Wednesday, 8 August 2018

Glass on Drop Rings

When glass drops through a ring, you need to check on some things relating to the placement and firing.

When thinking about the relationship between the size of the flat glass and the size of the aperture, you need to remember how the glass behaves as it heats up toward the drop temperature.

Glass behaviour
The glass begins to sag at the middle of the aperture, however the glass is still relatively stiff.  The weight of the rim is not enough to keep it from rising from the ring. The rim of the disc maintains the angle from the centre of the drop to the edge, until it gets hot enough for the weight of the rim to allow the edge of the disc to settle back down onto the ring.  This is the source of a lot of the stretch marks at the shoulder of drops.

Rim width
To avoid the glass dropping through, you need to have an adequately sized rim.  The width of the rim sitting on the ring, needs to be related to the size of the hole.  

The consequence of an inadequate rim

I have found that for apertures up to 300mm diameter there needs to be at least 35mm on the rim.  The consequence of this is that your blank diameter needs to be 70mm more than the hole diameter.  For larger apertures – up to 500mm – you need 50mm, or 100mm added to the diameter of the hole.  I do not have the experience to say how much more is required for larger diameter drop rings.  There is more discussion on blank sizes here. 

The rate at which you heat the glass and the top temperature both have effects on the possible drop through.  

High temperatures. The higher temperature you perform the drop out, the more likely you will need larger rims or other devices to reduce the drop through possibilities.  It also promotes excessive thinning below the shoulder. 

Fast rates. The surface will become hotter than the bottom, but at different rates.  The glass over the hole is heating from both top and (to a lesser extent) bottom.  The rim is sitting on the ring and so heats only from the top.  The differential in heat may cause a break.

Weight. The thickness of the glass effects when the drop will begin.  The heavier the glass and larger the hole, the effective weight will be greater.  In these cases, you can use a lower temperature for the drop.

Additional methods.  You can use other methods to reduce the chance of a drop through.  Two of them are:

Weights. You can put kiln furniture on the glass rim to keep it from rising during the initial stages of the drop.  These must be placed symmetrically. Four or six pieces of kiln washed props or small dams would be sufficient up to 300mm diameter.  More would be required for larger apertures.  Of course, these will mark the rim, meaning that it must be cut off.

Inclined rings. Another possibility is to use an inclined ring, with the glass resting on the upward incline, so the glass is held above the aperture and is heating evenly until the drop begins.

Wednesday, 1 August 2018

Dichroic coatings


“Dichroic glass is a multi-layer coating placed on glass by using a … vacuum deposition process. Quartz crystal and metal oxides [such as titanium, chromium, aluminium, zirconium, or magnesium] are vaporized with an electron beam gun in …[a] vacuum chamber and the vapor then floats upward and … condenses on the surface of the glass in the form of a crystal structure….  [As] many as 30 layers of these materials [are applied] yet the thickness of the total coating is approximately 35 millionths of an inch.”

“This coating that we commonly call dichroic glass today, is actually an “interference filter” permanently adhered to the surface of a piece of glass. The technology used to manufacture the optical interference filter has been in existence for many years. It is known as vacuum thin film deposition“  Howard Sandberg.

“The total light that hits the dichroic layer equals the wavelengths reflected plus the wavelengths passing through the dichroic layer.  A plate of dichroic glass can be fused with other glass in multiple firings. Due to variations in the firing process, individual results can never be exactly predicted, so each piece of fused dichroic glass is unique.”  Wikipedia

Care in Use

Dichoric glass can be used in stained glass as well as kilnforming.  There are some precautions to be observed when handling dichoric coated glass.

Determining Coated Side
The coating is a thin film that can be damaged easily. So, the first thing is to determine which is the coated side when the film is on a clear base.  One way is to look at the glass at a very acute angle.  If you see the colour above the clear, the coating is on the top.  If the clear is above the film, the coating is down.  Another way is to put a sharp point in contact with the glass.  View at a sharp angle.  If the point appears to touch the surface, the coating is up.  If there appears to be a small space between the point and the surface, the coating is on the bottom.  It is normal to check both sides to confirm the first impression.  Of course, if the dichoric is on black, the coated side is obvious. A more complete description of the method is describe here.

