Types of Glass

Glass Types by manufacturing method

There are several ways of categorising glass and this overview of glass types looks at the way the glass is manufactured.

Crown Glass

Crown glass is the oldest method of producing sheet glass and continued to be used until the 19^th century.  This method consisted of blowing a very large bubble of glass.  It was then spun rapidly over a pit until the bubble collapsed into a disc that ranged from 1500mm to 1800mm diameter.  

This gave the thinnest and least marked glass at the outer portion of the disc.  The centre was the thickest and became known as the bullseye.  The glass was cut to provide the best use of the disc.  This limited the size of panes to what could be cut from the disc.  Diamond shapes were often cut from the remainder and the central bullseye was used in less expensive glazing.

Corning Museum of Glass

Cylinder Glass

Cylinder Glass is a handmade process that includes broad sheet glass. It was widely used from the 17^th to the 19^th century, and now is limited to a few manufacturers.  

"Among the Glass Workers" Harry Fenn, 1871

An elongated bubble was blown.  The top and bottom of the bubble are broken off and annealed.  Later the cylinder is placed in the lehr for reheating.  It is scored and when it breaks open along the score, the glass is flattened. Characteristically, it has a gradation of thickness with thicker edges where the top and bottom of the cylinder were cut off.

From IdoStuff

Flashed Glass

A development in cylinder glass was to make the bubble of two colours, with the dark colour gathered first and then encased in clear (or sometimes other pale colours) and blown into a cylinder.  This made dense colours more transparent and enabled more detail through abrading and etching.

Drawn Glass

Industrialisation of glass production began with the development of drawn glass.  This method of mass production of window glass was invented and developed by Emile Fourcault in Belgium. Full scale production began in the early 1900’s.  

The glass is drawn upwards from a vat of molten glass until it cools enough to be cut into sheets at the top of the tower.  The process is subject to slight variations in thickness due to uneven cooling and gravity. It enabled much larger panes of glass without the astragals that are common in Georgian and later houses.  It was the most common method of producing window glass until the 1950’s.

Table Glass
Table glass is the process of putting molten glass onto a flat surface (the table) and rolling the glass flat.  This has been used from the latter part of the 19^th century to the present.  It enables textures to be pressed into the glass from the rolling cylinder.  It is easier to produce streaky and wispy glass by combining different colours on the table. 

Kokomo Glass Co.

This can be done as single sheets or further mechanised to roll out long ribbons of glass.  This is now mostly referred to as machine or hand rolled glass depending on the amount of mechanisation.

Float Glass

The glass that we now rely on for large clear windows began with the development of experiments by Alastair Pilkington and the company named after him.  This consisted of floating near molten glass on molten tin, hence the name, float glass.  This has been the standard method of glass for windows since the 1950’s.

Annealing Large Pieces

A question was asked about how long to anneal a large piece in relation to smaller pieces.

“Large” is in relation to the size of your kiln.  A large piece for a 300mm square kiln would be something 250mm square.  For a kiln of 600mm square, 250mm would be a small piece.  It would contain a large piece of 500mm square as a large piece. 

Large also relates to the distance from the edge of the kiln.  Although some kilns have much more even heat than others, all have areas that are relatively cooler than others.  It is important to know where those are, so that you can avoid those cool areas, by placing pieces to avoid those spots or by altering the rate of cooling.  Bullseye has a tip on determining the relatively hot and cool temperatures are in your kiln. 

In a rectangular kiln, there are usually cool spots in the corners.  Front opening kilns often have cooler areas at the front of the kiln.  Knowing where these are will give you the information to know the area of the kiln that has even heat.   This area tells you what the size of a large piece for your kiln is.

You can alleviate many of the differences in temperature in your kiln by remembering that annealing is not simply a given temperature.  It is a range. 

The popular perception is that the soak at the annealing temperature is all that needs to be done to anneal.  The soak at the annealing point equalises the temperature throughout the glass. But it does not complete the annealing. That continues through the gradual cooling of the glass down the next 110°C.

Simply soaking longer at the annealing point, in the circumstances where the temperature in not equal all over the glass, “locks” the stresses of uneven temperatures into the glass.  Instead, a gradual, slower than usual annealing cool is required.

Of course, the rate of cooling is relative to the thickness of the piece and the degree of temperature variation in your kiln.  If you must utilise the area of the kiln with slightly cooler temperatures, the minimum requirement would be to use a cooling rate for a piece at least two times thicker than the thickness of the one you are annealing at present.


But, to answer the original question - how long to anneal a large piece in relation to a small one of the same thickness?   Given the precautions above, the size of the piece is not the major determining factor.  The thickness of the piece is the important dimension when considering annealing.

