Wednesday, 17 January 2018

Annealing Point and Range

A question has been asked about whether the statement that “annealing longer never hurts” is true.

To understand why this statement is not always true, you need to be aware that annealing is not just the soak at the stated annealing point.

The annealing point has a mathematical description, but in lay terms it is the temperature at which the stresses in the glass are most quickly relieved.  Annealing at this point is only possible in large industrial processes.  It is reported that float glass manufacturers can anneal glass in 15 minutes because of excellent temperature control in their lehrs.  For those of us who do batch annealing such speed and accuracy is not achievable.

As we cannot achieve such accuracy with our kilns, annealing for kiln formers consists of a temperature equalisation soak at the annealing point and then slow cooling through the lower strain point.  That is the point where the glass becomes so stiff that no further annealing is possible. 

Most kilns have relatively cool areas.  They mainly are in the corners and at the front of top hat or front-loading kilns.  You should know where these cool spots are.  They can be checked for by a simple test as described in Bullseye Technote 1.   This will enable you to know if and where any cool spots may be.  In smaller pieces, you can just avoid those areas in the placing of your pieces.

Annealing of large pieces, parts of which must be in the cool areas, is possible.  But not with excessively long anneal soaks.  If the kiln has temperature differentials, a long soak will impose those variations in temperature upon the glass. This means that the glass will begin its annealing cool with variations in temperature across the piece.

During the anneal cooling, research at Bullseye Glass Company has shown that to achieve as stress free a piece of glass as possible, the temperature variation across and through the piece should not vary more than 5°C. This is relatively difficult to achieve if you have cool areas in your kiln.  But it is possible.

To alleviate the possible difficulties of temperature variations in the kiln, the anneal soak should not be extended beyond that recommended by its thickness.  What should be extended is the anneal cool. The rate of cooling should be slowed to the rate for a piece at least twice the thickness of the current piece.

If it is a tack fused piece, this reduction should be for a piece four times the thickness of the thickest piece you are annealing.

The conclusion is that it is possible to anneal too long, if the piece is large and the heat in the kiln is not uniform. If you are concerned, remember that the soak at annealing point is to equalise the temperature throughout the substance of the piece. The annealing cool - the first 110 degrees Celsius - is very important. If you are concerned, it is best slow that rate of decrease dramatically. This provides a safer option for an adequate annealing of large pieces.

Wednesday, 10 January 2018

Flat Kiln Shelves

A question has been asked about using tiles in addition to standard kiln shelves to fire glass upon.  Yes, you can use the unglazed backs to fire on, assuming they are not ridged or in other ways not a regular surface.

It is important to have flat shelves, as ones with even small shallow depressions can promote bubbles at higher temperatures. Tiles for walls and floors do not need to be flat to do their intended job and so are not checked for be flatness.

A magnified view of a shelf surface that is not perfectly even

You can do a quick check for flatness, by placing a ruler on edge across the tile or shelf to see if any light comes through underneath the ruler.  The light areas are the places where the surface is lower than the rest.  If these are few and small you can make corrections in the surface of the tile by grinding.

You can make sure they are flat by putting two tiles back to back and grinding them together. The initial grind will show you the high spots as they will have the grinding marks there. 

You can eliminate these higher areas by rubbing the tiles together with a coarse grit (ca. 80) between the tiles to speed the grinding. If you are concerned about the dust or don’t have good ventilation, you can make a slurry of the grit by adding water. When the whole surface has the same marks, both will be flat. To double check, scribble with a paint marker over one and let it dry.  Then add grit between to grind again. When all the paint marks have come off they are both flat on the back.

This sounds time consuming and lots of effort, but you will be surprised at how quickly you can achieve flat smooth surfaces even on larger tiles.  This also works for larger kiln shelves.

Wednesday, 20 December 2017

Preventing Devitrification on Cut Edges

“Question-when cutting up a Screen Melt, using a tile saw. How do you NOT get devitrification when laying the slices cut sides up?”

Devitrification occurs where there are differences in the surface.  This means that the surfaces exposed to the heat must be both clean and smooth.  It is not enough for only one of these to be the case, both are required.

First, the sawn edges need to be clean.  A good scrub with a stiff bristle brush is essential.