The dichoric film is strong, but very thin.  This means that anything that could scratch the glass will also scratch the coating.  Avoid the use of abrasives when cleaning the coating.  This means that steel wool and harsh abrasive cleaners should not be used.

Scoring and Breaking
As the film is very thin, it is best to cut on the non-coated side.  This avoids any chipping as you score the glass, and provides a clean break.

Also, when grinding the edge, you should use a fine grit to avoid chipping off the dichoric.

Fusing Notes
The dichroic coating is a strong thin film that does not expand and contract to the same extent as the glass being fused.  

Avoid movement
When there is a lot of movement in the glass, the coating can split. If the dichoric is on a clear base, you can fire it facing down to reduce the fracture of the film. You can also fire it with clear glass above to reduce the stretching and tearing of the dichoric film.

Over firing
Firing too hot causes additional movement in the glass, so you can think about reducing the temperature to avoid that over firing, which causes lots of movement of the glass.  You should also think about the volume.  If there is more than 6mm of glass, it will begin to spread to reach that thickness.  The spread causes a stretching stress in the dichoric film that can cause it to break apart.

You should not fire with the dichoric faces together.  The films do not fuse together, so the glass bases and tops will act as single layers and pull in, creating multiple fractures in the coatings.

In addition to dichoric coated sheet glass, there is also dichoric coated frit from CBS.  They have designed a proprietary process that allows the frit to be coated on approximately 80% of the surface area of the frit. Due to this high ratio of coating versus glass, the dichroic frit responds very differently under heating/hot working conditions.  Based on:

Wednesday, 25 July 2018

First Kiln Selection

Glass fusing works best in top fired kilns.  Glass casting and some tall work are best with side or bottom elements too.  Compromises can be made of course.  The comparison of glass and ceramics kilns is important to understand.  

Most of the following types of kilns are available for glass purposes.
Front loading.  These are good basic, multi-purpose kilns with good viewing properties.

Bell kiln.  This is where the whole of the heating chamber lifts up from the firing bed.  This is more common with very large kilns and is usually combined with lifting gear.

Clamshell kiln.  This is where the firing chamber is hinged, usually on the long side.  This kiln provides access from three sides. It can become too large to reach to the back of the kiln, so these tend to be rectangular.  The lid can also become too heavy for ease of movement and support.

Top loading.  Often called a coffin kiln, there are very good for casting or deep work, but are hard on your back while loading.  They need peep holes at appropriate levels to be able to monitor progress of the firing. These tend to have smaller floor areas than the clamshell.

Car kilns.  These are those where the firing chamber lifts like a bell kiln, but has the firing base on rails or tracks to move multiple firing bases under the firing chamber.

Modular kilns.  These are normally rounded kilns where each ring is controlled separately and can be placed on top of one another.  This is good for large heavy castings, as the refractory and glass reservoir can be placed on the base and the rings built up around the work.

All these kilns come in a variety of sizes.

Choose a kiln relevant to your current work.
The first thing you must decide is the kind and scale of work you intend to do in the near future.  It is too difficult to predict how your work might progress based on experiences with your current work.  It is better to by a smaller kiln that is ideal for the current work and then move to a different kiln, if necessary, or a kiln for different styles or scales of work.

The general advice is to buy as large a kiln as your budget and space and electrical installation will allow.  This remains the case with some precautions.  Think about how often you will fire - daily, a few times a week or a few times a month.   Think about how long it will take to fill the kiln.  A large kiln can take days or even a week to fill with small works. This would really limit the variety of things you could do in that period.  You would have to wait to slump until you had enough things fused to fill the space.  Indeed, you would need to have more moulds than if you had a smaller kiln.

I’m sure you can envisage a time when you will want to work larger than at present, but your first kiln will not become redundant.  It will continue to be useful throughout its long life.

Factors in the choice

Size. As already alluded, the size needs to fit with your current style and scale of work. 

