Edge Treatment in Cold Working

Frequently people who are grinding the edges of bowls, aperture drops and other vessels that need to have a smooth rim find that they are getting small chips of glass coming from the edge of the ground part of the glass.

There is a way to prevent theses unwanted chips  

The long established practice of glass workers has been to give the glass an arris at the end of each grinding stage before they change to a finer grit.  This small area of angled glass, allows the continued smoothing of the glass without creating such a sharp edge that the glass there is not strong enough to resist the grinding action.  

You will notice on a bowl or other rounded vessel, that the chips are almost always on the outside. The inside of the rim normally has an oblique angle to the rim, and the outside an acute angle.  The explanation is held in the angle.  As the rim is ground down, the outer acute angle becomes very thin as well as sharp.  At some point the glass is thinner than the grit used to grind the surface.  This causes little chips of glass to break off the edge.

By creating an oblique angle at the edge of the grinding surface, the glass will remain thicker than the grit being used to grind the glass.  If you feel you are taking off a lot of glass, it is advisable to check that the arris is still in place.  If not, give it a light grind to maintain the arris while using that grit. 

At the end of each stage of grinding, you need to add an arris for the next stage.  The reason for doing it with the coarser grit rather than the one you are about to proceed to, is that it maintains all the grinding at the same stage, enabling the whole piece to be finished to the same level of polish.

Wipe the surface dry and add marks with a paint marker.  Allow this to dry while you change grits.  The purpose of the marker is to assist you in determining when you have ground out all the previous marks, by the elimination of the paint.

Low Slumping Temperatures.

It is possible to have glass overhanging slumping moulds if you use low temperatures. The glass has the appearance of behaving differently at these low temperatures than at fusing temperatures.

Glass at low temperatures is affected largely by weight.  

At low temperatures it cannot quickly form exactly to the mould. It falls first in the middle.  Because the glass is not very plastic, the edges rise up from the mould at first, because the weight there is not great enough to allow the unsupported glass to bend.  The edges stay in line with the beginning of the bend in the middle. (apologies for the quick and dirty drawings)

If you have a mould with a rim, you will be able to observe this effect.  As the glass in the middle begins to slump, the glass at the edge rises from the rim.  This allows the central portion of the glass to settle and draw the excess glass into the mould.  

It only settles back to the rim with the heat work of the slump as the slumping soak continues.

It is useful to record the temperatures and times of these effects for different moulds.  It tells you when the slump begins, the intermediate stage(s) and completion of the slump.

Using the lowest practical slumping temperature gives the best results.

·         It allows glass with small overhangs of the mould to be successfully slumped. 
·         Low temperature reduces the mould marks on the back of the glass.
·         Fewer stretch marks are in evidence. 
·         Low slumping temperatures with long soaks reduce the uneven slump that is sometimes in evidence with deeper moulds.
·         Low temperatures allow different colours to heat more evenly.
·         Low temperatures reduce the thinning effect of a high temperature slump.

Large Shelves in Small Kilns

Sometimes you want to put a larger than usual piece in the kiln.  But your standard shelf is just too small.  There are some things you can do to allow the firing of the larger piece.

You can put a two part or multiple part shelf into the kiln. However, joining two or more small shelves to make a larger one will continue to show lines where they join.

 

You can put in a single larger shelf. If this is a mullite shelf, you need to be sure you can get your fingers into the gap to get enough purchase to lift the shelf back out.

You can put a large fibre or vermiculite board over the existing shelf and so extend the area.  You need to prepare these boards by firing and preparing them for firing with glass on top.

But, there may be problems with adding more shelf area. 

If you have a side firing kiln, the glass will be much closer to the elements, which will require baffles and certainly slower firing schedules.

The reason for the gap between the sides and the shelf are to allow air circulation underneath the shelf to get even heating and even cooling.  Restricting this air flow requires slower schedules. Some experts suggest cooling from one side – which this now will exhibit – requires annealing at half the rate of cooling from both sides, i.e., with air space along the sides of the shelf.  This means doubling the anneal soak, and a reduction by half of the annealing cool rate.