Flattening a Bubble

Sometimes a large shallow bubble appears from under the glass.  If it has not thinned there are some things you can do. 

First – do not drill holes.

One flattening method is to place the piece on 1mm to 3mm fibre paper and fire to a slump temperature.  The fibre paper of these thicknesses will allow air out from under the glass.  With sufficient time, the bubble will flatten.  It will take some time as the weight of the bubble is slight.

Another method is to fire upside down.  It does not matter whether the bubble is central or not. This will likely take less time than the first method, but requires an additional firing.  To use this method, place the glass upside down on the shelf with an appropriate separator underneath.  Take slowly to around 620C maximum for as long as it takes to flatten. A low slumping temperature will reduce any marking that later needs to be fire polished away.

When flat and cool, clean and fire polish.

If the bubble has become large and thin, this proposed process will not work. My suggestion for these is to avoid the effort to do an unsatisfactory repair.  Instead use it for one of the many inventive process that use unsuccessful projects.

Glass Stuck to Element

First consideration you need to think about when you discover glass stuck to an element is the nature of the metal of the elements.  Once fired, kiln elements become brittle.  This means that they are likely to break if disturbed when cold.  So, you need to make sure you absolutely must do something to rescue the kiln.  It may be that you can just leave the stuck glass alone.  Where the glass is, and how much of it, is stuck to the elements is important when considering what to do.

Where

This brittleness of the elements means that the location of the glass in relation to your firings needs to be considered.  If the glass is on an element below your normal firing position, you can think about just leaving it.  This applies to glass stuck to the side elements too, unless you are in the habit of firing very close to the side elements. The heating elements of the kiln form an external layer of oxidisation that protects the inner metal.  This means that small amounts of glass will not affect the operation of the elements, nor your future pieces.

If the glass is stuck to top elements, you are likely to be more concerned about future drips of the glass onto your future work.  The glass is not likely to become hot enough to detach or drip onto your work except at extended full fuse or casting temperatures.  This means that you can observe the progress of any possible drip at each firing and only remove the glass when it begins to begin to hang down from the element.

How Bad

How much glass is stuck to the element?  Normally, if it is only a small amount, it can be left.  Ceramics kilns often have a bit of glaze (a glass carrier of the colour) stuck to the elements and continue to be fired for years without damage.

If there is a lot of glass stuck to the elements you will need to remove most of it to avoid dripping onto future work. 

Methods of Removing

In most cases where there are significant amounts of glass stuck to the element, it is on the brick or fiber lining of the kiln too. 

My recommendation is to heat the glass just a few centimetres from where it is attached to the element. Use a hand-held blow torch to do this. When the glass is red hot - enough to begin moving - you can pull it away between the lining and the element with long handled tweezers.  Do not attempt to pull it off the element right away.  You can later chip the glass off the lining without damaging the element as the connection is separated.

As the element has begun to be warmed by the heat used to separate the glass on the lining and the element, you can continue to warm the element, moving the torch in a slow waving motion at least 10cm each side of the stuck glass.  When the glass and element are red hot, you can begin to pull the glass off with long handled tweezers, bit by bit.  Keep re-heating the element and glass as much as necessary so the temperature does not drop below cherry red.  This ensures the elements continue to be flexible and will not break.

Of course, glass can be melted onto its kiln furniture and there are different considerations for those circumstances.

Reducing a Bubble

A query about reducing a bubble appeared on the internet recently.  The bubble was from between the shelf and the single layer glass.  It was a relatively shallow dome that did not seem to have thinned the glass much.

There is quite a bit of information on reducing the incidence of bubbles. Among them are my blog posts on the subject.

Eliminating bubbles 
Preventing Bubbles  

Avoidance   

My view is that large thin bubbles cannot be repaired successfully.  As the bubble forms and grows, it pushes a proportion of glass to the side.  This thickens the glass at the edge of the bubble.  Bursting the bubble and filling it with something (e.g., a piece of glass, or frit) leaves marks at the thickened edge of the bubble, so it remains a mark in the finished piece.

I recommend two ways to deal with shallow bubbles.

Method 1

Glass with a low uprising between the shelf and the glass can be successfully repaired, if the uprising is low and the glass has not thinned. In the case mentioned, the risk in simply re-firing right side up is that the bubble will increase in size. The weight of the glass may not be sufficient to pull it down except at higher temperatures – which is where the risk of increasing the size of the bubble occurs.