Access.  How big a kiln can you get through the doorways?  How much bigger than actual external dimensions will the packaging make it upon delivery?  It is no use buying a kiln that must be taken apart, or all the packaging removed, to get it into your studio.  Of course, the wider the entrance(s) to your studio the easier it will be to get a larger kiln.  If you really need to have a large kiln, you might have to alter or move your studio space.  You also need to think about the kind of access to the studio.  Does the kiln have to come along the side of the house? Is the path paved or gravel? Stairs? Lift size? Parking for the delivery vehicle?

Space. The kiln also needs to fit into the space you have.  You will need about 15cm all around the outside dimensions for safety purposes.  This applies to ceramic kilns also, even though they routinely reach higher temperatures. The skin of the kiln does get hotter than is comfortable for your hand, but normally not hot enough to burn paper. You can reduce the front to back storage space by putting the kiln on wheels.  But the 15cm saved is not worth the time required to once again ensure that the kiln and shelves are level each time you move it. 

Accommodation also needs to consider access around the kiln to place work in the kiln, especially if you build elements in place on the shelf. 

Location within the studio is important, as the kiln needs to be near a power supply and in a place where it is away from the movement within the studio.

Power supply.  The nature of your power supply will also determine what size of kiln you should buy.  Note both the wattage and amperage required for the kiln and determine whether your household supply can cope with the energy requirements.  Usually a kiln can be run on household supply until it reaches the 1 metre2 size, where three-phase power is required to have efficient use of the electricity.

Wattage. Kilns below the 1 metre2 size have a need for at least 0.6 -1.2 watts per cm2, or 4-8 watts per inch2.  Once the kiln is larger, more power is required per area to accommodate the greater mass of the kiln.

Insulation.  All kilns require insulation.  This can be fibre or light weight brick, or a combination of the two.  These insulating bricks can be red hot internally, but only warm to the touch on the outside.  Generally, the refractory fibre – whether board or blanket – requires less energy to heat and cools more quickly in the critical devitrification range.  Most often the kiln floor will be made of brick to provide a firm base to support the kiln furniture.


All kilns come with a range of features, many of them relevant to the size, but not all have the same ones, or the ones important to kiln forming.

Viewing ports.  These are variously called vents, ports, bung holes, etc.  Their importance is at least three-fold. 
·        These provide an opening(s) for you to view the progress of the firing, so you can add more time or heat, or skip to the next segment when adequate heat work has been completed earlier than expected.
·        They provide a means of venting the kiln.  This is important in the burn out of any fibre paper binders, and in allowing enough air to promote the oxidisation and maturation of the hot enamel colours.
·        These openings allow the kiln to safely cool more quickly at lower temperatures, say 300°C, but lower for thicker or more delicate pieces.

Opening.  The way the kiln opens is an important consideration.  Some kilns do not allow the kiln to be opened at all during firing.  This is not a desirable feature on a glass kiln.  It is important to have a switch that will turn the kiln off after a certain degree of opening, so that no contact can be made with a live element. 
·        A front opening kiln allows maximum flexibility to view the progress of slumps, drapes, tack and full fuse kilnforming.  It should have a switch to turn off the power to the elements after a certain degree of opening.
·        A top loading kiln allows you to add glass during a casting process, but is not suitable for working the glass during firings – E.g. combing, manipulation of a slump or drape.  This type of kiln occasionally has no allowance to open the top without turning off all the power to both the controller and the elements.  Avoid this, or have it changed.
·        A clamshell or bell kiln allows maximum accessibility during the loading phase and the forming stages.  Although a lot of heat is dumped forward, it is the easiest to use for combing and other manipulation of the glass during the firing. Again, this kiln needs a lid operated switch to cut the power to the elements when opened beyond a certain point.

Controller.  Although essential, controllers are often given as options, especially on smaller kilns.  There are at least two reasons for this.  There are a variety of controller styles and costs.  The buyer may already have their own controller, or wishes to specify the kind.  Controllers are significant costs involved in smaller kilns – sometimes being at least one-sixth of the price.  In general, the more features a controller has, the more it costs.