Kiln Washing Kiln Lids

It is frequently recommended that the bottom of the kiln should be kiln washed to prevent any spilled glass from sticking to the kiln brick.  You should remember that this is applicable to brick lined kilns.

This in itself is a little clue.  You do not need to kiln wash any insulation fibre in the kiln. If any glass were to stick to the fibre, it would come away easily.  In any case, most insulation fibre blanket will not stick to the glass.

The recommendation often goes on to advocate kiln washing the sides.  There is a caution that the side elements (if any) should not be kiln washed. The caution comes from the knowledge that water and electricity should not be mixed.  The kiln should not be on when applying kiln wash anyway.  If kiln wash is splashed onto the elements, it is simply a matter of letting the whole kiln dry naturally with the lid open before firing.

The extension to this series of recommendations is that the whole of the kiln should be kiln washed, including the lid.  This is not a good idea.  The wash on the lid will soon fail and drop dust and debris onto and into your work.  The glass should never touch the top of the kiln anyway.  If the elements are in contact with the glass, the glass will either stick to them or break.  You have to ensure you do not put glass nearer than about 20mm to the elements or lid. In any case, the glass will fall to the bottom of the kiln, not the top or sides – unless the kiln is not level.

BUT

The whole idea of kiln washing the interior of the kiln is suspect in some ways.  Anyone who has had glass drip off the shelf and onto the brick during an over-firing will know the glass eats into the brick through the kiln wash.  Kiln wash will only protect the brick at full fuse or less temperatures. But it is a good precaution to keep the pieces of frit that fall off the shelf from sticking to the brick. It does not do much more than that.

The application of kiln wash to the kiln creates another source of dust within the kiln.  Dust and general uncleanliness in the kiln is a main potential source of devitrification. Thus, the application of kiln wash should be the minimum necessary and does not need to go up the side beyond the elements or the lowest shelf height, whichever is less.

There is a strong argument to be made that laying a sheet of 0.5 mm fibre blanket on the floor of the kiln will provide better protection of the kiln than any amount of kiln wash.  It is less likely to fail, it is not a source of additional dust, it provides a better protection during any kiln runaway, and it is easily replaceable.

Thinking About Design

To think about design, you need a vocabulary to describe the object. This needs to be combined with a structure of principles. What follows is an outline to structure your thinking about design.  This is based on the writing of Burton Wasserman in Spark the Creative Flame, Making the Journey from Craft to Art, by Paul J Stankard, 2013.

First there is the vocabulary to structure the conversation about design. The elements of this are “… point, line, plane, texture, colour, pattern, density, interval, … space, … light, mass, and volume”

There are then principles of good design.  They relate to:

Unity – all the elements form a whole

Balance – note, not only symmetry

Rhythm – this can be repetition with or without variation

Emphasis – or contrast between a main element and the rest

Harmony – all the elements work together

These five principles of design together with the vocabulary of elements form the language of design and assist your critical thinking about expressing your design and realising it in the best way you can.  This thinking can be applied usefully to the critical appreciation of others’ works.

Heavy Metals in Glass

Some concern has been expressed about the metals used in colouring glass.  This centres around the temperatures used in fusing and whether kiln workers may be of risk from these heavy metals vaporising.

First of all, let’s get some sense of perspective. This is from Greg Rawles, an acknowledged expert on the hazards of working with glass.

Understanding Exposure:

In reality, unless you are doing:
High-volume production work that exposes you to a health hazard all day long
You are exposing yourself to high levels of a health hazard for a brief time
You are working with a very toxic material
You are not working responsibly

You are not really at risk for an unacceptable exposure when working in a glass studio  http://www.gregorieglass.com/chemicals.html

Now, let’s think about how likely it is to have heavy metals vaporise at kiln forming temperatures. How stable would glass be if the metals that colour it vaporised when we fired it? the colour would vary with the heat and number of times we fired it.

Now, let’s think about how likely it is to have heavy metals vaporise at kiln forming temperatures. How stable would glass be if the metals that colour it vaporised when we fired it? the colour would vary with the heat and number of times we fired it.

