Hot patinas on bronze

Weed or flower!

When is weed a flower? When the viewer defines it as such.

I, like many amateur gardeners, take great pains to ensure my red trumpet flower plant flourishes but  go to even greater lengths to remove the blue Morning Glory weed.

This odd behavior takes on an irrational element when it comes to the garden at my workshop, as here, I have planted and am nurturing an orange variant of Morning Glory that I look upon as a beautiful flower. Clearly what we define as a weed is completely subjective and arbitrary.

Hot Patinas on bronze

Patinas by contrast are not irrational or subjective but driven by science.

When creating color on bronze we work with various nitrates which we apply to a piece pre-heated by blow torch to a temperature of 180 to 200⁰. The metal is first sand blasted and cleaned and the liquid oxide is then either brushed or sprayed onto the hot surface. Heating the metal enhances and accelerates the chemical reaction that occurs when the oxide is applied. Think of having a facial, the skin pores like the metal surface expands and this allows the nitrate to penetrate the surface. Upon contact the nitrate begins to break down and the oxygen becomes active and begins to react with the copper component of bronze to create a thin patina. The color of the resulting patina depends on the nitrate used with cupric nitrate for blue green hues and ferric nitrate for red orange tones.

As a rule of thumb, hotter temperatures will create more vibrant colors, and the process can be repeated once the surface is completely dry for a deeper richer color.

 The hot patina adds  color and durability when sealed with a UV resistant lacquer or wax. While these sealants help maintain the color of the patina, as with any finish, environmental pollutants like sulfur, chloride and cleaning fluids can reduce their effectiveness and can cause the patina to change.

Patinas For Hardware and Sculpture

In previous pieces we have covered  our standard light and dark antique patinas. For custom orders we also offer a hot patina, which is applied to pre-heated bronze. We limit these finishes to just a  few pieces, that include the  small butterfly pull, which can be special ordered in amber- yellow; the frog knob, which can be ordered in a blue green, and the lizard lever or knob which we can patina pea green.

Sandblasted brushed - patina ready Cold antique bronze patina Pea-green hot patina

With our new bronze sculptures, we are developing patinas made with ferric nitrate as well as liver- of sulphur. Ferric Nitrate is a chemical compound of iron powder, nitric acid and it produces a  rust red patina. Liver-of-sulphur  is a chemical compound of powdered sulphur, sodium, and water. They are applied to the sculpture with a spray bottle or brush and the sculpture is preheated to a temperature of approximately 100°to 200°depending on the chemical and dilution. As the sculpture is hot, the wet patina attaches to the surface and the moisture evaporates quickly leaving behind a layer of color.

The colors these patinas produce on bronze can vary from gold to brown to red to dark red to black, depending on how hot the piece is, and how many coats are applied. The chemicals below can be mixed and diluted to produce a wide range of hues. They can also be applied over each other provided the previous patina has sufficiently dried. By layering the patinas and by using brushes and different spraying methods it is possible to develop deep mottled colors and effects. It has taken a lot of practice and years of trial and error to create the look we are seeking for each sculpture.

1. Scotch bright surface 2. Sulphur dioxide spray mist onto surface 3. Diluted bismuth white splatter spot spray

4. Ferric nitrate fine spray cover surface fine coating. 5. When dry wax

Words of caution to anyone thinking of learning how to patina bronze, exercise caution and a generous helping of common sense. The patinas we use are applied in a very well ventilated area and masks and gloves and protective aprons are a must. We also exercise considerable caution when heating up bronze as temperatures above 100° can burn exposed skin.

Liver of sulfur – yellow brown to black tones

Ferric nitrate – red tones

Ferric chloride – darker red tones 

Bismuth nitrate and titanium dioxide – white

Cupric chloride  – pea green or apple green

Cupric nitrate – blue green

A very good site for reading more about patinas and for practical advise on formulas can be found at David B. Bowman an artist in the San Francisco Bay Area.