• de
  • es
  • en


Glossary of important terms for the material equitable use of the OPTUL Products

AK (english: COE):

German abbreviation for the expansion coefficient, technical correct thermal expansion coefficient, linear or cubic. This coefficient is measured with a dilatometer and correlates to a certain temperature range (see classification by AK/COE).

Heat tinting:

Glasses coloured with gold (0015, 4005, 4015, 4025, 4026) with silver (0165, 0185) or with selenium (all colours whose number begins with “1”, like the transparent tinting colour 1015 red or “2”, like opaque tinting colour 2015 red-opal). They change colour and/or transparency in different temperature ranges, in different dwell times at high temperatures, in repeated heating and/or cooling. The glass expert calls this process “Tinting”.


The crystallization of the surface of a fused flat glass, of a completely fused flat glass or of the smelting of a piece of Pâte de Verre is called “devitrification”. This develops if the smelting commodity is exposed to long the so called “critical devitrification temperature” or if substandard glass was used or a combination of both described factors. The critical devitrification temperature lies according to the used type of glass between 600°C and 700°C. To avoid the described devitrification you have to stride quickly through the mentioned temperature range while you are heating and cooling. The matt surface of a devitrificated product can be made shiny again by applying our gloss ULLSOLL-BF/II and reheating to not less than 700°C. If it is known that a certain flat glass devitrificates, applying of ULLSOLL before the fusing process is promising – a second burning is not necessary. The feature of the “erroneous devitrification” is used in the production of glass ceramics.


The “secret” of producing double-sided high-luster float glass is the floating on molten tin of the red-hot infinite pane before slackening. By this treatment the bottom (= tin-side) is polished. The upside receives this feature by fire polishing (= fire-side). It is possible to find tin residue on the tin-side especially by bulky panes. These residues are able to tamper with the colour of selenium panes. Because of this coloured glass should be fused in principle on the fire-side. The tin-side is to ascertained by ultraviolet light or by using the brush/water contraction.
It persists the common opinion that “float is float”. Anything but this is true. We detect differences in dilation in numerous samples of glasses of different origin.

On one's own account:

The required chunky crude glass for OPTUL products is produced according to our specifications in glass factories in Europe and in the USA. Normally, this is a complete port with a capacity of 300 to 400 kg, depending on the port size and the specific gravity of the colour glass. Before we call off the chunks, we test a sample of each melt on colour and hardness. The colour is compared with our colour standard, the hardness is determined with a Schneider voltage tester. Only when both parameters meet our standards, the chunks are retrieved and delivered to the refinement at OPTUL.

Classification by COE:

Of course, we know that the compatibility of glasses, depends not only on the linear, thermal expansion coefficient, but is also deteriorated by homogeneity, surface tension, viscosity, relaxation quality, etc. After recently the "COE-classification" was introduced, we also use this figure/denomination.
Normally the expansion between 20°C and 300°C is measured. For example COE 82 means, as measured by our standard float glass 82.83 in the thermal range 20/300 ° C and 87.09 in the range 20/400 ° C - determined by a approved Analysis Center. Our FF colours are set in this expansion. Compatibility testing is still essential.


Gold and selenium rubies appear dipped in water - the first step to be processed into granules - crystalline to bright yellow, greenish to bluish, the devitrificated similar colour, or a mixture of all these variants.The actual colour appears only after proper heat treatment. Mishandling causes discoloration - and opaque discoloration. We glassmakers call this “Lebrigwerden” (german).

Lötz colours:

The OPTUL colours 5010 yellow/gold, 5020 bluegreen/gold, 5030 green/gold and 5040 grey/gold were the base colours of the renowned art nouveau glass factory “Joh. Lötz Wwe.” in Klostermühle in the southern Bohemian Forest. They are smelted according to original recipes of our ancestor Johann Gottlieb Schneider, a supplier of Lötz. In order to achieve the gold effect granules are applied to the outer surface of the (blown) glass.
In order to increase the natural gold effect it is iridesced with "cottage Iris". This is done by spraying the completed piece of glass in a iridescent drum with the iridescent compound – this might also be repeated - and reheating.
The primordial recipe of the iridescent mixture - each glassmaker favours "his" special mixture - is in water dissolved tin chloride, diluted to an extend with alcohol that it can be sprayed smoothly according to the used spraying technique. The rainbow colours achieved by the tin chloride can be shifted to brown by adding ferric chloride, to blue by the addition of gold chloride, etc.
The usage of pure ferric chloride results in “Carnival glass” common in the USA. Pure gold chloride was used by Tiffany and Carder for their “Aurenegläser” (german).

Opal glass:

These are glasses whose label begins with a “2” or a “3” and the gold-pink-opal 4025. They can lose their opacity to partial or complete transparency at different temperatures or temperature treatments, but retain their original shade of colour. A typical example of these characteristics are the "jade and lapis Art Nouveau glasses" of Josef Riedel in Unterpolaun, the "Hyalith- and Lithyalingläser" by Friedrich Egermann in Blottendorf in Haida and more recently the ring mottles- and cats paw flat glasses from the USA.

Discoloration of selenium glasses:

In addition to the previously described tinting effects (heat tinting), the glasses change colour -, to a dirty gray and brown tone -for processing in highly reducing or strong oxidizing atmosphere in contact with glasses containing metal oxides or with organic substances - eg exposure to excessive use of fusing glue. This "error" can result in interesting colours, if properly implemented.

Compatibility test with a voltage tester (polarization device):

Two glasses are fused and observed under polarized light. Stresses appear in the white zone in rainbow colours, in the gray area in various shades of gray. This type of testing is the only one that provides accurate results - but polarization devices are expensive.

Compatibility test sample by string test:

Two small batches/chunks of the to be used glasses are melted above the other in the same amount and pulled into a string of about 300mm in length and a thickness of approximately 1x2mm and kept in suspense until the glass is completely cooled down. If the double string is staying straight, both glasses can be processed. If the string is bent, the glass of the concave side is too soft, respectively the convex side is too hard. This type of testing should only be applied to very small quantities of glass, for example, Wrapping beads.

Compatibility test by ring sample:

During the processing of multi-colored hollow glass is the "ring test" a necessary "must". Into a cylindrical blow mold of a diameter of about Ø 80-100 mm and the same height, a hollow body is blown to an approximately 4 mm wall thickness - the two glasses to be tested are to be used in the same quantities if possible. After a good relaxation time rings of about 25 or 30 mm height are to be blown out of the cylinder and cut in one place. If the ring stays closed, the hardness of both glasses are matching. Is the ring slideing over itself or opens, the glasses do not fit together. How far such a glass is usable, must be determined empirically.

Compatibility test of flat glass developed by the OPTUL developed simple and inexpensive method the EASY TEST stripmerger:

In four indentations of the ceramic mold (3 + 4 + 5 + 6 mm, corresponding to the most used flat glass thickness) as accurately as possible a matching stripe flat glass is placed and then either another flat glass is placed above or filled with the test colour granules. If the strip remains straight after smelting and cooling (relaxation), both glasses are compatible. Is the strip concave - seen from the support side - the lower glass is softer; is it convex, the lower glass is harder. The deflection is measured with a simple gauge and empirically tested how much deflection can still be called compatible.