Even if the metal were to evaporate, how much is in the glass. Apparently, Bullseye uses less than 3 pounds of cadmium for a pot of glass. We can tell from the sheet numbers that a pot of glass gives at least 2000 sheets of glass, so there is ca. 0.0015 lbs or .07 grams or less of metal in a sheet of 3mm glass. There is very little there to "vaporise", so even it were able to evaporate, it is in such small quantities as to be negligible, and the exposure so low as to be of extremely low risk. There is however, no risk in protecting yourself with dust masks. Just remember that the risks from vaporised heavy metals is much less than most of the other studio practices involving glass. If you need breathing protection for metals (and you may feel it is not worth the risk) then you need to be wearing a mask all the while you are doing glass work. It is about relative risk.

For complete information, the melting and boiling points of various metals relevant to glass colouring are given below.  The vaporisation will be somewhere above the melting point and toward the boiling point.  You will be able to see the relevant temperatures and take any precautions you feel are necessary.  Remember that the metals are not used in their pure forms, but as oxides.  These may have different melting and boiling temperatures.  In general, the oxides used in colouring glass have higher melting and boiling points than the pure metal.

Antimony -for whites

Melting point: 630C

Boiling point:  1635C

Antimony Oxide

Melting point:  380-930C

Boiling point:  1425C

Cadmium 

Melting point: 321C

Boiling point:  767C

Cadmium sulphide - yellow

Melting point: 1650-1830C

Boiling point:  2838C

Chromium 

Melting point: 1907C

Boiling point:  2671C

Chromic Oxide – for emerald green

Melting point: 4415C

Boiling point:  7230C

Cobalt 

Melting point: 1495C

Boiling point:  2927C

Cobalt Oxide- blue to violet

Melting point: 1900C

Copper 

Melting point: 1084C

Boiling point:  2562

Copper Oxides - for blue, green, red

Melting point: 1232-1326C

Boiling point:  1800-2000C

Gold

Melting point: 1337C

Boiling point:  2970C

Gold Chloride - red

Melting point: 170-254C

Boiling point:  298C

Iron

Melting point: 1538C

Boiling point:  2862C

Iron Oxide – for greens and brown

Melting point: 1377-1539C

Boiling point:  3414C

Lead – for yellows

Melting point: 327C

Boiling point:  1749C

Manganese 

Melting point: 1246C

Boiling point:  2061C

Manganese Dioxide – purple and a clarifying agent

Melting point: 535-888C

Neodymium

Melting point: 1024C

Boiling point:  3074C

Nickel 

Melting point: 1455C

Boiling point:  2730C

Nickel Oxide – for violet

Melting point (II - for green): 1955C

Melting point (III - for black): 600C

Selenium

Melting point: 221C

Boiling point:  685C

Selenium Oxide – for reds

Melting point: 118-340C

Boiling point:  350C

Silver 

Melting point: 961C

Boiling point:  2162C

Sodium

Melting point:  370C

Boiling point:   882C

Sodium Nitrate – a clarifying agent

Melting point: 308C

Boiling point:  380C

Sulphur

Melting point: 115C

Boiling point:  444C

Sulphur oxide - for yellow to amber

Melting point:  17C

Boiling point:   45C

Tin 

Melting point: 231C

Boiling point:  2602C

Tin Oxides – for whites

Melting point:  1080-1630C

Boiling point:  1800-1900C

Uranium

Melting point: 1132C

Boiling point:  4131C

Uranium oxide – for fluorescent yellow, green

Melting point:  1150-2765C

Boiling point:  1300C

Reducing Stress Points in Tack Fusing

Stress is greater in tack fused pieces than in full fused. Tack fused pieces to some greater or lesser extent behave independently from the base and surrounding pieces.  This means that more care must be taken in the anneal cooling of the glass.

Stress is dissipated more evenly in rounded tack fused pieces so the stress is not concentrated as individual points around the edge of the glass.

Stress is however, concentrated in corners of rectangles and in points of triangular and the ends of thin pieces.

By nipping the corners off these sharp angled pieces, the amount of stress concentrated there can be reduced.  Very little needs to be removed to have the effect. So the appearance of the angles is hardly affected.

