Showing posts sorted by date for query gas kiln. Sort by relevance Show all posts
Showing posts sorted by date for query gas kiln. Sort by relevance Show all posts

Wednesday 31 May 2023

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.

 

 


Wednesday 29 December 2021

Mineral Wool Fibres


Refractory Fibres


The general name that includes refractory fibre is mineral wool. It is any fibrous material formed by spinning or drawing molten minerals and ceramics.  These are used as thermal insulation, filtering, soundproofing and as a hydroponic medium, in addition to high temperature insulation as in kilnforming and furnaces.

The initial manufacture of mineral wool was in Wales in the mid-19th century, but the process was so dangerous that it was abandoned. The first commercial production was in 1870’s Germany, manufactured by blowing air through a fall of molten slag metal.  At the end of the century an American developed a technique to turn molten rock into fibres, so initiating the rock wool industry.  The high temperature versions were developed during the second world war, but not commercially available until the 1950’s.

Current manufacturing involves a flow of molten minerals (at ca 1600°C) through which air is forced.  This creates fibres of amorphous structure that can be compressed together without binders.  More advanced production rapidly spins molten minerals similar to the production of candy floss, or cotton candy. This results in a mass of fine, intertwined fibres with a typical diameter of 2µm to 6µm (microns).


Credit: Knauf.com


High-Temperature Mineral Wool


High temperature mineral wools are rated for about 650°C to 1600°C and are made in similar ways to the lower temperature versions.  However, they are more expensive and so are used in refractory circumstances including kiln forming.

The three main types of HTIWs include:

Low Bio-persistent (LBP) Wool, including Alkaline Earth Silicate (AES) wools and others:

Alkaline earth silicate (AES) wool
       Calcium magnesium silicate wool
       Calcium silicate wool
       Magnesium silicate wool
Alkali metal silicate (AMS) wool
       Potassium alumino silicate wool

Alumino Silicate Wool (ASW), also known as Refractory Ceramic Fibres (RCF)
       Aluminium silicate wool
       Aluminium zirconium silicate wool

Polycrystalline Wool (PCW)
       Aluminium oxide wool
       Mullite wool

The main forms that kilnformers are interested in are blanket, paper and board.  The paper and board normally contain binders ranging from latex to cellulose. There are other forms: bulk fibres, modules or blocks formed ready for installation, vacuum formed shapes, cement mastics, textiles, yarns and ropes.


A brief description of these kinds of refractory mineral wools are:

Alkaline earth silicate wool (AES)

AES wool consists of amorphous glass fibres that are produced by melting a combination of calcium, magnesium oxides and silicone dioxide.  Products made from AES are generally used in equipment that continuously operates and in domestic appliances. AES wool has the advantage of being bio-soluble—it dissolves in bodily fluids within a few weeks and is quickly cleared from the lungs and so has been excluded from carcinogenic classifications. It is generally rated up to 1200°C.

Alumino silicate wool (ASW)

This is also known as refractory ceramic fibre (RCF), again consisting of amorphous fibres produced by melting minerals and blowing air across the flow.  In this case, a combination of aluminium oxide and silicon dioxide.  It has a low thermal conductivity, and good resistance to chemicals. Alumino silicate wool is generally used at temperatures from 600°C to 1300°C  for intermittent operation, making it good for kilnforming. 

This was classified in Europe as a carcinogen category 2 – “Substances that should be regarded as if they are carcinogenic to humans” under the Dangerous Substances Directive in 1997. This was translated under CLP Regulation into a carcinogen category 1B “Known or presumed human carcinogen; presumed to have carcinogenic potential for humans, classification is largely based on animal evidence”.

Some of the trade names used are:
  • Kaowool®, a high-temperature mineral wool made from kaolin. It was one of the first types of high-temperature mineral wool and continues to be used. It can withstand temperatures to 1250°C. 
  • Cerablanket®, is a spun blanket manufactured from a high purity blend of alumina-silica and is classified up to 1315°C.
  • Cerachem® and Cerachrome® provide chemical stability and strength and have acoustic as well as thermal insulation characteristics. They are classified to 1426°C.

