Gas Fired Kilns

Installing a Gas Fired Kiln

There are a number of considerations about the location of a gas kiln.

Air exchange

The kiln should be placed in an area where there is good air exchange. For a number of reasons, it is usually convenient to place the kiln near an outside wall – ventilation and canister storage are the two most important.

Ventilation

There should be a low level vent to the outside to allow air to rise for the use of the combusting gas. There needs to be high level vent for the gas to escape. Ideally a hood with powered extraction would be installed.

Space

As with any kiln, you need to have sufficient space around the kiln to avoid heating any flammable materials. In the case of a gas kiln you need to be careful to avoid storing any thing that might ignite above the kiln. You can put heat resistant materials around the kiln if desired.

Location and storage of gas canisters

The safest place to put the gas canisters is outside the building. This does mean going outside to turn the valve on the gas canister on and off. However this is the safest place, should any fire start in the building, because the fire brigade can find and remove it from the fire. This placing applies to both the in use canister and any other full or empty canisters.

Detectors

Finally, even with these precautions, you should install a carbon monoxide detector. Carbon monoxide has no smell and can overcome you very quickly. Even a few minutes of exposure can leave you feeling ill for days.  

Keeping Copper Inclusions from Oxidising

The colour change in the copper foil is due to oxidisation - if the copper foil is completely deprived of oxygen it stays shiny and copper coloured. If you leave copper exposed at all it will go metallic blue or even bottle green, mostly it turns a lovely burgundy red colour- an intermediate oxidisation stage.

Klyr fire or borax solutions may help the copper stay bright.

Through doing some experiments with art school students, I have found the speed of firing is critical in an electric kiln. In a gas kiln the speed is normally fast anyway and produces better results than an electric kiln. It also is a kiln with a reducing atmosphere rather than oxidising one of an electric kiln.

Summary:

The main elements in keeping copper inclusions (and by extension, other metals) bright is to keep the metal from oxidising. Two elements are important in this:

• Keep oxygen from the metal
• Reduce the time the metal is exposed to high temperatures

Various methods are used to keep the metal from exposure to oxygen. Some of these involve: 

• coating the metal with fluxes to reduce the amount of oxygen in contact with the metal. 
• using a reducing atmosphere, such as a gas kiln. 
• placing an oxygen hungry material in the kiln with the glass and metal. 
• coating the metal with glass powder before encasing it within the glass.

Reducing the heat exposure of the metal also indicates that firing fast would provide better results. This requires very even heating within the kiln to avoid heat shocking the glass.  This is where a gas kiln is most advantageous - it can be fired fast without breaking the glass and it has a reducing atmosphere within it.

In general, it is easier to make use of the effects of the oxidised metal rather than striving for bright metal inclusions.

Super Glue - Kiln Forming Myths 29

The use of super glue in the kiln causes cyanide gas

This is not true.  But because it is such a persistent belief, a lot of detail is given below.  In short the precautions are: 

• use the minimum amount, 
• use an organic gas face mask, 
• do not wear natural fibres or gloves, 
• let the glue cure before placing it in the kiln, 
• have the solvents at hand while using the glue.

Super Glue Safety

Super glue is frequently used as a temporary fixative in assembly of kiln forming projects. There is some concern about safety, as it is known that super glue is made from cyanoacrylate, which it is feared will break down in the kiln into cyanide gas.

Greg Rawls, a certified industrial hygienist says

"I looked at the MSDSs for several forms of super glue. The main component is Ethyl 2-cyanoacrylate, which has a TLV of 0.2 ppm which is relatively toxic. [However,] the thermal decomposition products are carbon monoxide and carbon dioxide. I did not see a reference to cyanide gas. However, as I recall cyanide gas dissociates into elemental carbon and nitrogen at about 800 F. Since you use it in such small quantities, I would not worry about it. In my opinion the worst thing that could happen is you glue your fingers to the glass."

Safety issues

To treat the safety issues seriously and determine if you feel Greg Rawls' view is justified, you need to look at the issues of toxicity, reactions, adhesion of tissue, ventilation, first aid and decomposition products in the whole context.

Toxicity

The fumes from cyanoacrylate are a vaporized form of the cyanoacrylate monomer that irritate sensitive membranes in the eyes, nose, and throat. They are immediately polymerized by the moisture in the membranes and become inert. These risks can be minimized by using cyanoacrylate in well ventilated areas. About 5% of the population can become sensitized to cyanoacrylate fumes after repeated exposure, resulting in flu-like symptoms. It may also act as a skin irritant and may cause an allergic skin reaction. On rare occasions, inhalation may trigger asthma. There is no single measurement of toxicity for all cyanoacrylate adhesives as there is a wide variety of adhesives that contain various cyanoacrylate formulations.