Second, devitrification sprays of whatever kind do not seem good enough to prevent the devitrification. This is probably due to the thin covering of the differences (scratches, pits, etc.) on the surface.

Beyond that, I know of two ways to prevent or reduce devitrification. That is, providing a smooth surface to resist devitrification.

1 – Grind
This can be done with hand pads, grit slurry or machines such as a Dremel with damp sanding pads or belts, wet belt sanders, or a flat lap.  The grinding should go down to at least 400 grit before cleaning and arranging to fire.

2 – Clear glass
This method relies on putting a layer of clear glass that is less likely to devitrify than the cut edges over the whole surface.  You could use a sheet of glass, although that would promote a multitude of bubbles due to the spaces between the strips and the naturally uneven heights of the strips.

Placing a layer of fine frit on top of the arranged pieces before firing is a way of allowing air out and forming a smooth upper layer by filling the gaps. It is best to avoid powder, as this promotes a multitude of fine bubbles, giving a grey appearance. The layer you apply needs to be an even layer and at least 1mm thick. If you are concerned at getting lots of bubbles, you could use medium frit instead.  In this case, the layer will need to be thicker than 1m to get an even coverage. The whole of the surface of the piece needs to disappear under the layer of frit, and that may be a good guide to the thickness of frit to apply.

Sunday, 17 December 2017

Composition of Glass

Glass can do most anything. From bottles to spacecraft windows, glass products include three types of materials:
  • Formers are the basic ingredients. Any chemical compound that can be melted and cooled into a glass is a former. (With enough heat, 100% of the earth's crust could be made into glass.)
  • Fluxes help formers to melt at lower temperatures.
  • Stabilisers combine with formers and fluxes to keep the finished glass from dissolving, crumbling, or falling apart.
Chemical composition determines what a glass can do. There are many thousands of glass compositions and new ones are being developed every day.


Most commercial glass is made with sand that contains the most common former, Silica. Other formers include:
  • Anhydrous Boric Acid
  • Anhydrous Phosphoric Acid
But melting sand by itself is too expensive because of the high temperatures required (about 1850°C, or 3360°F). So fluxes are required. Fluxes let the former melt more readily and at lower temperatures (1300°C, or 2370°F). These include:
  • Soda Ash
  • Potash
  • Lithium Carbonate

Fluxes also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. So stabilizers need to be added. Stabilisers are added to make the glass uniform and keep its special structure intact. These include:
  • Limestone
  • Litharge
  • Alumina
  • Magnesia
  • Barium Carbonate
  • Strontium Carbonate
  • Zinc Oxide
  • Zirconia

Based on an article from the Corning Museum of Glass

Float Glass

A reported 90% of the world's flat glass is produced by the float glass process invented in the 1950's by Sir Alastair Pilkington of Pilkington Glass. Molten glass is “floated” onto one end of a molten tin bath. The glass is supported by the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a lehr. The finished product has near-perfect parallel surfaces.

An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch.

Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

More information on float glass in the kiln is here.

Figure Rolled Glass

The elaborate patterns found on figure rolled glass are produced by in a similar fashion to the rolled plate glass process except that the plate is cast between two moving rollers. The pattern is impressed upon the sheet by a printing roller which is brought down upon the glass as it leaves the main rolls while still soft. This glass shows a pattern in high relief. The glass is then annealed in a lehr.

Rolled Plate Glass

The glass is taken from the furnace in large iron ladles and poured on the cast-iron bed of a rolling-table. It is rolled into sheet by an iron roller. The rolled sheet is roughly trimmed while hot and soft and is pushed into the open mouth of a lehr, down which it is carried by a system of rollers.  The method is similar to table glass, except in size and thickness.

Table Glass

This glass was produced by pouring the molten glass onto a metal table and sometimes rolling it. The glass thus produced was heavily textured by the reaction of the glass with the cold metal. Glass of this appearance is commercially produced and widely used today, under the name of cathedral glass, although it was not the type of glass favoured for stained glass in ancient cathedrals. It has been much used for lead lighting in churches in the 20th century.