There are bio-soluble fibres produced under trade names such as Superwool® with temperature ratings of 1300°C and 1450°C.  Superwool® fibres are exonerated from carcinogen classification within Europe and not classified as hazardous by IARC or under any national regulations throughout the world.

Polycrystalline wool (PCW)

Polycrystalline wool was commercialised in the 1970’s and consists of fibres that contain more than 70% aluminum oxide. It is produced by sol–gel method from aqueous spinning solutions. The water-soluble green fibres obtained as a precursor are crystallized by means of heat treatment. This is produced in small quantities for specialised applications.  Its characteristics are that the fibres are of regular defined dimensions, it is chemically and thermally stable, with low shrinkage and high tensile strength, all with less dust produced in handling.  It is a more expensive process than producing RCW papers and blankets.

The polycrystalline wool is generally used at temperatures above 1300°C.  One trade name is Denka Alcen with a temperature rating up to 1600°C. Denka blankets are more resistant to acid and alkaline solutions than conventional alumino-silicate fibre blankets and have good thermal insulation characteristics.

Other than kilnforming, applications are in the ceramics, metals, petrochemicals, aerospace and automotive industry sectors. Typical PCW applications include use as support mats in catalytic converters and diesel particulate filters to reduce exhaust emissions, and as insulation in industrial high temperature furnaces for energy conservation, particularly in high temperature and/or chemically aggressive environments.

Credit: Alibaba.com


Kilnforming Refractory Papers

There are two fibre papers widely used in kilnforming: Papyros and Thinfire.  These are special cases of the RCF papers and deserve particular attention, although they are subsets of the previously described RCF wools.

Papyros
This is a fibre paper similar in thickness to cartridge paper.  It consists of  aluminium hydroxide, hydrated magnesium silicate (hazard classification: irritant), alumina borosilicate glass (hazard classification: irritant), wood pulp and resin (both binders).  None of the materials used in the composition of Papyros are classified as a possible carcinogenic substance.  It is recommended that eye, breathing and skin protection be used when handling the fired residue to reduce any irritation.  Washing after handling the dusts is recommended.


Thinfire
This fibre paper is also like cartridge paper in thickness and has a slightly finer texture than Papyros.  Its constituents are aluminium hydroxide, glass fibre, polyvinyl alcohol, cellulose, and polyamide resin.  Only the glass fibre is classified as an irritant.  The dust can be an irritant to eyes and skin.  If either are irritated, wash with large amounts of water. It is sensible to use breathing protection while handling the fired residue.


The materials used place both these fibre papers in the AES group of refractory fibres, which are biosoluble.  The use of hydrated magnesium silicate in Papyros gives an extremely small increased health risk over Thinfire.

Credit: cdc.com

Fibre Paper – Health and Safety

Mineral wool fibres and refractory ceramic fibres have been  classified as "possibly carcinogenic to humans" (Group 2B).  In contrast, the more commonly used vitreous fibre wools produced since 2000, including insulation glass wool, stone wool, and slag wool, are considered "not classifiable as to carcinogenicity in humans" (Group 3). The International Agency for Research on Cancer (IARC) elected not to make an overall evaluation of the newly developed fibres designed to be less bio-persistent such as the alkaline earth silicate (AES) or high-alumina, low-silica (ASW) wools. 


Bio-soluble fibres are produced that do not cause damage to the human cell. These newer materials have been tested for carcinogenicity and most are found to be non-carcinogenic.

Due to the mechanical effect of fibres, mineral wool products may cause temporary skin itching. To diminish this and to avoid unnecessary exposure to mineral wool dust, information on good practices is available on the packaging of mineral wool products with pictograms or sentences. Safe Use Instruction Sheets like safety data sheets are also available from each producer.

People can be exposed to mineral wool fibres in the workplace by breathing them in, skin contact, and eye contact. … The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 5mg/m3 total exposure and 3 fibres per cm3 over an 8-hour workday [the highest existing standard].  The equivalent European standard is set by the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH).