The United States National Toxicology Program and the United Kingdom Health and Safety Executive have concluded that the use of ethyl cyanoacrylate is safe and that additional study is unnecessary. 2-octyl cyanoacrylate degrades much more slowly due to its longer organic backbone that slows the degradation of the adhesive enough to remain below the threshold of tissue toxicity, so the use of 2-octyl cyanoacrylate for sutures is preferred.

Reaction with cotton

Applying cyanoacrylate to some materials made of cotton or wool results in a powerful, rapid exothermic reaction. The heat released may cause serious burns, ignite the cotton product, or release irritating white smoke. Users should not to wear cotton or wool clothing, especially cotton gloves, when applying or handling cyanoacrylates.

Adhesion of the Skin

Various solvents and de-bonders can be used. These include:

·         Acetone, commonly found in nail polish remover, is a widely available solvent capable of softening cured cyanoacrylate

·         Nitromethane

·         Dimethyl sulfoxide

·         Methylene chloride

Commercial de-bonders are also available.

Warnings include:

·         It is a mild irritant to the skin.

·         It is an eye irritant.

·         It bonds skin in seconds.

·         Any skin or eye contact should be copiously flushed with water and medical attention be sought immediately.

·         Do not attempt to separate eye tissues – the bond will separate naturally within a few days.

Precautions

·         Use goggles.

·         Do not wear cotton or wool clothing while using super glue

·         Ventilate the area well. Since cyanoacrylate vapours are heavier than air, place exhaust intake below work area. Activated charcoal filters using an acidic charcoal have been found effective in removing vapours from effluent air so the bench top air filters are suitable for use while using super glue.

·         Avoid use of excess adhesive. Excess adhesive outside of bond area will increase level of vapours.

·         Assemble parts as quickly as possible. Long open times will increase level of vapours.

Evaporation Effects

·         The effects of heating cyanoacrylate are not completely known. The flash point is known to be greater than 85ºC. As a precaution do not remain in the area of the kiln after that temperature has been reached.

·         The decomposition products are carbon monoxide and carbon dioxide. There is no reference in the literature to cyanide gas. It is highly unlikely that heat will cause the release of cyanide gas at any time during the heating. To be certain, you should make sure the evaporation of the glue is complete before firing the kiln.

See this tip for the use of super glue in kiln forming. http://glasstips.blogspot.co.uk/2013/12/super-glue.html

Kiln Characteristics Investigation

Many people ask about the best kiln to buy.  Sometimes they mean the cheapest, but mostly they mean the best for their favoured processes. To get the best from your proposed kiln, you should be aware of its characteristics and how it fits your proposed kilnforming practice.  There are a range of factors that interact to give the special conditions of your kiln.  They range from the purpose, the materials of construction, the placement of heating elements, how it opens, and its shape.  All these can affect the degree of even heating of the kiln bed or shelf.

Kiln types

There kilns for many purposes. Some of them are powder coating of metals, enameling of metals, vitreous painting of glass, glass forming, ceramics, casting of glass and metals, lehrs for annealing, and furnaces among many others.  

Large powder coating kiln

Large enameling kiln

Jewellery enameling kiln

Electric glass painting kiln with multiple shelves

Example of a sheet glass annealing lehr

For our purposes we are concerned with the glass and ceramics kilns.

Ceramics vs Glass Kilns

In general ceramics kilns are made to lose heat slowly, while glass ones are designed to lose heat relatively quickly.  There are many glass kilns based on ceramic ones.  You should be aware of the differences between kilns designed exclusively for glass and those based on ceramics kiln designs.

Small ceramic kiln

Small glass kiln

Construction Materials 

The materials used in constructing kilns are refractory insulation and a steel structure of a design to hold all the refractory materials together. 

Refractory bricks for glass kilns are light weight and usually designed for temperatures under 1200°C (dense bricks rated much higher are normally used in ceramic kilns). 

Light weight refractory brick

Bricks tend to be used in most glass kilns on the floor as well as the walls (some smaller ones use only refractory fibre).

Small fibre kiln

Kilns derived from ceramics tend to have brick walls and lids.  Most kilns designed for kilnforming have fibre walls and lids.  In the cases of top hat opening kilns, fibre is a necessity to reduce the weight of the lid.

