Working With GLASS KILN = Part I



Dear Readers,

Typical Glass Kiln

Glass can be worked upon at three ranges of temperature.

  1. Cold Working

When working on glass at room temperature, this type of processing is used in making Stained Glass, Glass Etching, Glass Engraving and Sand Blasting etc.

  1. Warm Glass

Warm Glass is a misnomer. At temperature range of 6000 C to 925 0  C the glass is hot not warm. This work is done with the help of Kiln. Glass Slumping, Fusing and Combing are examples of this segment.

  1. Hot Glass

When working with glass above 10000 C. This working is done with furnace or Glass blowing.

In this article, we will discuss working with Warm Glass. Warm Glass working is the art of working at temperature range between 6000 C to 9250 C. This is the temperature at which the glass is in a semi solid to fluid state.

  1. Glass Fusing
Glass Fusing is the work, by which two or more pieces of glass are joined together permanently by heating them in a kiln.   Kiln is a hot oven with refractory lining wherein the temperature of the article, kept in it, can be controlled and raised up to 12000C. The heating is normally done Electrically, but other means of heating like Gas (LPG), Oil and Wood can also be used. With rise in temperature, the glass tends to soften, with further rise in temperature, the glass turns fluid and flows.  Two pieces of glass when heated together, will soften, then melt and become liquid and flow into each other. Thereafter, on cooling, they would have amalgamated to become a non separating single unit

Fused Glass Pendant


A phase of fusing is “Slumping or Sagging”. This is the process, by which, the glass is made to flow and take shapes of the moulds on which it is kept, by gravity. The glass during slumping can be one glass or an already fused two or more glasses. The molten glass will take the shape of the mould. You can do slumping (Flow due to Gravity) of Sheet Glass (Large Size), as well as small pieces. Bowls, Ashtrays, Door Panels, Cups and many similar items can be made with this process.

The moulds, used for slumping or sagging are made of stainless steel, ceramic material, mixture of moulding plasters, fibre papers and also at times of clay.

Glass Slumping

The mould for slumping can be for one use or multiple uses. Depending upon the number of times the mould is to be used, the material of construction is decided. Clay or rock moulds cannot be used for a number of times.

Other additives added to change the colour of glass are Metal and Metal Oxides.

  1. Iron (IO2) Oxide : Bluish Green Glass

Add Chromium   : Red Green Glass.
Used in making bottles in Wine and Beer industries.

  1. Sulphur :Used with Carbon and Iron PolySulphides Amber Colour Glass sssss.

More addition can result in Yellow to Black Glass.

  1. Manganee Can remove Green Colour tint due to Iron. It gives puple Colour.
  1. Selerium  Small Quantities are used to decorative glass. Large quantities give Pink to Reddish Colour. Add Cadmium Sulphide for Brilliant Red Colour. This Glass is Called Selerium Rub.
  1. Cobalt Added to give Blue Glass. The result are very good if it is Potash Glass.
  1. Copper Oxide Added to give Turquiose Colour.
  1. Pure Copper Metal to get Dark Reds Colour, Almost OPAQUE Glass, Also called “Ruby Red Glass”.
  1. Metal As the concentration increases colour firm Blue to Voilet to Black is obtained.

However Nihol and Cobalt is used to  “Decolourise Lead Glass”.

  1. The Oxides Antromany and Arsenio Oxides.Added to make “White Glass  looks


  1. Chromium with Tin Oxide and Amonio Added to get Emerald Glasss.


  1. Cadmium and sulphur Added to give Deep Yellow Colour.
  1. Thanium  Added to get Yellowish Green Glass.


  1. Gold Added with email concentration (.001%) provides Rioh Ruby coloured Glass.
  1. Uranimum added to get Flurorecant Yellow and Green Colour.


  1. Silver added in compound form or Silver Nitrate, Produces Orange Red Yellow Glass.

Chemical composition in term of % (percentage ) by weight of various common type of glass.


Type of glass



Soda lime



Borate Silicate.



60 %– 70%

70% - 81%


54% - 65%




2% – 7%





5% - 12%












12% - 18%

4%  - 8%


13% - 18%










7% - 12%








18% - 30%


Chemical Properties of Glass: -

The Glass is mostly chemically inert. The corrosion of glass in contact with atmosphere and aquous solution takes a very long period.

The change (Chemical) depends on the composition of the solution reacting to it.

  1. Glass is a good thermal conductor.
  2. Glass is an Electrically Insulating Material.It does not conduct electricity.
  3. When light falls on glass , partly it is reflected at the surface, partly it is absorbed and part of light is transmitted.
  4. Coloured glass transmits only selective wave length of the light spectrum.


Glass normally does not transmit Ulra Violet Radiations (of short wave length) .Glass transmit infrared Radiations (Long Wave Length) – This turns the glass hot on other side also.

Mechanical Properties Of Soda – Lime – Silica Float Glass

Module of Raptue ,Mean Mor (50% Probability of breakage).