AES, ASW and PCW have been registered before the first EC deadline of 1 December 2010 and can, therefore, be used on the European market.
ASW/RCF is classified as carcinogen category 1B.
AES is exempted from carcinogen classification based on short-term in vitro study result.
PCW wools are not classified; self-classification led to the conclusion that PCW are not hazardous.

Based on the total experience with humans and the findings of scientific research (animals, cells), it can be concluded that elongated dust particles of every type have in principle the potential to cause the development of tumours providing they are sufficiently long, thin and bio-persistent. According to scientific findings inorganic fibre dust particles with a length-to-diameter ratio exceeding 3:1, a length longer than 5μm (0.005 mm) and a diameter smaller than 3μm (WHO-Fibres) are considered health-critical.

High-temperature mineral wool is processed into products containing fibres with different diameters and lengths. During handling of high-temperature mineral wool products, fibrous dusts can be emitted. These can include fibres complying with the WHO definition.

There is concern about the silica content of refractory fibres.  The silica that is of concern is of a crystalline structure.  The method of production does not produce crystalline silica. The process used to create the fibres is:
Amorphous high-temperature mineral wool [fibres] (AES and ASW) are produced from a molten glass [or mineral] stream which is aerosolised by a jet of high-pressure air or by letting the stream impinge onto spinning wheels. The droplets are drawn into fibres; the mass of both fibres and remaining droplets cool very rapidly so that no crystalline phases may form.

The potential effects on health of the materials in refractory fibres have been tested and found to be non-hazardous.

In after-use high-temperature mineral wool crystalline silica crystals are embedded in a matrix composed of other crystals and glasses. Experimental results on the biological activity of after-use high-temperature mineral wool have not demonstrated any hazardous activity that could be related to any form of silica they may contain.

Thus, no crystalline silica is produced and the risk of silicosis from refractory fibres does not exist.  Certain sizes of any fibre present other risks.

Risks


Consideration of risks and therefore precautions, relate to three factors: Dimension, Durability and Dose.

Dimension

Fiber dimensions are critical, as only fibres of a certain size can reach the lungs…. Mineral fibres with a diameter greater than 3 microns are, in humans, “non respirable”. … Even below this respirability threshold only the finest fibres may be deposited into the gas exchange region of the lungs.

While respirability is determined by fiber diameter, fiber length is also important. Short fibres behave as if they are compact particles and can be cleared by the normal mechanisms which involve cells called macrophages. However long fibres [greater than 5 microns] frustrate this mechanism and, for some still unknown reason, are more biologically active.

Durability

Durability in this context describes the ability of a material to persist in the body and so is more accurately called “bio-persistence”. …  Fibres can dissolve or they may break into shorter pieces which can then be removed to the airways or through the lymphatic system. The rate of removal of different fibres is typically measured … and expressed as their “half-life” – that is the time it takes to reduce the number of fibres in the lungs by 50%.

Dose

The [dose] is the result of [dimension and durability] and is often referred to as “lung burden”.  With chronic exposures the lung burden is the result of … [continued exposure] and … bio-persistence. If the exposure is high enough and clearance slow then a sufficiently large dose will accumulate for adverse health effects to result.


The scientific knowledge about fiber toxicity allows comparison of fibres in terms of their toxicological potency and has also driven several initiatives to reduce potential risks in the workplace.  This has led to development of manufacturing processes for thicker fibres, although this is limited by the lesser thermal efficiency of thick fibres.  Thicker fibres are also more likely to cause skin irritation.  A lot of effort has been put into the development of bio-soluble fibres such as the AES wools which are increasingly available.

Recent research has shown a gradation of increasing bio-persistence is in the order of – least to greater –
AES (Calcium Silicate);
AES (Magnesium Silicate);
PCW;
RCF. 
This same research shows that fibres longer than 20 microns cannot be easily cleared from the lungs.  Breathing protection must filter out all particles larger than 20 microns. 

The WHO research shows that lung health effects can be produced by particles down to 3 microns. This means that filters used must be able to eliminate particles larger than 3 microns to provide effective protection against high exposure.