Controllers are often classified as “three-key”, or as full number pad.
·        The three-key controller – even if they have many more than three keys – is one where the numbers must be cycled through by holding an up or down arrow to change the numerical information.  This includes the programme number, segment number, time, rate, temperature, and sometimes other information. 
·        The full number pad controller will allow direct entry of numbers at each segment of the programming.  It will often have additional features, such as calculating the firing cost or kilowatts used, elapsed time, additional capacity for more saved programs, ability to control different areas of the kiln heating, etc.

There are often things which will be worth considering purchase along with the kiln, but are not usually included in the base price.

Stands.  Smaller kilns range from table top - which do not need stands at all – through medium sized – which have optional stands – to larger ones that come with the stand integral to the whole kiln. Unless you intend to move your kiln about, it is not necessary to buy one of the metal stands. Even so, most of these stands come without wheels, so check that they do have wheels already attached.  If you will not be moving the kiln, you can use a wooden table with a refractory fibre board between the stub legs of the kiln and the table surface.  If the kiln does not have stub legs, you can set it on 4 house bricks. 

Kiln furniture. This consists of the refractory props and dams that will be needed in kilnforming.  The most essential are short (2.5cm) kiln posts to support the shelf.

Shelves.  Most shelves require a mullite/cordierite shelf to fire on.  This is a robust shelf that does not have the quartz/crystobalite inversions that ceramic shelves and tiles used for shelves have.  It is a good idea to buy one of these to fit your kiln at the time of purchase. Smaller kilns can use fibre board or vermiculite board as the shelf.  These can be purchased later.

Extractor fans.  These are available on many kilns. They are unnecessary on smaller kilns as they cool quickly anyway.  Larger kilns in a production environment may need quicker cooling, and these arrangements are very useful in those circumstances, but not others, as most kilns will cool in 8 – 16 hours without drawing air through the kiln.

There are a lot of other considerations in buying a kiln, but these are among the important ones, especially in selecting the first one.

Wednesday, 18 July 2018

Multi Stage Slumping

Deep slumps cannot be done in one slump. Usually, multiple slumps are required to get an even rim with even thickness along the sides.

Special three stage moulds have been developed for deep slumps. The set is expensive even if you have the shallow starting mould already.

When deep slumps are tried in a single stage, uneven sides, hang ups at the edge, needling at rim, and distortion of the image are common in addition to some thinning and significant distortion.

Do it yourself
This leads to investigating whether it is possible or reasonable to try do it yourself methods.

The DIY process involves using two moulds and filling the deep mould with powdered separator.

·        First stage – slump the glass blank into a shallow shape first.  The starting diameter of the blank will need to be about one third larger than that of the finished vessel.  This can be determined by measuring the diameter of the deep mould and adding one third. This means that if your deep mould is 300mm, you will need a 400mm diameter starting disc and an equivalent size of mould. Fire this slump at your standard slumping schedule for large shallow pieces.

·        Second stage – Add powdered kiln wash or whiting to the deep mould.  Fill the mould to half or two thirds of the volume.  Smooth a shallow depression in the powder.  It should rise to meet the curve of the mould shoulder, even if it does not fully match it. This firing is probably the most critical in the DIY process.  The shallow shape will be considerably larger than the diameter of the mould on which you are placing it.  This means that you must fire slowly and you should peek frequently.  As the glass begins to slump, the outer edge will begin to rise at first.  As soon as the outer edge begins to relax, you must advance to the annealing segment.  If you allow the rim to sag, it will not sit very well in the mould at the next stage.

·        Third Stage – This may require more than one firing to achieve the intermediate shape.  In preparation, remove about half of the powder from the previous firing. Shape the remaining powder to a smooth curve. Fire the glass, again watching and advancing to the anneal when the rim begins to flatten.  If the glass has not touched the powder at the bottom, you will need to do another firing.

·        Fourth stage – Remove all the powder from the deep mould. Place the glass and fire.

Keep the kiln wash powder for future use. Its composition will not have changed as you have not fired it to tack fusing temperatures.  Dispose of the whiting, if you used it.  It may be fine for further use, but since it is cheap, it is not worth the risk of it sticking to the glass in subsequent firings.

