Working with Glass Kiln – II


Dear Reader,

In our last technical article we have learnt about the use of kiln in glass. In this technical article , the focus is on compatibility of glass  for fusing.
For fusing we require  two or more sheet of glass. What is very important ,Is to see that the glasses we fuse , are ‘Compatibility’ .
Two different type of glasses are said to be compatible when they expand and contract with temperate change in the same pattern. The co-efficient of expansion (COE) of the two glasses should be he same. This property is called ‘Compatibility’ of those glasses .
For fusion , the compatibility test of the glasses must be done .The two incompatible when fused can result in cracking and breaking during cooling .The two pieces can be rendered waste after the whole process of fusion due the glass being incompatible, resulting in total loss.
When we heat glass in kiln, depending on the co-efficient of expansion, the glass expands and reverse happens when it is cooled, the glass contracts
The increase in linear length per degree rise in temperature is called Co-efficient  Of Expansion (COE) of that solid.

Type of Glass

Co-Efficient of Expansion

Expressed in whole number

Bulls Eye






Float Glass



Effetre (Morrith)



Spectrum System 96






Armstrong ,Bulls Eye, Desag, Freemant ,GNA kokomo , spectrum wissmach, youghiogheny ,are well known manufactures of fusion tested compatible glasss.

If we calculate effect of  ‘CEO ‘ of different types of glass,we find that one type of glass cannot be fused with the other, Example : Bulls Eye cannot be fused with spectrum glass. Lets calculate the change ;A 10 inch long glass when heated from 300C to 9000C the expansion in spectrum  Glass would be 0.083 inch and expansion in similar length of Bulls eye glass will be 0.0783 inch.
This difference may on first sight look to be too small or insignificant. This difference is quiet enough to break the glass and thus, Bulls Eye glass cannot be fused with spectrum Glass. At the Molecular level, this difference in ‘COE’ is very significant.

Glass manufacturing company make glass which are compatible to each others, at the same time , have different colours ,pattern and design. These glasses are stumped and marked for being compatible. The manufacturers test the glass for fusing compatibilities before making them as “Compatible”.

There can be two ways to ascertain the compatibility of glass.

  • To the pretested compatible glass by the manufacturer.
  • To test the different glasses to  be fused for compatibility, before going for the final project.


It is  always recommended that the compatibility test should be done by the artist even if the glass is marked as “Compatibility” as the temperature / Condition of the Laboratory
Of the manufacturer can be different to that of artist’s laboratory.

Even Glass with the same ‘COE’ cannot be fused together. It is not a big surprise

The fusion compatibility of tested glass is more expensive than non- tested glass. The manufacturer charge premium to do this test and mark it as ‘Tested compatible’.
Spectrum , American art Glass manufacturers have a range of coloured glass under a system called ‘SYSTEM 96’ i.e 96 COE glass. All glasses sold in this system, have 96 COE.

Spectrum iss Co-operated with Uroboros Glass studios in to produce glass which is compatible for fusing harmoniously with their range of System96 glass.

The glass under ‘SYSTEM 96’ of spectrum is ‘Tested Compatible’ for hot working. The products under ’SYSTEM 96’ are not only compatible but also formulated for multi – fire devit resistance.

The ‘tested Compatible’ (TC) glass is more expensive than the stained glass products. The ‘TC’ glasses are fundamentally different in their basic chemistries. These glasses are carefully manufactured to give stability to glass fusing , slumping and other hot glasses at work.

To differentiate of ‘TC’ glass with other glass . the ‘TC’ glass has a fled ‘SYSTEM 96’ logo.

The colour stability of fusing glass is also a big factor to be considered. The colour stays stable even after multiple fusing is to be ensured. Some colours are inherently more stable then other. So when selecting compatible fusing glass the colour stability at higher temperature is to be ensured to get desired results. Some colours become darker when fused at high temperature. To minimize this colour change effect, avoid keeping glass at peak temperature for long time.

Wissmoch CEO 94 has a similar range of compatible glass with s’CEO 94’. The colours and patterns available make a choice for the artist  to decide for any particular rang.

Float glass is also widely used for fusion. Float glass , manufactured by one manufactured at times, may not be compatible with float glass manufactured by the other. The range of CEO of various float glass manufacturers is from 83 to 88. it is advisable to fuse glass from the some sheet or preferably from the some batch of the manufacturer.

The ‘TC’ glass should be marked at short intervals indicating its compatibility with other glasses marked similarly.

The manufacturers of both tested compatible glass and non tested compatible glass and non tested compatible glass do not want any responsibility, they always mention that the user must test the glass for compatibility at his end even when the glass is marked ‘TC’.

So, the artists normally test small samples of glass before going for the final project.

To conclude, it is imperative that the glass be tested before fusing to avoid breakage and loss during fusion.

In the concluding TECHNICAL ARICLE PART III we will discuss about other material and glass products that can be fused.



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