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.

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

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

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

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

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

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

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



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

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.
Avoidance   

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


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

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

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





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

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

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

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

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

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

Curves with angles

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

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

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

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


Removing kiln wash from glass

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

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




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





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





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

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.


Wednesday, 25 October 2017

CoE of Paints for Fusing


CoE of the glass carrier for paints is a distraction.

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




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


CoE in relation to paint does not matter.

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




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





Wednesday, 18 October 2017

Slumping Glass that is not Tested Compatible

Is it Possible?

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

Preparation of the Glass

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

Cleaning

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

Firing the Slump

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

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

Observation

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

Altering the Schedule

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

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

Annealing

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

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


Sunday, 15 October 2017

White solder beads

It is relatively common for questions about white deposits on the solder beads of copper foiled pieces to be raised. In reflecting on the cause of the white deposit on solder beads, I recalled that some people use baking soda to neutralise the flux.  I put this together with some work on lead corrosion.


I have been doing a bit of research on lead came corrosion in another context.  One of the forms of lead corrosion is white lead corrosion, or lead carbonate.  It has the chemical compound PbCO3.  It is a curious compound, as it is soluble in both acid and alkali.  This much you will have seen from a previous posting about lead corrosion.  


In that it is possible for excess whiting left on lead cames to give rise to this form of white corrosion. Baking soda has a chemical formula of NaHCO3.  Solder contains a significant amount of lead – usually 37-40%.  The chemical reaction of lead and baking soda gives lead carbonate - PbCO3 and NaH -sodium hydride.  The sodium hydride is soluble in water, leaving the white deposit of lead carbonate as a corrosion product on the surface.


Putting these things together leads me to recommend that baking soda and other carbonates should not be used in cleaning solder beads.  Some other non-carbonate neutralising or rinsing agent should be used instead.

Wednesday, 11 October 2017

Separator Cost Comparisons


Many people are concerned about the cost of kiln forming, but use fibre paper rather than kiln wash or powders, although it is many times more expensive. This may be a matter of convenience.  This leads me to an exercise in comparing relative costs and benefits of various separators.

Separators are essential to keep the glass from sticking to the shelf or mould that supports the glass. There are several forms of separators –
·         papers,
·         liquids and
·         powders.

Papers
The papers include the very smooth Thinfire and Papyros papers and the rougher papers ranging from 0.5mm to 6mm.  All these contain a binder of some kind. 

·         Papyros, Thinfire
·         Refractory fibre papers - .5 to 6mm

These are mainly suitable for flat surfaces.

Liquid
·         Kiln wash – there is a variety. Most have kaolin - china clay - as a binder.  A few do not.  These you can just brush off the shelf or mould after firing.
·         Colloidal Boron Nitride – a popular formulation is called Zyp.

These are suitable for both flat and curved surface applications.


Powder separators include:

·         Chalk
·         Talc
·         Alumina hydrate

These have applications directly onto the shelf or mould and onto refractory separators.  If used between glass sheets as in bending, very little is required.  This is similar when applied to existing refractory papers.  As a shelf bed, much more is required.

This analysis of separators shows the first choice is about the application, as some are not useful in a given situation.  But in all cases, there are choices in what separator to use.

I used a popular UK website to obtain comparative prices for the various materials.

Papers
Papyros paper is listed at £18.46. This is enough for 5 shelves at 52 cm sq.  The per shelf cost, assuming two uses per sheet, equals £1.85.

Thinfire is listed at £10.16. This is enough for 5 shelves at 52c m square. The per shelf cost, assuming one use, equals £2.03.

Liquids
400 g Bullseye kiln wash is listed at £3.96.  This enough for about 80 shelves at 52cm sq.  The per shelf cost equals £0.05.

400g of Primo primer is give as £6.06.  This also is enough for about 80 shelves at 52cm sq.  The per shelf cost equals £0.075 (i.e., 7 and a half pence).

Boron Nitride enough for about 25 shelves at 52cm square is listed at £63.93.  The per shelf cost equals £2.56.

Powders
25kg calcium carbonate is listed at £14.61. This is a one-use material.  Applied at half a centimetre thickness, it is enough for 700 shelves at 52cm square.  The per shelf cost is £0.02.

300gms talc is listed at £2.99.  this is enough for 8 shelves.  As this is a multi-use material, assume 10 uses.  This gives a per shelf cost of £0.035.

Alumina hydrate is listed at £9.99 for 500gms. Again, this is a multi-use material, so assume 10 uses.  This gives a per shelf cost of £0.04.