6,000 pel (41 MPa) Annealed
12,000 pel (83 MPa) Heat strengthened
24,000 pel (165 MPa) Tempered

Design Stress for 0.8%
Probability of breakage

2,800 pel (19 MPa) Annealed
5,600 pel (39 MPa) Heat Strengthened
11,200 pel (77 MPa) Tempered

Module of Elasticity (Youngs)

10.4 X 106 pel  (72 GPa)

Module of Rigidly (3hear)

4.3 X 106 pel (30 GPa)

Poieean’s Ratio


Bulk Module

6.2 X 10n pel (43 GPa)


156 lb/Fo (2530 kg/ Mn)

Coefficient of Thermal Stress

50 pel /FO (0.62 MPa /OC) 

Thermal Conductivity of 76 FO

6.5 Btu.In/hr. OF. (0.937 W.m/m2 .C2)

Specific heat at 76FO

0.21 btu/lbnF (0.66Kl/Kg.C)

Coefficient of linear Expansion (75 – 575F)

4.6 X 104 in /in Fo (8.3 X 10-4 mm/mm .OC)

Hardness (Moh’s Scale)


Saffening Point (ASIM C336)

1319OF   (716OC)_

Annealing Point (ASIM C336)

1018OF   (548OC)

Strain Point (ASIM C336)

952OF   (511OC)


Fusing has other variation also


It is a technique,by which the shape of the glass is distorted or changed while it is not, to create intricacies.

Fine Polishing

Polishing of glass surface to make it shiny and smooth, during healing of glass in the kiln is called ‘Fine Polishing’.

Kiln Casting or Casting of Glass

This is a technique , by which glass is melted in the kiln in a mould. The glass gets casted as per the shape of the moulds. The molten glass can be poured into various moulds and allowed to cool at room temperature to get shape of the moulds, Firozabad in U.P is well known for the glass casting art. Very beautiful artistic glass articles are made by casting of glass and are exported to countries all over the world.
The basic machinery and equipment, required to work with warm glass are:-

  • The kiln.
  • The Moulds and
  • Separators.


The kiln is the most important machinery for warm glass working.
Most of the kilns are electrically heated but due to economical and geographical reasons, Gas Fired or Oil Fired Kilns are also in use.
The kiln should be energy efficient. To make it energy efficient ,loss of heat is controlled by using Refractory Bricks     and other heat resistance insulating material.
Glass kilns normally have a high temperature achievement rate with in short period, Most of the kilns varies according to requirement. Small kilns normally have a working area of 30 Sq. cm. Large kilns can be of the size of 5 meters X 5.5 meters working area.
The very important feature require in the kiln is the temperature indicator and controller. Modern kiln have programmer and sequencers which are computer controlled. The helps to maintain. The gradual increase (during melting) and decrease in temperature (during cooling) of the kiln can be maintained and controlled as per the need to achieve the desired results.
The refractory, the mould and the shelves (where glass is kept) in the kiln are suitable for repeated use. They can take temperature upto 7600C and subsequent cooling also to room temperature.

Separators or kiln wash

During warm glass working. The most important problem that needs absolute precaution, is to prevent the glass from sticking to the moulds. Fibre paper is commonly used for this purpose.
Fibre paper is a special kind of paper that acts as a separator. It can with stand glass fusing temperature without sticking to glass, on walls of the kiln and at the same time not being burnt or so itened.
Some emulations also known as ‘Kiln Wash’ can also be applied to the shelves and moulds to avoid glass sticking to moulds and shelves.
To accomplish the non sticking requirements sprays are available to be put on the moulds before fusing.

Safety Equipment likes gloves and protective glasses must be used while working with warm glass. Some kilns have sight glass through which you can watch the activities inside the kiln.

Bending of Glass can also be achieved by heating the glass in the kiln. This is a kind of slumping and sagging along the mould designed to bend glass to the required angle. The glass when hot ,get softened and due to gravity takes shape along the bending can be controlled by regulating the temperature and the soaking period.

Stages of Slumming, Blending And Fusing.
The complete process can be divided into five period or stages.

  1. Healing stage.


This is the initial heating stage. The temperature is raised by heating from room temperature to required temperature for fusing and slumming

  1. Soaking Stage.


This is the constant  temperature zone where temperature is maintained for a particular period of time .

  1. Fast Cooling Stage


The temperature is made to decline from the heating temperature to the annealing temperature.

  1. Annealing Stage.


The slow gradual cooling is required to released the stress in the glass.

  1. Room temperature cooling stage.


The glass of this stage is allowed to cool to room temperature. We must understand these stages in order to obtain perfect fused glass without undue stress.
The complete fusing . meaning thereby the getting of two or more glasses into each other to form one piece. This takes place of above
(815 0C). To obtain this temperature the glass in heated from room temperature to (815 0C) . At this temperature the glass acquires a fluid state and is able to flow.

The reaching of the temperature (8150C) can be divided into futher three stages.