Remember that long – low and slow – slumps are required at all stages of creating a deep slump.  As a comparison, think about the hours required for a free drop to form and still keep the glass at the shoulder thick.  Deep vessels require long hours of watching just as aperture drops do.

Friday, 13 July 2018

Fire Polishing

Polishing of glass can be done in the flame, in the kiln, by acids or by grinding with successively fine abrasives depending on the nature of the piece and the equipment available.  Fire polishing in the kiln is widely popular as it utilises existing equipment, avoiding purchasing additional cold working equipment. This post indicates some elements about fire polishing in the kiln. 

Fire polishing is the technique most often available to kiln formers. This is the process of heating the glass to less than a full fuse to achieve a smoother texture on the glass. It is often used after sandblasting or hand sanding a piece in order to give a smooth shiny surface to the glass without extensive cold working with successively finer grits to get a polish. It also can be used to give a variety of textures from a sealed but almost unchanged sandblasted surface, through a satin-like finish to a very subtle difference between full polish and slightly textured surfaces in the same piece.

Fire polishing range
The temperature range that this occurs between slumping and tack fusing. The normal range is 650C to 750C depending on the glass, the soak time and the speed of advance.  The purpose of this kind of firing is to get the surface of the glass hot enough to form the desired surface without soaking long at higher temperatures, as this is also the devitrification range (700C - 760C).  Normally there would be a minimal or no soak at the top of the temperature range.

When to fire polish As this temperature range is above the slumping temperature, fire polishing should be done after fusing and before slumping. As this will be the last operation before forming, you also should do any work to shape the edges and deal with any other imperfections, before fire polishing. After doing any grinding or other work on the edges or surface of the piece, thoroughly wash and polish the piece dry.

You can take the fused piece that has been treated to remove the devitrification up at the same rate as for slumping the piece to the tack fuse temperature.  The higher you go, the less soak time is required. Of course, the higher you go, the longer you are in the devitrification zone.  

Some people advocate a quick fire polish.  This is achieved by firing at a relatively slow rate until a low slump temperature is achieved.  Then fire very quickly to the tack fuse temperature with no soak and return to annealing temperature as quickly as possible.

The quick fire polish does achieve a minimum of time in the devitrification zone, but it eliminates all subtely in the surface.  A long soak of up to 90 minutes at a moderate slumping temperature will give a satin appearance to an abraded or sandblasted surface.  A shorter soak will seal sandblasted work without eliminating the texture of the sandblasted image.

In all the cases of fire polishing you need to peek at intervals to determine when the desired surface has been achieved.  This requires careful placement in relation to the place from which you will be able to peek at the surface.  For a fully polished piece, you will see the reflections of the elements.  For more subtle textures, you need to think about what you want to see, peek, close the lid or observation port and think about what you saw.  If it is not yet what you want, peek at another interval in the same way, until you observe the surface you want.

Combining fire polish and slumping It is sometimes possible to fire polish and slump at the same time, but this is a risky technique often leading to changes in shape or an uprising of the glass at the bottom of the mould. It is possible to fire polish glass as low as 630 with a long soak – 60 minutes or more. If you are determined to fire polish and slump at the same time, it's essential that you watch the piece very carefully to prevent over-firing.

Fire polishing already slumped items Similarly, re-firing already slumped items to a fire polish rarely succeeds. Distortion of the piece is more likely than achieving a fire polish on an already slumped item.

Again, in these more difficult circumstances, you must observe at intervals to ensure you do not over fire and distort your piece.

The reason that no indicative schedules are given is that different glasses, and different lay ups require different firing conditions.  These are dealt with elsewhere in the blog.

Alternatives Alternatives to fire polishing include acid polishing, which can present a health hazard, and is normally an industrial process. The other common method of polishing is to cold work the piece. This often requires specialized equipment, but can be done by hand if you have the time.

Wednesday, 11 July 2018

Smooth Kiln Wash on Shelves

There are a number of ways of applying separators to the kiln shelf.
These go by a variety of names - kiln wash, shelf primer, batt wash, etc. - all are separators to keep the glass from sticking to the shelf. They are all combinations of alumina hydrate and china clay (or kaolin or EPK) in various amounts.  The china clay provides a high temperature binder for the alumina hydrate which does not stick to glass.