Fibre board and fibre blanket are used widely.  The floor tends to have a floor consisting of steel, fibre board on top and brick on top of the board. Fibre blanket tends to be used on the walls and ceilings of rectangular glass kilns. Oval and circular ones tend to have brick walls and ceilings.  The use of fibre board and blanket walls and ceilings leads to a more rapid cooling than those with brick ones.  This will affect the scheduling of the kiln firings.

The steel used to contain and support the refractory materials is important.  Many kilns use mild steel in sheet form to fill the spaces between the heavier structural support steel.  The higher quality kilns use stainless steel sheet, even though they may use mild steel for structural support.  The stainless steel lasts much longer than mild steel, especially when there is liable to be moisture involved in the kiln processes, such as pate de verre or casting.

Opening Method

This post gives a description of the common methods of opening the kiln.  

The purposes for which you want to use the kiln relate to the firing characteristics needed.

Top opening

Top opening kilns have the advantage of depth, normally with elements around the sides.  This makes them good for casting, but not so good for processes that need observation or manipulation.  The depth is most useful in casting  and deep slumping work, but requires a lot of experimentation to make use of multiple shelves in one firing.

Front opening kilns have the advantage of being able to observe the whole depth of the firing, if you protect yourself from the heat that will be dumped from the kiln.  They often have elements on the sides which is an advantage for drops and melts (when observation is necessary).

Top hat opening kilns are those that have the whole heating chamber hinged at the shelf level.  These are very good for placing of work, as you can work directly above the pieces.  These are one of the best types of kiln for combing or any other manipulation of the glass during the firing. You can also observe by opening the kiln a little during the firing.

A range of top hat and a bell kiln

Bell kilns are those where the whole of the heating chamber lifts above the bed.  These are often equipped with two bases which can be wheeled in turn under the chamber which is lowered before firing.  These tend to be very large kilns.

Small gas fired kiln

Heat source

Most kilns are heated with electrically powered elements, either exposed or in quartz tubes.  The quartz tube contained elements provide more even heating than the exposed ones.  The most even heat is provided in gas fired kilns, although these are generally more expensive and less widely available.

Element Placing  

The location of the heating elements can have a significant influence on the way you fire your glass.

·        Top fired kilns are generally the easiest to use as the glass is most affected by radiant heat.

·        Side fired kilns provide the radiant heat to the edges of the glass first, before the air temperature can begin to affect the surface of the glass.  This means more caution is required in the heat up of the glass.  However, side elements are very useful in drops and casting processes.

·        Some kilns have both top and side heating elements.  This provides flexibility in heating up and in cooling evenly.

·        A few kilns have elements around the sides but below the shelf.  This promotes even cooling of glass from both the top and bottom. It is most useful in dealing with the cooling of thick slabs.

Kiln sizes and shapes

Kiln sizes have an effect on the behaviour of the kiln.  Smaller kilns (depending on the refractory materials) generally heat and cool quicker than large ones.  The mass of a larger kiln takes more energy to heat up and more time to release the heat than smaller ones do.  This will influence the scheduling for different sized kilns.

 

The shape of the interior of the kiln affects the distribution of heat within the chamber.  Rectangular kilns tend to have cooler corners than circular ones (as there are no corners).  Oval kilns tend to give space for longer pieces and reduce the cool corners.

 

The height of the kiln also affects the heat distribution within the kiln.  Taller kilns are cooler at the bottom than the top, even with side elements.  They are especially good for casting and drop processes.  Deeper kilns, even if rectangular, require more energy to complete any given process, because of the distance between the radiating elements and the glass.

Hot and cold spots can be tested for by using this method.  The actual operating temperatures can be tested by the use Orton cones to measure heat work. This depends on the speed used to get to the process temperature.

There are many factors that make up the characteristics of kilns. The main ones are style, construction materials, opening method, shape and depth. These need to be considered in relation to the kind of kilnforming you intend doing, to make the selection optimum for your practice.

More information is available in "Your New kiln" from Etsy shop VerrierStudio: https://www.etsy.com/uk/shop/VerrierStudio

or direct from stephen.richard43@gmail.com

Super Glue Safety

Super glue is frequently used as a temporary fixative in assembly of kiln forming projects. There is some concern about safety, as it is known that super glue is made from cyanoacrylate, which it is feared will break down in the kiln into cyanide gas.

Greg Rawls, a certified industrial hygienist says "I looked at the MSDSs for several forms of super glue. The main component is Ethyl 2-cyanoacrylate, which has a TLV of 0.2 ppm which is relatively toxic. [However,] the thermal decomposition products are carbon monoxide and carbon dioxide. I did not see a reference to cyanide gas. However, as I recall cyanide gas dissociates into elemental carbon and nitrogen at about 800 F. Since you use it in such small quantities, I would not worry about it. In my opinion the worst thing that could happen is you glue your fingers to the glass."