  • From Room temperature to 5400C. At room temperature, the glass is solid , hard and brittle. As the temperature increases s, it expands due to thermal expansion. This is a stage where glass can develop cracks or break or deform. Precaution must be taken to control the gradient of increase in temperature increase. This breaking / cracking of glass during this stage is called “Thermal Breaking” Which is the correct gradient to be used for temperature rise depends on factors like type and thickness of glass (This topic was discussed at length in my previous article “Property of Glass” which you may refer. ) at 5400C,there is no moisture or contamination in glass as it gets burnt off. The surface of glass turns glossy is also not very  hard but softens to an extent. After this temperature is obtained the worry of thermal break age / thermal shock is over as there is no rigidly.
  • The second stage is form 5370C to 7000C. The glass is glossy and very so it, its shape is now affected by gravity. Glass tends to move along the mould and needs a support. The colour of glass is yellowish red giving a hot burning look. The different pieces of glass are now sticky and tends to penetrate in each other. If working with a single single glass . it will result in Slumming and Sagging stage. Slumming take place at this temperature range in most of the cases.


  • The last phase is obtained by continued heating above 7320C to 8200C . the conners get rounded off and glass has thrown its weight on the mould. Glass moulds itself into the shape of the mould and stretches out to reach its limit. The glass is liquid at this temperature and is free flowing.

Other form of art of fusing called Pate de Verre, takes places at higher range than 8160C . if glass is allowed to heat above 8160C , the glass is completely liquid and can be moulded in pots . this is the art of fusing a paste, made with small pieces of glass in the mould. The paste in embedded in glass or layered on the surface of glass .

Glass Combing

As the name suggests , in the temperature range of 9000C to 9500C, this action is undertaken. With the help of tools. The heated glass is manipulated in various shapes. This is also done to introduce metallic or other external object into the glass.
The glass can be raked and made to flow in unconventional direction by combing.

Heating Stage

When we go further to temperature range (8160C – 9160C). The glass can be poured into the moulds like other metals. The glass can be casting into various shapes. Glass casting is very popularly done in India. Large crucibles of molten glass are prepared and glass is then poured in small moulds to be cast intricate shapes.
This is the first and most energy consuming stage of fusing. Called the Heating Stage.

Soaking Stage

This is the stage, that starts at the peak temperature of the process. So the starting of this stage is at 6480C 7000C, in case of slumping and 8160C for fusing. The soaking time period will  vary with the type of work , thickness of glass and type of glass. Increase is soaking time during slumping. Will make glass adhere more to them mould . in case of fusing the enhancement of soaking time will result into making the fused glass smooth and flatter.
The duration of soaking is between 5 minutes, to 60 minutes depending on the work activity and property of glass.
The need of soaking time is to allow the molecules to realign as the glass moves from solid stage into liquid stage. Soaking period is the time , in which , the glass attains its final position and shape.

Fast Coating Stage

The red hot glass is made to attain its original colour at this stage. To do this, the door of the kiln is opened for a short duration to allow the trapped hot air to escape, the need for such fast cooling is to avoid devitirication.


Devitrification  is formation of crystals on the surface of glass, during cooling. Devitrification  makes the surface of the glass  ugly. In the cases , the glass is devitrified as some artist have a liking for it. This is stage must be avoided , unless being done purposely. To devitrification  is done by spraying borax and water and other devitrifying sprays available for this purpose, before fusing.

Fusion glass manufacturers also produce glass that resists devitrification.

Annealing Stage

This is an important stage. It helps relieving the stress in the heated glass. As the annealing stage starts, the glass has already attained its colour. Stress is caused by rearrangement of glass molecules when heated. As the glass cools and solidifies, (change from liquid to solid) the molecules tend to reach their original position and glass attains hardness. Stability of glass depends largely on the annealing process.

Temperature is divided into three most important points.

Upper Annealing Point

The temperature at which the glass changes the phase from liquid to solid. .

The Annealing Point

It is an intermediate temperature, at which, the molecules start realigning themselves thereby releasing the stress between them.

The Strain Point

The nearing end point of annealing zone. The glass now solidifies. The strain is being removed.

Any amount of stress, left after the strain point/annealing zone, cannot be removed. It would require reheating and again annealing to remove these stresses.

The annealing is the best, when done at the temperature very near to annealing point. The temperature at the annealing point can be reduced slowly until it reaches below the strain point. The slower reduction of temperature, ensure that the stress does not get reintroduced during reaching the strain point.

Annealing zone and annealing temperature have to be tested for a particular type of glass. Glass manufacturers mention these temperatures in the technical data of the glass.

The best results are obtained by having a very slow temperature reduction gradient. This will overcome the complications of annealing but take longer time in the process.

Therefore, a good glass technician maintains a log of these parameters for various types of colour, thickness and manufacture of glass being fused.

The Room temperature Cooling Stage

Upon completion of  Annealing this stage to be tackled. The glass is allowed to cool slowly to room temperature. This is not difficult. No external induced drought of air is used. Natural cooling process is the best with adequate ventilation. The need is to prevent thermal breaking/thermal shocks and air traps. The cooling rate will also depends on the temperature difference between the glass temperature or annealing zone and the room temperature. If there is rapid cooling, the kiln can be fired for short intervals to avoid thermal shocks.

In the continued article, part second, we will learn about compatibility of glasses to be fused, the items that can be used to decorate fused glass like Fritz, Decals etc, which needs an indepth understanding, while working with the warm glass.



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