is to be indoors or outdoors. Make sure the sculpture is completely dry so you do not trap moisture under or between coats. Buff between coats with a soft, clean cloth.

Recommended Books

Patrick V. Kipper, Patinas for Silicon Bronze

Ronald D. Young, Contemporary Patination

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Dan@RotblattSculpture.com
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