These are some examples of glass separators.  The Primo Primer has very little china clay, and is easy to remove.  It is particularly good for small casting moulds.


The object in applying the separator is to achieve a smooth surface a possible. Remember there will always be some texture because of the particle size of the wash.  For the smoothest surface, use the finest powder you can find.  You can, if you want to spend the time and effort, put the powder into a rock tumbler with ceramic balls to get an even finer powder.  Avoid shelf primer that is intended for ceramics, as it is coarser than that sold as a separator for glass.

It also is important to prepare the mixture some hours before application to ensure all the particles of the powder are wetted.  Immediate use often leads to a gritty surface.

There are several methods for applying the kiln wash to the shelf.  The two I use are spraying and brushing.  Which I use depends on circumstances - spraying requires more set up time.

Spraying the separator onto the shelves can give an even coating without brush marks, runs or ridges.  In this example a mould is being sprayed.  To ensure an even covering on a shelf, it should be horizontal and leveled so the kiln wash is evenly distributed.  Numerous light passes with the sprayer is best, as in air brushing.

Applying the kiln wash with a very soft brush such as a hake brush in a variety of directions will ensure full coverage. The brush should lightly touch the shelf and provide a number of thin layers.  Applying in four directions - horizontal, vertical, and the two diagonals will ensure full even coverage. There may be some residual brush marks.

To reduce the application marks further, you can brush or spray hot water over the still damp kiln wash. This helps to remove brush marks or the stippling that often comes from spraying and brushing.  It is important that the shelf is perfectly level for this operation.

Another way to reduce the texture after the shelf primer dries is to lightly polish the kiln wash with a ball of old nylons or rub a flat piece of paper with the palm of your hand over the shelf.  Be sure to remove the dust that may be left behind from this polishing.

Boron Nitride

Another separator that has become popular in spite of its expense is boron nitride, often referred to by the trade name Zyp.  This is a high temperature lubricant for industrial kiln operations that has been adapted for the generally lower glass forming temperatures.  This is not suitable for kiln shelves, as it completely seals the porous surface of the shelf.  It is difficult to go back to the cheaper kiln wash separator as the water of the kiln wash solution will not be absorbed into the shelf, leaving a patchy coverage of the kiln wash.  Although both separators should be renewed after each firing (above low temperature tack fusing) the boron nitride is much more expensive and cannot provide a smoother surface than the shelf already has.  My recommendation is that boron nitride use should be confined to moulds or other surfaces where the glass may slide or move in the forming process.

Adding Colour to Slumped Pieces

Sometimes an already fused and slumped clear piece needs colour for interest, definition, etc. The problem is how to do it without altering the fused piece.

You can use cold paints. Both acrylic and oven baked paints can be applied, but often they are unsubtle, harsh colours.  These are not permanent.

You can apply enamels.  These can be the ones produced for glass fusing, if fired carefully. The curing temperature of 780°C means that you need to fire slowly to about 600°C and then quickly to 780°C with no soak and AFAP to annealing.  This fast rate of advance is to preserve the shape as much as possible at temperatures above that required for slumping. This will need to be done in the mould, of course.

You can more safely use traditional glass stainer colours, which are also called enamels, although they are slightly different from the traditional ones.  These cure between 520°C and 580°C so can be fired as normal in one steady climb to the top temperature with no soak and quickly down to annealing. To be sure the shape is retained, the glass should be in the mould during the firing.

Use of frits and powders requires the higher temperatures that will distort the piece unless fired in the mould. When firing to tack fuse in a mould, you need to be careful that you do not damage the mould during the higher temperature firing, nor get the separator incorporated into the powder.  If you can place the powder or frit on top of the glass, you will get a better result at a lower temperature as the frit will heat and spread more easily on top than on the bottom. 

In general, it is not recommended to add colour to slumped pieces with frits and powders.  It is hard on the mould, and risks the glass sticking to the mould. Even if successful, the slumping mould will have to have the existing kiln wash removed and new added to avoid the kiln wash sticking to the next piece to be fired.  