Using your grozing pliers, you can take a small piece of the corner off.  It needs not be much more than a large grain of sand. This should be done at all corners and points.  It will not reduce the amount of annealing or the rate of cooling, but will assist in reducing the possible stress built up in the tack fused piece.

Filling Gaps in Fused Pieces

Often your cutting is not as accurate as you would like so there are small gaps between the pieces as you assemble the piece.

Tammyhudgeon.com

One solution that is often used is to grind the edge of the too large piece to get the fit desired.  The problem with this is that thorough cleaning is required to avoid devitrification lines appearing on the final piece.  Also, even with extensive grinding, the fit is not perfect.

The alternative is to fill the gaps with fine frit or powder. 

artistryinglass.com

Assemble the whole piece and assess the gaps.  If they are very large, you need to adjust the glass. If they are only millimetres wide, powder and frit can fill the gap to disguise the join.  I generally use powder for almost perfect joints, and fine frit for anything larger.

I first cover the gap with powder or frit and with a soft brush work at right angles to the line of the join.  This ensures that I have filled the gap to the height of the glass.  However, the frit and powder have air spaces, and so will fuse to a lower level than the height of the glass.  So, once gap is filled, I build a small ridge over the gap trying not to extend beyond the gap.  This mound compensates for the lack of density of the frits.

The frit and powder colour must match the glass exactly to become invisible.  It can be made from your scraps or purchased at the same time as the glass. I find it more successful to do these fills with the darker glass.  It provides a more distinct edge to the joint.  It also conceals the base glass better.

It can also be used to conceal the joint in a single colour where the piece cannot be cut as one and needs several pieces to make the whole.  This is more simple as any overspill will not be noticed when fused.

This method only works with full fusings.  At tack fuse temperatures the frit will not fully combine with the sheet glass to form a smooth invisible join surface.

Devitrification on Repeated Firing

 Devitrification is defined as the crystallisation of the glass, making it a non-vitreous substance.

Molecular level difference between vitreous and devitrified silica
from Digitalfire.com

You can see that there is not much difference between the the two states of the glass in structure, but mainly the arrangement of molecules.

The appearance of devitrification has a range of appearances from a mild smeary look through a dull surface to a crazed, crumbly aspect in severe cases. 

Mild devitrification

Medium level devitrification requiring abrasive cleaning

Causes of devitrification are related to slow changes of temperature (up or down) and most importantly nucleation points such as dust, oils, or cleaning residues. So, thorough cleaning is most important. 

Causes in repeated firings of the same piece relate to:

        Cleaning. 

It is important to thoroughly clean the piece before each subsequent firing.  Many times abrasive cleaning such as sandblasting is important to clean out impurities from the previous firing.  The resulting surface from any abrasive cleaning requires further cleaning with lots of clean water and a thorough drying with clean cloths or paper.

        Slow cooling or heating. 

Devitrification normally occurs in the range of 670⁰C to 750⁰C. This is the reason for the rapid rates of advance in this temperature range rather than other factors.  It can form both on the rise and on the fall in temperature. Slower rates in the devitrification range allow enough time for the crystallisation to begin.

        High temperatures.

Both high temperatures and long soaks can promote devitrification.  It is not just the slow rise or fall in temperature, but long periods at high temperature can lead to devitrification even though other precautions have been taken.

Changes in the composition

High temperatures and many repeated firings of the piece can lead to changes in the glass.  Some metals and fluxes are more likely than others to change composition or oxidise at extended soaks at high temperatures.  This can reduce the ability of the glass to resist devitrification.

Prevention/Correction

Prevention relates to thorough a) cleaning and b) firing rates.

All correction of devitrification relates to the modification of the surface.  If the problem is only at the surface, you can use either abrasive cleaning or the addition of fluxes to the surface, or a combination of the two. 

Where you have a mild dulling of the surface due to devitrification you can apply a flux.  This softens the surface by reducing the melting temperature of the glass and so reverses the crystallisation at the surface. The devitrification solution can be a proprietary spray such as Super Spray. Be aware that some sprays use lead particles as the flux, so are inappropriate for pieces intended to be food bearing. You can make your own devitrification solution by dissolving borax in distilled water.  When the devitrification is wide spread or deep, abrasive cleaning is required.