Modern example of rolling glass. The operator is waiting to take the rolled sheet off the table

Broad Sheet Glass

Broad sheet is a type of hand-blown glass. It is made by blowing molten glass into an elongated balloon shape with a blowpipe. Then, while the glass is still hot, the ends are cut off and the resulting cylinder is split with shears and flattened on an iron plate. (This is the forerunner of the cylinder process). The quality of broad sheet glass is not good, with many imperfections. Due to the relatively small sizes blown, broad sheet was typically made into leadlights.

According to the website of the London Crown Glass Company, broad sheet glass was first made in the UK in Sussex in 1226 C.E. This glass was of poor quality and fairly opaque. Manufacture slowly decreased and ceased by the early 16th Century. French glass makers and others were making broad sheet glass earlier than this.

Drawn Sheet Glass

Drawn sheet glass -sometimes called window glass or drawn glass – is made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardens just out of the vat. This film or ribbon is pulled up continuously and held by tractors on both edges while it cools. After 12 meters (40 feet) or so it is cut off the vertical ribbon and tipped down to be further cut.

This glass has thickness variations due to small temperature variations as it hardens. These variations cause slight distortions. You may still see this glass in older houses.

In more recent times, float glass replaced this process.

Flashed Glass

Red pot metal glass is often undesirably dark in colour and very expensive. The method developed to produce red glass was called flashing. In this procedure, a semi-molten gather of coloured glass was dipped into a pot of clear glass. As the bubble became enlarged, the red glass formed a thin coating on the inside. The formed glass was cut, flattened and annealed as any other blown sheet.

There were a number of advantages to this technique. It allowed a variety in the depth of red – and other deep colours - ranging from very dark and almost opaque, and sometimes merely tinted. The other advantage was that the colour of double-layered glass could be engraved, abraded, or etched to show colourless glass underneath. 

Other base colours are also used in making flashed glass, for example red flashed onto a pale green base.  Also see this post on finding the flashed side of glass.

There still exist a number of glass factories, notably in Germany, USA, England, France, Poland and Russia which continue to produce high quality glass by traditional methods primarily for the restoration of ancient windows.

Cylinder Glass

Cylinder blown sheet is a type of hand-blown window glass. Large cylinders are produced by swinging the cylinder in a trench or blown into a cylindrical iron mould. The glass is then allowed to cool before the cylinder is cut. The glass is then re-heated and flattened. The result is much larger panes and improved surface quality over broad sheet.

Trench method

Cylinder blown sheet glass has been manufactured in France, Germany and Poland since the 18th Century, and continues today. It began to be manufactured in the UK in the mid 19th Century, although the only small remaining company has ceased manufacturing in the late 2010's.

Mould method

Machine drawn cylinder sheet was the first mechanical method for "drawing" window glass. Cylinders of glass 12 m (40 feet) high are drawn vertically from a circular tank. The glass is then annealed and cut into 2 to 3 m (7 to 10 foot) cylinders. These are cut lengthways, reheated, and flattened. This process was invented in the USA in 1903. This type of glass was manufactured in the early 20th century (it was manufactured in the UK by Pilkington from 1910 to 1933).

Crown Glass

Crown glass: The earliest style of glass window

The earliest method of glass window manufacture was the crown glass method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force forced the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and cut into small sheets.  

This glass could be made coloured and used for stained glass windows, but is typically associated with small paned windows of 16th and 17th century houses. The concentric, curving ripples are characteristic of this process.

At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain. They are known as bull's eyes and are feature of late 19th century domestic lead lighting and are sometimes used with cathedral glass or quarry glass in church windows of that date. Optical distortions produced by the bullseye could be reduced by grinding the glass. The development of diamond pane windows was in part due to the fact that three regular diaper shaped panes could be conveniently cut from a piece of crown glass, with minimum waste and with minimum distortion.

This method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet and rolled plate processes, but it is still used in traditional construction and restoration.

Saturday, 16 December 2017

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 19th 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 17th to the 19th 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 19th 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.

Wednesday, 13 December 2017

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.

Wednesday, 6 December 2017

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.

Wednesday, 29 November 2017

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.

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.

Wednesday, 22 November 2017

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.

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.

Method 1
However, 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 620°C maximum 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 560°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 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.

Wednesday, 15 November 2017

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.

Wednesday, 8 November 2017

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 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 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.

Wednesday, 1 November 2017

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.