Safety issues

To treat the safety issues seriously and determine if you feel Greg Rawls' view is justified, you need to look at the issues of toxicity, reactions, adhesion of tissue, ventilation, first aid and decomposition products in the whole context.

Toxicity

The fumes from cyanoacrylate are a vaporized form of the cyanoacrylate monomer that irritate sensitive membranes in the eyes, nose, and throat. They are immediately polymerized by the moisture in the membranes and become inert. These risks can be minimized by using cyanoacrylate in well ventilated areas. About 5% of the population can become sensitized to cyanoacrylate fumes after repeated exposure, resulting in flu-like symptoms. It may also act as a skin irritant and may cause an allergic skin reaction. On rare occasions, inhalation may trigger asthma. There is no single measurement of toxicity for all cyanoacrylate adhesives as there is a wide variety of adhesives that contain various cyanoacrylate formulations.

The United States National Toxicology Program and the United Kingdom Health and Safety Executive have concluded that the use of ethyl cyanoacrylate is safe and that additional study is unnecessary. 2-octyl cyanoacrylate degrades much more slowly due to its longer organic backbone that slows the degradation of the adhesive enough to remain below the threshold of tissue toxicity, so the use of 2-octyl cyanoacrylate for sutures is preferred.

Reaction with cotton

Applying cyanoacrylate to some materials made of cotton or wool results in a powerful, rapid exothermic reaction. The heat released may cause serious burns, ignite the cotton product, or release irritating white smoke. Users should not to wear cotton or wool clothing, especially cotton gloves, when applying or handling cyanoacrylates.

Adhesion of the Skin

Various solvents and de-bonders can be used. These include:

Acetone commonly found in nail polish remover, is a widely available solvent capable of softening cured cyanoacrylate

Nitromethane

Dimethyl sulfoxide

Methylene chloride

Commercial de-bonders are also available.

Warnings include:

• It is a mild irritant to the skin.
• It is an eye irritant.
• It bonds skin in seconds.
• Any skin or eye contact should be copiously flushed with water and medical attention be sought immediately.
• Do not attempt to separate eye tissues – the bond will separate naturally within a few days.

Precautions

• Use goggles.
• Do not wear cotton or wool clothing while using super glue
• Ventilate the area well. Since cyanoacrylate vapours are heavier than air, place exhaust intake below work area. Activated charcoal filters using an acidic charcoal have been found effective in removing vapours from effluent air so the bench top air filters are suitable for use while using super glue.
• Avoid use of excess adhesive. Excess adhesive outside of bond area will increase level of vapours.
• Assemble parts as quickly as possible. Long open times will increase level of vapours.

Evaporation Effects

• The effects of heating cyanoacrylate are not completely known. The flash point is known to be greater than 85ºC. As a precaution do not remain in the area of the kiln after that temperature has been reached.
• The decomposition products are carbon monoxide and carbon dioxide. There is no reference in the literature to cyanide gas. It is highly unlikely that heat will cause the release of cyanide gas at any time during the heating. To be certain, you should make sure the evaporation of the glue is be complete before firing the kiln.

See this tip for the use of super glue in kiln forming.

Causes of Large bubbles

 Let’s think about moisture and large bubbles from under the glass. It is not the water, but the gasses created by the decomposition of materials that can cause the bubbles. There are other causes of large bubbles too. The most common causes are discussed here.

The usual explanations are:

• ·        Uneven shelf
• ·        Heat resistant particles under the glass
• ·        Uneven heating
• ·        Glues
• ·        Organic material
• ·        Moisture
• ·        Amount of gas

 

image credit: Warm Glass

Uneven shelf

Shallow depressions in shelves can cause large bubbles. Occasionally, the shelf can be damaged in various ways causing scratches or dings in the shelf. Air can be trapped in these depressions. And it does not take much volume of trapped to be a problem. The heat of kilnforming causes the air to expand. As the glass becomes less viscous with increased temperature, the pressure from the expanding air forces the glass upwards. The amount of air and the amount of heat work combine to create bubbles from simple uprisings to large thin walled or even burst bubbles.

There are some things that can be done to detect and avoid bubbles from forming. It is possible to screed powdered kiln wash over kiln washed shelf. This gives pathways for the air to escape. It does leave a more marked bottom surface than kiln wash.

Using 1mm or 2mm fibre paper allows air from under glass. You can maintain a relatively smooth surface with Papyros or Thinfire over the fibre. Even Thinfire or Papyros on its own will allow air from under the glass.

