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Melting Oxblood or Haematinum red glass |
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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 metalic 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.
Just about every possible color or phase is encapsulated in the sample shown here. 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. CuΉΉ→CuΉ→Cu In the case of
oxblood glass, the main coloring phase comes from cuprous oxide, or
Cu₂O. 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 1920s, 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. My oxblood melting
experiments I have wanted to make my own oxblood glass since the early 1990's. Slowly this idea sat simmering on the back burner until 2006. Starting around January 17, 2006 - I added black copper oxide to my clear cullet to create a copper blue I then stirred in some SiC. It reduced the copper but made a super fine-grained goldstone color with extremely small-localized bits of oxblood starting to show. In order to get the appropriate amount of copper into solution along with the SiC, a glass batch must be used. My melts were all 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 I started. I would add my colorants to this batch. Sand 10 Soda Ash Dense 3.5 Sodium Nitrate 0.8 Calcium Carbonate 1.75 Borax 5 mol 0.8 Kryolite 0.07 16.92 pounds Batch Experiments conducted February 11th through 16th - 2006. Number 1 Crystal Batch 16.92 Black Copper Oxide 3 Red Iron Oxide 2.25 Notes: Was hoping the Fe would act as a reduction agent alone made a nice emerald green. Number 2 Crystal Batch 16.92 Black Copper Oxide 0.6 Red Iron Oxide 0.9 Zinc Oxide 0.4 Notes: Emerald green again except this time it was snotty and full of seeds from the addition of Zn. Number 3 Crystal Batch 16.92 Black Copper Oxide 0.22 Red Iron Oxide 0.12 Black Tin Oxide 0.22 Notes: Added the tin oxide to see if it would act as a reducing agent it did not. Made a dark green. Number 4 Crystal Batch 16.92 Black Copper Oxide 0.50 Red Iron Oxide 0.22 Black Tin Oxide 0.22 Silicon Carbide (600) 0.3 Notes: Made a nice copper ruby that was somewhat localized and fugitive the batch was streaked with oxblood. These colors were contained in a dark green matrix. Reduction was finally supplied by the SiC. number - 5 February 16th, 2006 Crystal Batch 16.92 Black Copper Oxide 1 Red Iron Oxide - 0.9 Silicon Carbide (600 grit) - 0.9 Notes Perfect! 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. For more reading on this same subject, see the following articles: A Red Opaque Glass from Sardis and some Thoughts on Red Opaques in General Journal of Glass Studies- Vol. 30 (1988) page 16-27 Mark Taylor and David Hill - Roman Glassmakers Coloured Glasses Woldemar A. Weyl - Society of Glass Technology 1951 |
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