Instead, flip the piece over. Allow the weight of the glass to flatten the uprising. You can use a much lower temperature to flatten the glass by taking advantage of the weight of the whole piece.  This lower temperature means that you will not mark the surface so much as at higher temperatures. Don’t worry if the uprising is not central, you do not have to balance the glass on the point of the bubble for this process to work.

Take the piece to 650°C for as long as it takes to flatten. The rate of advance should be slow – not more than 100°C per hour.  This steady, slow input of temperature will allow the glass to relax at lower temperatures than rapid increases. 

You should use the smoothest separator surface that you can – Thinfire or Papyros, or a smoothed kiln wash.  This together with a low temperature will give minimum markings. 

You must observe the process from about 600°C to be able to stop the slump when the piece is flat and advance to the annealing segment of the firing.

Method 2

This post gives a further alternative. Use two shelves to compress the uprising flat. Although the post is talking about thinning a pot melt, the principle is the same.  Place fibre paper around the edge equal to the thickness of the glass piece and place another prepared kiln shelf on top. You do not need to invert the surface of the piece to do this.  It may be that you will later need a fire polish to remove any marks on top.

A plea

Do not drill holes. Especially not in the case of a shallow bubble.  The glass has not significantly thinned and so can be rescued.  Drilling a hole will only leave an unwanted mark.

Removing Fibre Paper Marks

We all at times take short cuts or economies which lead to less than desirable outcomes.  One of these is to piece together fibre paper.  Often the marks of the join – which are always there – are just too obvious to leave.  The question becomes whether the fibre paper join marks can be eliminated.

Yes, there are at least two ways to remove these marks.  

One is to cold work the bottom with a flat lap or wet belt sander.

The other is to use the kiln to re-fire the piece.

One method would be to put fresh fibre paper or kiln wash on the shelf and fire.  This will require temperatures near the full fuse to achieve enough heat at the bottom of the glass to effect a significant change in the markings.

My suggestion for removing fibre paper marks - while it is still flat - is to fire upside down to fire polish to get rid of the marks without much changing the desired final texture of what will be the top. This is because the underside of the glass will not have the same heat effects as the top side. This also has the advantage that you can observe when the marks are eliminated.

When fired, flip over, clean the piece well to remove any fibre or kiln wash, and take to a quick fire polish to remove any marks - if necessary - caused by the upside-down firing.  This quick fire will be a slow rise to ca. 600°C, and then quickly to the 740°C to 770°C range.  This will cause the minimum change in the surface of the piece.

You will need to observe when both the evidence of the line disappears, and in a subsequent firing, when the final top surface of the piece is fire polished.

Kiln Wash Removal

There are a variety of ways to remove kiln wash.  Many depend on whether the surface is flat, smooth curves, angles or textured.  Some are applicable to both.

Flat surfaces are the easiest to deal with.

Abrasive methods work well with a variety of tools. 

They can range from large paint scrapers to smaller ones with a Stanley blade inserted. 

Coarse open mesh plaster board (dry wall) sanding sheets are very useful. There are frames that you can fix them to, but sanding without the frame works well too.

Using power tools to sand the shelf is not advisable.  It is too easy to remove lots of material, including the surface of the shelf – even the hard, ceramic ones.  This leads to minor depressions in the shelf and consequent bubble difficulties when firing.

Do not be tempted to sandblast as that will, almost certainly, create small depressions in the surface of the shelf.  Sand blasting is only possible on steel moulds.

Wet

Wet methods are applicable if you are concerned about the dustiness of the process.  You can dampen the kiln wash on the shelf and sand or scrape as above.  You will create a paste or slurry in front of the scraper which can be bagged and put in the waste.

You can also use a lot of water and the green scrubby washing up pads.  Unless you use a lot of water, the kiln wash builds up in the scrubbing pads.

Some people use vinegar or chemicals such as lime away with the water. Both are acids – lime away being much the strongest.  I am sure these are used on the basis that kiln wash is based on lime.  However, the material that makes the kiln wash stick to the shelf is china clay which is barely affected by the chemicals.  In addition, the alumina hydrate is impervious to many chemicals available to kiln workers.

One drawback to using wet methods, is that the shelf is wetted and needs drying before use.  The amount of water used in applying kiln wash is minor in relation to washing or soaking the shelf to remove the kiln wash.

Do not be tempted to use pressure washers. Yes, they will remove the kiln wash, but also leave little divots in the shelf which will cause later problems.

Smooth curves

Kiln wash on moulds with smooth curves can be removed with flexible sand papers or the plaster board sanding screens.  Normally, the coating of kiln wash is thin and does not require a lot of pressure or effort.