 

Handling practices

Sensible precautions when handling refractory fibre papers are eye, breathing and skin protection.  This can be safety goggles, dust mask (see filter size above), and long gloves and long sleeves.  Higher levels of protection can be used, but are not indicated as necessary by the research and classifications of health and safety organisations in the western world.

During clean-up the fibres should be dampened before any brushing of the residue, or vacuumed with HEPA filters to reduce the movement of fibres into the air.  You should also wash exposed skin after handling any of the dust.  Clothes should also be cleaned and washed frequently. 

Do not smoke, eat or drink in areas where the fibre dust is present.


More detailed information is available in the e-book: Low Temperature Kilnforming.

The understanding of the composition and manufacture of refractory fibre papers and blankets should help assess the small risks of using these materials, and the precautions that should be taken in handling both the un-fired and fired forms.

 


Wednesday 16 June 2021

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.


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

Saturday 2 November 2019

Schedules for Steep Drapes

I have been asked for a schedule for draping in the context of a tip on steep straight sided drapes.

What you are trying to do with a steep drape is two things. One is to compensate for the heat sink that the glass is supported by, and the second is to compensate for the relative lack of weight at the outer edge of the glass.



The supported glass transmits its heat to the support, leaving it colder than the unsupported glass. This often leads to breakage due to heat shock at much lower temperatures and slower rates of increase than glass supported at its edges. My experience has shown that - contrary to what I recommend for other kinds of firings - a slow rise with short soaks at intervals up to the working temperature works best. The reason for these slow rises and soaks is to try to get the support and the glass to be as nearly as possible at the same temperature throughout the rise in temperature. The soaks help ensure the mould is gaining heat without taking it from the glass.


The other problem with steep drapes is that the edges of the glass begin to drop more quickly than the area between the support and the edge. This leads to the development of an arc that touches the mould side near the bottom before the glass between the edge and the and the support. Extended soak times are required to allow the glass to stretch out and flatten. If this is done at high temperatures, the glass will thin - possibly to the extent of separating.


So the requirements for a firing schedule on this kind of drape are slow increases in temperature with soaks to avoid thermal shock, and an extended soak at the (low) forming temperature.


Whether using steel or ceramic moulds, I use a slow rise in temperature to 100C with a soak of 15 minutes. I then increase the rate of rise by 50% for the next 100C and give a 15 minute soak there. For the next 200C I raise the temperature at twice the original temperature rise, again with a 15 minute soak. The glass and mould should now be at 400C. This is still at the point where the glass could be heat shocked, so I only increase to 2.5 times the original rise rate but use this rate all the way to forming temperature.


Each kiln has its own characteristics, so giving schedules is problematic. 


  •  A side fired kiln will need slower heat rises than a top fired one. 
  • The closer the glass is to the elements, the slower the rate of increase needs to be. 
  • The kind of energy input - electric or gas - has an effect. 
  • The thickness of the glass is also a factor in considering what rate to use. 
  •  The size of the glass in relation to the size of the support is important - the greater the differential, the slower the heat rise should be. 


So in making a suggestion on heat rises, it is only a starting point to think about what you are doing and why you are doing in this way.

I have usually done this kind of draping in top fired electric kilns where the elements are about 250mm above the shelf, and about 120mm apart. In the case of a 6mm thick piece about three times the size of the support area, I use 50C/hr as my starting point. This is one third of my usual rate of temperature rise. However you must watch to see what is happening, so that you can make adjustments. You should observe at each of the soaks, so you know how the glass is behaving. It will also help you to pinpoint the temperature range or rate of advance that may be leading to any breakages.


On steep slumps, the temptation is to use a high temperature to complete the drape. This is a mistake as the glass will be more heavily marked and tends toward excessive stretching and thinning. What you really need is a slow rate of advance to a relatively low temperature. If you normally slump at about 677C, then you want to do this steep, straight sided drape at 630C or less. It will need a long soak - maybe up to an hour. It will also need frequent observation to determine how the drape is progressing. So plan the time to make yourself available during this forming soak.


Annealing is done as normal, since the mould and glass are more closely together and will cool at the same rate.


The original tip on the set up of a steep straight sided slump is here.

Wednesday 6 September 2017

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.