Checking for depressions can be done by spreading kiln wash powder over shelf and drawing a straight edge over the shelf. Depressions will be shown by the presence of the powder. It can also be done with powdered glass frit.

Particles under glass

Any particle resistant to kilnforming temperatures holds the glass up while it is forming so creating an air space. It is important to ensure the shelf is clean as well as flat. Small pieces of grit or dirt that are resistant to high temperatures will hold the glass up from the shelf enough to create a bubble – small or large depending on the temperature. Vacuuming the shelf before adding anything to the surface before each firing is important to bubble free results.

Uneven heating

This is sometimes cited as a cause of bubbles. If so, the heat would need to be very localised. This is possible if the glass is very near elements. In general, the temperature is equalised at a distance equal to the width of the elements.

Glues

A wide variety of glues are used in kilnforming. Those available to enthusiasts all burn away leaving gasses between layers. These gasses - if trapped - can thin the glass below as well as above the glue’s position. This will give the impression that the bubble has come from between the shelf and the glass. Most often the bubble forms between the glass layers, pushing a bubble only into or through the top layer. The solution is to avoid using glue or minimise it and place it only at the edges.

Organic material

Organic materials can be a problem. When you are using a large or thick fibre paper sheet under a piece of glass, occasionally the gasses from burning out of the binder can be great enough to create a bubble. Although normally, this only leaves a grey to black mark on the underside of the glass. Vermiculite boards need to be fired before use, as they contain significant amounts of binder.

Inclusion of organic materials such as leaves, twigs, or bones, leads to bubbles. Very long soaks below the softening point of the glass are required to allow the organic material to burn out of the objects.  The time required increases from an hour for leaves to 24 for bones.

Moisture

Moisture is very often cited as the source of bubbles. It is possible that the steam from water may be trapped in shelf depressions, or the areas held up from the shelf. And anytime there are no precautions to allow the air from under the glass, or between sheets bubble formation can be promoted. If adequate precautions are taken (flat shelf, clean shelf, bubble squeeze) the moisture will evaporate before the glass is hot enough to form a seal around the edges and trap any steam. It is another good reason for moderate ramp rates at the beginning of a firing.

Amount of gasses

Of course, if there is a lot of moisture there can be problems. Simply applying kiln wash in four coats does not leave enough water in the shelf to be a problem.

If you have washed the kiln wash off a mullite shelf, there will be a lot of water in it even after it feels dry. Then it does need to be kiln dried before use. To avoid breaking the shelf you need to fire slowly to 99°C/210°F and soak there for a couple of hours with the vents open or lid propped up a little to allow the moisture out of the kiln.

 

 

Boron Nitride

What is boron nitride? What makes it a good separator?

Boron nitride is a heat resistant refractory compound of boron and nitrogen with the chemical formula BN. It is also chemically stable at elevated temperatures.  It exists in various crystalline forms that are similar to a structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN forms.  It is the form most useful in kiln forming as a smooth release separator, especially for steel.  It is also used as a high temperature lubricant, and has a wide use in cosmetic products.

There is a cubic form that is similar to diamond (called c-BN), but softer.  It has a superior thermal and chemical stability.  There is a harder form called wurtzite, but which is rare. Neither of these is of much use in kiln forming.

Hexagonal BN

Hexagonal BN (h-BN) is the most widely used form of boron nitride. It is a good lubricant at both low and high temperatures (up to 900C, even in an oxidizing atmosphere). Another advantage of h-BN over graphite is that its lubrication properties do not require water or gas trapped between the hexagonal sheet layers. So, h-BN lubricants can be used even in vacuum, e.g. in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads.  In kiln forming, the high temperature lubricating properties are made use of as separator between metal, ceramic and other supporting materials for the glass.

“Hexagonal BN was first used in cosmetics around 1940 in Japan. However, because of its high price, h-BN was soon abandoned for this application. Its use was revitalized in the late 1990s with the optimization h-BN production processes, and currently h-BN is used by nearly all leading producers of cosmetic products for foundations, make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products.”   
https://en.wikipedia.org/wiki/Boron_nitride

It has wide application in materials to give them self-lubricating properties.  Boron nitride has the properties of stabilisation of materials, reducing expansion and resistance to electrical conduction, making for wide use in plastics and electronics among a wide variety of other products.

Health and Safety

There are some health issues related to its use.  It is reported to have a weak association with the formation of fibrous material in the lungs and so result in pneumoconiosis when inhaled in quantity in particulate form.  It is best to wear a dust mask when applying and to do it outdoors, as simple ventilation will not prevent dust settlement indoors.