It is possible to dampen the kiln wash and take it off with scrubbing pads.  Make sure you do not use excessive pressure.  If you have wetted your ceramic mould, you need to dry it very carefully, to avoid having the mould break in the next firing.  This is because trapped water can turn to steam and the pressure will break the ceramic. It is best to let the mould air dry for a week or so before putting it into the kiln to thoroughly dry at about 90°C for a couple of hours.

Do not be tempted to use a pressure washer or water pick, as both can erode the surface of a ceramic mould.

Curves with angles

Moulds with angled areas such as at the bottom or corners of a rectangular mould need a flexible abrasive to clean out the angles.  You can fold a piece of sand paper to use the folded edge to do the final cleaning out of the angles.

The same can be done wet, but all the precautions about wet removal of kiln wash need to be observed.

Textured

Textured moulds require much more care in cleaning the kiln wash away, to avoid damaging the images and textures.  The flat upper surfaces can be dealt with as though it was a flat kiln shelf.  The indentations need to be more carefully treated.  Folded pieces of sand paper can be used to clean the delicate areas.

To ease cleaning of textured moulds it seems best to use kiln washes without china clay as the binder.  These will brush out of the mould with a fibreglass bristled brush.  It is now popular to use boron nitride - often sold as Zyp - as a coating for these moulds.  This needs to be brushed out and renewed with each firing.

Removing kiln wash from glass

Kiln wash stuck to the glass can present greater problems, because you want to avoid marking the glass.  It is best to start with the least aggressive abrasive, such as a green scrubby, and progress toward more aggressive and abrasive methods.  When using the more aggressive methods, try the finest grit first to see if that will work, as it makes for less work cleaning up the grinding marks from the glass.

For flat glass, you can work with a succession of finer loose grits, or a succession of finer diamond hand pads.  

Flexible diamond impregnated sheets can be used for curved surfaces.  Again, this requires a succession of finer grits to get to the polished stage.

You can use small hand held rotary tools with diamond and felt pads to polish out stuck kiln wash.  This helps to remove some of the labour of polishing the glass.

Some people advocate the use of acids to remove the kiln wash.  However, you must remember that glass is an alkaline material and acids will tend to mark the glass.  Vinegar is a mild acid, but prolonged exposure will etch the glass.  Strong chemicals such as lime away or etching cream or hydrofluoric are all strong acids and will mark the glass after brief exposure to them.

Holes vs. Elevation of Moulds

Drilling holes and raising the mould are not the same. They achieve different things.  Drilling holes allows air out from between the mould and the glass.  

There are some things you need to check about the vent holes in moulds.

            Are the holes in the mould at last touchdown point(s)?

Sometimes the vent holes in moulds are made at convenient points rather than at the places where the glass will last touch the surface of the mould.  On a simple ball mould, a hole at the centre will be appropriate, as this is the last place the glass will touch. 

 On a bowl with a square base, the last places the glass will touch are the corners, so that is where the holes need to be.

The vent holes in this could also be at the other two angles in addition to those at the top and bottom of the picture.

Are there holes in the side of mould to allow air out from under the mould?

If there is one or more, there is no need to elevate the mould.  The air will move out from under the mould through the hole in the side. In general, moulds are not so uniform on their base that they fit the shelf enough to seal the displaced and expanding air underneath the mould. But you can be safe by elevating the mould on pieces of 1mm or 3mm fibre paper.

This mould has side vents, although the holes at the base may be a little large.

Are the holes clear?

This is more important.  If the vent holes are not open due to kiln wash or other things blocking the space, there will be no escape for the air.  The vents need to be checked on each firing to ensure they are open.

  

Does the mould need holes at all?

There are shallow slumpers and other simple moulds - such as a wave mould or any cylindrical mould form - that do not need vent holes, either because they are so shallow, or because the air can escape along the length of the mould.

More information can be found in this and related blog posts.

Large thick bubbles at the bottom of the glass

Not all large bubbles at the bottom are the result of the lack of holes.  Sometimes they are the results of too fast or too high a firing. Some notes on this are given in this blog entry. 

What does elevating the mould do?

The purpose of elevation is not allow air to escape from under the glass, although that may be a by-product.  Elevating the mould allows marginally more even cooling of the mould and glass if it is on a thick kiln shelf. It will not create any problems, but you need to be careful about how near the elements it will place the glass.  The elevation does not need to be more than 25mm, just as for the shelf above the floor of the kiln.

CoE of Paints for Fusing

CoE of the glass carrier for paints is a distraction.

