Home Glass Chemistry Oxblood

Oxblood

Merriam-Webster

1) ox·blood noun \ˈäks-ˌbləd\

Definition of OXBLOOD: a moderate reddish brown

First Known Use of OXBLOOD = 1705

Encyclopedia Britannica

2) sang de boeuf, (French: “oxblood”) also called flambé glaze,  a glossy, rich, bloodred glaze often slashed with streaks of purple or turquoise used to decorate pottery, particularly porcelain. The effect is produced by a method of firing that incorporates copper, a method first discovered by the Chinese of the Ming dynasty, probably during the reign of Wanli (1573–1620).

Canal Fulton Glassworks

3) Oxblood is a red opaque glass containing copper particles of the same order of magnitude as the wavelength of light. Crystals of this size produce opacity, but are not large enough to produce metallic gloss, for example, aventurine.

Oxblood or Haematinum opaque red glass has been around since the Late Bronze Age (1600 – 1200 BC) in Mesopotamia, and later in Egypt during the New Kingdom period at Amarna.

The color of this beautiful and historic glass comes from copper. Copper can exist in a glass body as either an ionic solution or as a colloidal solution. An ionic solution of copper can range in color from colorless to a vivid transparent turquoise or "copper blue". When the colloidal state is dominating, the colors range from a transparent ruby, to opaque orange and then brick red (oxblood), and finally to gold stone or aventurine.

Copper generally comes with some oxygen: Cuprous Oxide and Cupric Oxide. When you strip away all of the oxygen from the copper oxide, you end up with an atom of copper. This is referred to as a redox reaction. Cu¹¹→Cu¹→Cu

In the case of oxblood glass, the main coloring phase comes from cuprous oxide, or CuO. In nature, the mineral cuprite has the same color. Cuprous oxide crystals are suspended in the glass matrix and either exists alone or more commonly in clusters connected together to form dendritic shaped crystals. When the crystals are very small, they tend to have more of an orange appearance while larger they appear more dark red.

In order to achieve cuprous oxide crystals in a glass melt, a redox reaction must occur. This can be achieved through adjustments in furnace atmosphere or through additions to the glass batch – generally both methods were employed. Traditional redox additions for glass batches include iron oxide, tin oxide (helps keep certain metals in solution in glass), or forms of carbon including sugar, coal dust, or sawdust. In more recent times, silicon carbide and even elemental silicon have been employed to achieve redox. Basically these additive substances latch onto the oxygen that is bonded with the copper, break the copper oxygen bond, and then are driven from the glass melt, most commonly as carbon dioxide. When all of the oxygen is stripped away from the copper, you end up with goldstone glass.

The glass melt while active, can react with the atmosphere and the redox reaction will start to reverse itself. The spoiling of a copper ruby glass is a typical example of this occurrence.

Traditionally in the glass industry, copper based opaque red glasses have been called “brick red”, "Sang de Boeuf" (French translation "blood of beef" or simply "oxblood"), "sealing wax red", and just “dark red”. These are actual titles in old glass batch books from the 19th century.

By the mid to late 1920’s, red colored glasses were more easily produced with selenium and cadmium. The more difficult to melt copper based reds just seemed to fall out of favor.

Making your own oxblood.

Melts should be conducted in an electric furnace, which basically has a neutral atmosphere. Any redox reaction in this case has to be based only on the batch constituents, not the glass furnace atmosphere.

Crystal Batch - base point from where you should start.

(All measurements are in pounds.)

Sand – 10

Soda Ash Dense – 3.5

Sodium Nitrate – 0.8

Calcium Carbonate – 1.75

Borax 5 mol – 0.8

Kryolite – 0.07

The crystal batch should total a weight of 16.92 pounds.

Add these colorants to this batch:

Black Copper Oxide – 1

Red Iron Oxide - 0.9

Silicon Carbide (600 grit) - 0.9

The amount of CuO should be about 10% of the weight of the silica content in the batch. The dark streaks that are a defining element of oxblood are just incidental. This would include streaks that look black, amber, and olive green. All of these colors come with the oxblood; they do not need to be added. The colored streaks are resultant from parts of the melt that have reversed the redox reaction and are moving away from the colloidal state towards the ionic.