Ratios of cost between the least and most expensive (given the assumptions) is as follows:
·         Chalk =1
·         Talc = 1.75
·         Alumina Hydrate = 2
·         Bullseye shelf primer = 2.5
·         Primo shelf primer = 3.75
·         Papyros fibre paper = 92.5
·         Bullseye fibre paper = 101.5
·         Boron Nitride spray = 128


This illustrates that convenience most often wins over expense, as the boron nitride, Papyros and Thinfire seem to be the most popular separators.

Monday, 9 October 2017

Lead Corrosion


There are three important versions of lead corrosion: Red, Brown and White.  In addition, there are other factors that can weaken the lead came.

Red lead is a corrosion product that appears as a bright red surface, is dangerous, and requires water, air and often wood, to form. Sometimes water in the manufacturing process can develop red lead.   The chemical composition of red lead (Lead (II, IV) or triplumbic tetroxide is Pb3O4 or 2(PbO.PbO2).  It is a bright red or orange crystalline or amorphous colour.

Red lead is virtually insoluble in water or in ethanol. But, it is soluble in hydrochloric acid as is present in the stomach.  When ingested, it is dissolved in the stomach’s gastric acid and absorbed, leading to lead poisoning. It also dissolves in undiluted acetic acid, as well as in a dilute mixture of nitric acid and hydrogen peroxide.

When inhaled, lead (II,IV) oxide irritates the lungs. In the case of a high exposure, the victim experiences a metallic taste, chest pain, and abdominal pain.

High concentrations can be absorbed through skin as well, and it is important to follow safety precautions when working with lead-based paint.

This means that anyone dealing with read lead needs protection against skin contact, and breathing protection.  Methods need to be implemented to ensure no dust is raised, or that the area is thoroughly cleaned after dealing with red lead. Clothing should be discarded or washed separately from all others.


White lead corrosion, Lead(II) carbonate, is the chemical compound PbCO3. It occurs naturally as the mineral cerussite.  It is a curious compound, as it is soluble in both acid and alkali.  It is possible, but rare, for excess whiting left on the lead to give rise to this form of corrosion. Generally, it will be neutralised by the weather.


Brown lead corrosion appears as a brown to dull red colour. 

Lead(IV) oxide, commonly called lead dioxide or plumbic oxide or anhydrous plumbic acid …, is a chemical compound with the formula PbO2. … It is of an intermediate bond type, displaying both ionic (a lattice structure) and covalent (e.g. its low melting point and insolubility in water) properties. It is an odourless dark-brown crystalline powder which is nearly insoluble in water. …. Lead dioxide is a strong oxidizing agent which is used in the manufacture of matches, pyrotechnics, dyes and other chemicals. It also has several important applications [e.g.,] in the positive plates of lead acid batteries.    Source: wikipedia

Air, water and salt are needed to form brown lead. This means coastal areas and those with driving rain are prone to this kind of oxidisation. Lead dioxide also forms on pure lead, in dilute sulfuric acid.  So, with the acid rain that we are all subject to, it can form in almost any situation, but will be more obvious on areas exposed to the prevailing wind.  The corrosion is soluble in strong acetic acid.


Tin corrosion also has a brown, almost copper appearance, very similar to brown lead.  The tin corrosion will be confined to the solder joint and surrounding area rather than all along the length of the came. 


Corrosion resistant lead
The ideal composition of lead to resist corrosion is 98.5% lead with up to 1% tin. This, combined with fractions of a percent of antimony and traces of silver, bismuth and copper provides a combination of metals and trace elements to resist corrosion of the lead as well as stiffening it.  Conservators indicate that, for whatever reason, cast lead incorporating trace elements is the most resistant to corrosion.  This is evidenced by the longevity of medieval lead cames.


Solder composition
Conservators also indicate that the higher the lead content of solder, and the better the match it is to the lead came, especially the almost pure lead came, the more resistant it is to lead came fracture at the margins of the solder joints.


Stretching the lead came, rather than simply straightening it, not only weakens the lead, it leaves very small pits in the surface of the lead.   These small pits allow the elements of the environment to penetrate the lead’s surface and act as sites for the beginning of corrosion.

Stretching also causes stress points near the solder joint.  The stretching creates stress along the length of the lead.  When the lead is heated in the soldering process the molecules of lead sort themselves into a stress-free arrangement.  As heat does not travel far or fast in lead, there is a stress point formed a short distance from the soldered lead joint where the already stressed and the stress-free lead meet.


Conclusion
Clearly there are a range of factors that relate to the resilience of lead came.  98.5% lead with trace elements including tin and antimony provides the greatest strength and resistance to corrosion.  Stretching the came can lead to general weakness and introduce pits into the surface forming sites for corrosion. Stretching can also lead to stress points near the solder joints.

All these indicate that resilient leaded glass windows can be produced by:
the use of lead came with 1.5% of trace elements,
the use of high lead content solders, and

the straightening (rather than stretching) of the came.