Paint has been applied to glass and fired for at least seven centuries – long before CoE measurement.  The earliest enamels were intensely coloured glass powders applied to depressions in the base metal (iron, gold, copper, brass, etc) and heated.  More detailed images began to be created when the powers were mixed with a liquid binder and painted on either in a single, or multiple layers onto glass and metals.

Silver stain became popular in the 16^th century and has continued since.  This is a different way of colouring the glass, as the colour does not laminate with the surface, but is chemically combined with the glass.  Various silver salts produce different colours and vary in intensity at different temperatures.  This can provide a variety of effects at fusing temperatures where it “metalises”, providing ambers and blues.

CoE in relation to paint does not matter.

The amount of paint is miniscule in relation to the mass of glass to which it is applied, and so any incompatibility would not have sufficient strength to break the glass. If the paint’s glass carrier was too incompatible, it would come off instead of breaking the glass, in any case.

The composition of the fusing glass paints is largely unknown, although commonly supposed to be powdered glass frit. Some may be the same as enamels used in metal enamelling. Some others may be the same as the on-glaze ceramic colours. They all have glass as the carrier of the colour.  Still, the amounts of glass involved are very small and compatibility is not a concern.

In any case CoE is not the most important element in compatibility.

Slumping Glass that is not Tested Compatible

Is it Possible?

It is possible to slump unknown glass. This glass might be art glass, window glass, bottles, or any other glass whose characteristics are unknown by you.  There are some suggestions about the characteristics of some glasses in this post that can be used as a starting point.

Preparation of the Glass

Prepare the edges to their final finish before slumping.  This because the slumping temperature will not be enough to alter the finish of the edge significantly.  This preparation can be done with diamond hand pads, or wet and dry sandpapers.  Start with a relatively coarse grit. You may wish to do the initial shaping on your grinder. This will be between 80 and 100 grit.  Continuing with a 200 grit and working your way through 400 and then 600 grit will give you an edge that will become shiny during the slumping.

Cleaning

Clean thoroughly.  This is especially important when using glass that is not formulated for fusing.  Devitrification is more likely on these glasses.  Water with a drop of dishwashing liquid can be enough unless your water has high mineral content.  Then distilled water or a purpose made glass cleaner such as Bohle or Spartan should be substituted.  Finish with a polish to dry with clean paper towels. More here. 

Firing the Slump

Fire up slowly.  You should advance at about 100°C to 150°C per hour.  Set your top temperature around 630°C for a simple slump, for soda lime stained glass.  For bottle or window glass you will need a temperature closer to 720°C although the also are soda lime glasses.

It is best to start with simple curves, as there are fewer difficulties in determining what the glass is doing.  It will help you to learn the characteristics of the glass before you tackle the difficult stuff, such as compound curves or texture moulds.

Observation

It is necessary to observe the progress of the slump as you do not yet know the slumping temperature.  You want to know when the glass begins to deform so that you do not over fire.  Start watching the glass at about 10 minute intervals from about 580°C for stained glass and 680°C for window and bottle glass.  There is not much light in the kiln at these temperatures, so an external light is useful.  You can also observe the reflections of the elements on the glass.  When the image of the elements begins to curve, you know the glass is beginning to bend.

Altering the Schedule

Soak for at least 30 mins at the temperature when the glass begins to visibly drop. This may or may not be long enough.  Continue checking at 5-10 minute intervals to know when the slump is complete.  If the glass is completely slumped before the soak time is finished, advance to the next segment.  If not fully slumped, you need to extend the soak time. This means that you need to know how to alter your schedule in your controller while firing.  Consult your controller manual to learn how to do these things.

Stop the soak when complete and advance to the anneal. Continue the slumping soak if not complete after the 30 mins.  In some cases, you may need to also increase the temperature by 5-10°C.

Annealing

The annealing point will be about 40°C below the point that the glass visibly starts the slump. If you want a more accurate determination of the annealing point, this post gives information on how to conduct a test to give you both the slump temperature and the annealing point.  It also helps to determine the lower part of the tack fusing range (the lamination state), since it is not far above the slumping point that you will observe.

The annealing soak for a single layer, 3mm glass need not be long – 15 to 30 minutes.  The annealing cool can be as fast as 120°C down to 370°C.  For thicker glass and slumped bottle glass you will need a longer soak – 30 to 60 minutes – and a slower cool.  The annealing cool in this case could be about 60°C/hour to 370°C.  You can turn the kiln off at 370°C, if you wish, or keep the temperature controlled to about 50°C.  The rate for the final cooling can be approximately double the first cooling rate.  For a single layer of stained glass this could be 240°C, and for thicker glass about 120°C