It is better to slump the piece to flat, if possible, and then add the frits and powders before fusing.  Then slump again.

These notes show that it is important to assess the flat piece critically before proceeding to the slump.  This can mean setting the piece aside for a few days to review your impression of the fusing result.  This little time elapse can give you a fresh view of what the piece requires, if anything. 

Wednesday, 4 July 2018

Grinding to Fit

In copper foiling, a considerable amount of work goes into getting the pieces to fit with just enough space to accommodate the copper foil and a thin space for the solder fin to join both sides.  This of course, promotes consistently narrow solder lines without the solder melting through to the opposite side.

Grinding to pattern
Many times it is necessary to grind to fit pieces together with this degree of accuracy.  Those who draw onto the glass or stick pattern pieces to the glass, often grind to the template or the drawn lines.  This can lead to inaccuracies in relation to the cartoon.

The object in scoring and breaking the glass is to be as accurate as possible.  This reduces the amount of grinding required.  It saves time. It makes the whole process easier.  Still, we all have to grind relatively often.

Grinding to cartoon
In my view, when grinding to fit, you should be trying the piece out against the cartoon, rather than the template or the drawing on the glass. This will tell you how well the current piece fits in with the rest of the pieces you have already fitted to the cartoon. 

The cartoon drives the assembly of the whole piece.  Thinking you can just make small adjustments as you work along, creates increasing difficulties in making the whole fit together.  If you follow this principle of fitting to the cartoon, you are judging the accuracy of the piece against the cartoon lines, rather than any template or drawing on the piece of glass.  This means that the fit will be correct and the whole will go together with the minimum of difficulty.

Centering Holes for Drilling

When using larger core drills, it is not possible to see the centre point for drilling. So a different arrangement for marking the place to be drilled is required.  This example is to locate the hole for a clock spindle accurately bewteen the points marked by the stringers.

Find the centre point and extend lines at right angles to each other across the centre point.

Then measure the radius on each arm and make a perpendicular mark on each of the radial arms. Depending on how long those lines were, you have something approaching a box.

Approximately center the drill over the center point. In this case it is a portable drill, but the principle is the same for a pillar drill.

Lower the drill bit over the hole. The radius marks allow the drill operator to see the edge of the hole and use any two of the marks to centre the drill bit within the hole.

This procedure ensure the accurate positioning of the hole.  This is especially important when fitting to existing fixing points rather than making new ones.

Wednesday, 27 June 2018

Over Annealing

 I hear the comment "you can't over anneal" all the time. Is it true?

My response to this may be controversial, and I do expect there will be some dispute with aspects of what follows.

My view of the statement “you can’t over anneal” is that it results from a lazy approach to thinking about the process.

The short answer is, in my view “yes, you can over anneal”.

  • ·         Lengthy anneal soaks can induce stress in certain circumstances. More later.
  • ·         Excessive annealing soaks waste time and money.
  • ·         Annealing is more than the soak.  It is a combination of equalisation of the heat within the glass (not just temperature) and the gradual cooling of the glass to below the lower strain point to ensure the glass does not incorporate differences of temperature of plus or minus 5°C.

There is both tradition and research to assist in determining the length of the anneal soak.  The tradition seems to embrace 30 minutes anneal soak for each layer of glass. The research has been done by Bullseye and they have developed a table to assist in accurately determining annealing soaks for thick glass. 

Although this is for thicker pieces, it will inform users of the relationship between thickness and annealing soaks.  The table starts at 12mm thick, but you can extrapolate that a 6mm flat piece cooling from both sides will need a one-hour soak, an initial cooling rate of 110°C, a secondary rate of 200°C.  It is safe to turn the kiln off at 370°C, as the kiln is unlikely to be able to cool faster than the 330°C suggested (although I programme to room temperature). The temperatures used need to be altered for glass other than Bullseye, but the rates remain valid. My advice is to use the research, rather than tradition.

Other considerations include the nature of the kiln.  If your kiln has significant temperature differentials across the shelf, long annealing soaks will incorporate those differences during the annealing cool and result in a stressed piece. You do know the temperature distribution within your kiln, don’t you?  This Tech Note #1 from Bullseye will give you the information to test for the temperature distribution. Using this information will enable you to avoid the cool spots when placing your pieces and utilise the areas where the heat is even.