Russell Andavall

4) The particles of light, called photons, are reflected off the Cupric Ion crystals in the glaze to produce a red wave length of light. In order to do this, first there must be enough copper present in the Glaze to produce the alpha Ions of Cupic oxide that light must refract off of in the red wave length. This may seem simple enough by just adding a sufficient supply of Copper to a glaze to produce the amount of Cupric Ion necessary for the reaction to take place. Unfortunately, it is not that simple. Copper has a low boiling point within the Silicate Solution that we produce to make a glass for our ions to reflect the light off of. The Copper will normally boil out of the glaze and not be present at the end of the firing cycle when heated to temperatures above Cone 9 . Indeed the best Oxblood Glaze that I have produced are Cone 12 to Cone 14 glazes and I will discuss how to keep the copper Ions from boiling out of the Glaze. By adding Iron and Boron to the glaze, we can keep the Copper Ions from boiling out of the glaze. So we make sure that our glaze has a supply of Iron and Boron to cling to and not boil out. I use about 1 %Red Iron Oxide to promote the Copper from boiling out of the glaze. I use a combination of Gerstely Borate and Borax as a substitute for Colemanite. Borax sends to dissolve into the water and form a soapy film on the pots. It still has additions that make it better than Gerstely Borate alone.

The next problem we face with the Oxblood Glaze is that the Copper and the Boron acts as flux to the Glaze and may run off the pot. We must stiffen the glaze with a sufficient proportional amount of Silica in the glaze to make it a stiff Glaze that will not run off the pot. In addition, I use this Oxblood over another stiffer glaze that will catch the running Oxblood. My Clear Boron Crackle Glaze is such a glaze.

The Oxblood Glazes also need to have the free Oxygen molecule removed from them so that they will become Cu++ ions and form the Ionic crystals that will refract the photon into a red wavelength. There are two methods to achieve the desired affect. The first is to starve the kiln for Oxygen when the kiln has reached a temperature where the glazes are molten. I bring the kiln up to Cone 10 and then starve it for Oxygen by restricting the flow of gases from the kiln and cutting back the flow of Oxygen to the burners to produce a yellow flame that seeks Oxygen from the glazes. In addition, I include in the glaze a local reducing agent in the form of FFF silicon carbide to help in the reduction process. This must be mixed well with the glaze because it settles out. I stir and mix up continually when I apply this glaze. I use an additional emulsifying agent in the glaze to help keep material in suspension in the form of Bentonite and the form of Epsom Salts. The Epsom Salts make the glaze smell like rotten eggs after a few days and when I am not using this glaze, I cover it or keep it in a jar.

The next problem that Copper Red glazes present is that because they are releasing Oxygen, they can have lots of pinholes in their surface. We need to add several ingredient that will degassify the glaze and minimize this effect. I add Calcium to the Glaze in the form of Whitting (CaCO3 ) and the form of Bone Ash or technically Calcium triphosphates (Ca 3Ph CaCO3 ) . These ingredients will flux the glaze as well and also cause the Copper to boil out of the Glaze, so a balance must be achieved between these ingredients and the materials promoting Copper Ions forming into the crystals that refract the light into a red wave length. In addition, I let the kiln sit at a lowered flame at the end of the firing cycle to allow the glaze to release the oxygen molecules from the glaze and heal the pitting over.

Finally, the red color shows best upon the white clays of porcelain rather than that of the brown stoneware. Porcelain has its own problems to work with as well. The fine Porcelain clay is harder to throw clean forms out of in uniformly thin walled pots. The surface of the pots made of porcelain become loose and cause the glaze to crack off and pull away from the surface of the pot creating a crawling effect. Great care must be taken to Glaze thinly and apply coats of glaze and smooth them onto the surface of the pot. A gum arabic solution added to the glaze will help stop crawling.

With all the problems associated with the Oxblood Glazes, we can still see the results of our work when we get beautiful red pots with a minimum of flaws to crawling, pitting, and running. These are truly magnificent glazes which enhance our work. I tend to use several other glazes with the Oxblood Glaze to give a mottled effect and ensure that there is some benefit for my efforts.