Abrasive cleaning can be by hand with sandpapers or diamond pads.  Be sure to keep them damp.  This keeps dust from rising, and the sanding surfaces clean for better working.  Sandblasting can be quicker, especially on uneven surfaces or where there are deep imperfections.  The surfaces resulting from abrasive cleaning need to be scrubbed clean with sufficient water, and then polished dry as for a finished piece.

It is possible to combine both these methods to be more certain of a shiny finish.  When combining, you need to do the abrasive cleaning first, then the wet cleaning and finally add the devitrification solution.

A fourth possibility is to sprinkle a fine but consistently thick layer of clear fine frit or powder over the piece.  This, when fused, provides the new surface concealing the devitrification below.  Again, this must be done at a full fuse, so it is not applicable to items you wish to remain tack fused.

However, if the devitrification has progressed to a crazed appearance, it is so deep as to be almost impossible to reverse.  The piece will also probably have developed incompatibilities. So the only real option in crazed pieces is to dispose of them.  They will not be useable in combination with any other glass. They will make any glass with which they are combined subject to devitrification and possible breakage.  These are pieces which truly cannot be cut up and re-used.

Annealing Multiple Levels of Tack Fusing

A question was asked of me about schedules for tack fusing multiple pieces – three layers thick in places – as a single unit, then placing on a 6mm fused base and tack fusing.  Special interest was in how the different thicknesses and the tack fusing would affect the scheduling of the annealing.

My response – edited – was as follows.

This is going to be a long reply.  I have written a general guide to tack fusing that will be useful, but this response will try to be more specific to your project.

First, tack fusing of pointed things is more sensitive to annealing than rounded things.  For up to 3 layers of triangles, I would be thinking of annealing for at least 12mm (four layers). This means a 2hour soak at 482⁰C, followed by a cooling rate of 55⁰C for the first 55⁰C degrees and then 99⁰C for the next 55⁰C. After this 110⁰C degrees of cooling the rate can be as fast as 330⁰C/hr.  This will apply whether Bullseye or System 96 is involved.

Second, from the description I take it that a 6mm clear under a 3mm layer of two colours side by side is being fused as a base.  [This was confirmed], so you could fire at 200⁰C/hr to a bubble squeeze of 30mins and then 300⁰C/hr to top temperature.  Anneal at 482⁰C for 60-90mins and cool for first 55⁰C at 65⁰C/hr and the next 55⁰C at 150⁰C/hr, followed by 300⁰C/hr to room temperature.

The third stage is to combine them.  Think about how thick this is physically – ca.18mm.  Then think about the differences in thickness – 9mm.  My rule of thumb is to add the difference between thicknesses to the thickest part – in this case to 18 plus 9 equals 27mm.  This is the “scheduling thickness” for this variation with rounded elements.  As your piece has lots of triangles, you need more care.  It is an additional level of difficulty.  So I add another 3mm to my “scheduling thickness” to accommodate the angular aspect of the piece, making a total of 30mm for putting the two fused pieces together. 

This thickness leads me to propose a relatively complicated schedule.  I suggest 70⁰C/hr to 250⁰C, 100⁰C/hr to 540, 120⁰C/hr to 620 and then 150⁰C/hr to top temperature.  The top temperature will be lower than your normal tack fuse temperature because this is a much slower rate of advance than normal.  This in turn, means that you will want to be checking at intervals on the tack fuse progress from at least 720⁰C.

The annealing will be long and slow. About 5 hours at 482⁰C, 11⁰C/hr to 427⁰C, 20⁰C /hr to 370⁰C and 65⁰C /hr to 30⁰C. This will be a schedule of about 35+ hours.

The two sources mentioned earlier give the rationale for this kind of schedule.  Think about the considerations I have listed, and then decide whether I am being too cautious or not.  The principle remains - as you increase the risk factors, you

·         slow down rates of advance and cooling rates, and

·         extend soak times.

You should note that I have used Graham Stone’s Firing Schedules for Glass, the Kiln Companion and the Bullseye chart for Annealing Thick Slabs in preparing the proposed schedule, although you will not find this exact schedule in either of them.