Economy is another reason that it is possible to over anneal.  Soaking at the annealing temperature uses a significant proportion of the electricity consumed in a firing.  This means an overly long temperature equalisation soak will use more electricity than necessary.  It also uses more kiln time than necessary, by delaying the anneal cooling and the following natural cooling rate of the kiln.

It is possible to under anneal
You need to learn about the effects of your project on annealing requirements, because it is possible to under anneal.  The research on annealing is based glass of uniform thickness. The most popular style of kilnforming appears to be tack fusing of one degree or another.  This is unfortunate for the novice, as it is the most difficult of styles to anneal adequately. There are a lot of factors to consider when setting the annealing schedule. 

I feel this is the origin of “can’t over anneal” thinking.  Instead of thinking about the specific annealing difficulties, many seem to just add more time in a generally random manner.  The post on tack fusing considerations (the link above) is designed to help in thinking about the requirements of the lay-up of your piece. The cumulation of factors can easily treble the annealing soak and slow the rates by three times. In some extreme cases, the annealing time can be extended by as much as five times.

What is the anneal?
Another problem is that most often annealing is thought of as merely a soak at the annealing point of the glass.  It is much more than that.  The annealing point is usually the temperature at which the heat within the glass is equalised in preparation for the anneal cool.  This is because the annealing temperature is that at which the glass will most quickly anneal.  Since the anneal is temperature sensitive, the equalisation of the temperatures within the glass will be most successful at getting a good anneal throughout the cool.

For two-layer flat fused items, the annealing point can be used as the heat equalisation temperature.  The soak is to get the glass to be + or - 5°C throughout the piece. 

Sometimes, especially with thicker or more difficult pieces, the annealing is done closer to the lower strain point. The reason for this is to save time in the annealing cool.  If you look at the Bullseye annealing chart, you will see how slowly thick pieces need to be cooled, so starting 35°C below the annealing point can save many hours of cooling.

Once the glass has equalised in temperature, the object is to cool the glass at a rate that ensures the internal temperatures do not vary more than plus or minus 5°C across and through the piece.  The rate can increase by approximately twice after the lower strain point has been reached (approximately 55C below annealing soak).  This second stage rate should take the glass to around 370C, where the rate can again be doubled to room temperature. 

Difficult pieces
Tack fused and other pieces with uneven thicknesses require more care in the annealing to ensure even cooling of the whole without a greater variation in temperature than +/- 5°C.  As said above, tack fusing is one of the most difficult of styles to anneal adequately.  The blog entry for tack fusing considerations indicates some factors that increase the requirements for more careful annealing.

As an example, I cite a piece 6mm thick, with two layers of rectangular and pointed pieces that are just barely rounded.  This adds five factors of complications for the fusing - two levels of tack fusing, rectangular pieces, pointed pieces, laminated tack fusing.  This number of complications increases the practical thickness to 21mm – 6mm of flat base, 3mm each layer of tack (6mm), 3mm for rectangles, 3mm for pointed pieces, 3mm for laminated fuse.  Because this is tack fused, the next practical step up in the table needs to be used. That is the one for 25mm, which requires a four-hour temperature equalisation soak, and 15°C per hour initial anneal cool rate.

Glass other than Bullseye
I have so far talked about Bullseye.  It is possible to apply these times and rates to any glass of which you know the annealing point.  The annealing soak can be set above the lower strain point, which is approximately 55°C below the annealing point.  To be safe, a point 35°C below the annealing point is used.

E.g., if you are annealing a 12mm slab of float glass, the annealing point of which (in the UK) is 540°C, you chose a temperature of 505°C to do your two-hour soak, followed by a cool rate of 55°C for the first 55°C and then 99°C for the second stage cool to 370°C.  The final cool of 330°C per hour to room temperature remains the same too.  So, you can see the rates and soak times remain the same regardless of the glass type.  It is only the temperatures that change.

A summary of this can be seen here.

Bullseye chart for Annealing Thick Slabs

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