Capping with Frit

Capping with a clear or tinted top layer is necessary in many cases of inclusions, or desirable when looking for depth or distortion in flat fused work.

Capping inherently has bubble creation potential.  The development of a bubble squeeze helps prevent the largest of bubbles.  It cannot eliminate all the trapped air that then turns into small bubbles around the inclusions or multiple pieces when covered by a sheet of glass.

An alternative is to do away with the sheet glass capping and instead use enough frit to provide the desired depth, or the necessary material to cover the inclusion.  In fusing with two large sheets, a fine covering of powder between the layers will help to eliminate bubbles.  However, this will not be enough to successfully cover metal or other inclusions, or provide the amount of glass to give an appearance of depth.

The size of frit to use in a given application can be determined from other styles of glass working. It is known from glass casting that the smaller the frit the greater number of small bubbles will appear in the fired piece.  This means that you need to use medium sized frit for cast work.  Fine frit is likely to produce many very small bubbles across the whole piece in fusing applications.  Large frit is likely to produce larger bubbles, as the pieces themselves trap air as they deform.  This means that medium frit is a good compromise between large and small bubbles in capping. 

The layer of frit should be at least 2mm thick.  This means a lot of frit is required to do the job.  To judge the amount, you can measure the area of a rectangle or circle in square centimetres and multiply that by 0.2 to give you the volume (in cubic centimetres) of frit required.  Multiplying the volume by 2.5 (the approximate specific gravity of soda lime glass) will give you the weight of frit needed to cover the area. 

Alternatively, if the piece is irregular, you can weigh the base and add the appropriate weight of frit on the top.  If the base is 2mm, no further work is required to determine the weight. Weigh the 2mm sheet and use the weight of frit to equal the base.  If the glass is 3mm, you need two thirds of the weight in frit, and so on for thicker glass.

Using frit to cap is unlikely to eliminate all bubbles, but it will reduce them to a minimum.

Steep Slumps

Not all steep slumps are deep.

An example of a deep, steep slump

An example of a soup bowl with steep sides

A square bowl with slightly less steep sides

A shallow plate or platter

Relative to the size, the above platter mould is a steep slump, although not deep. 

This can be slumped in two stages to obtain confirmation of the glass to the mould without distortion. 

One way to do this is to place powdered kiln wash in the mould so there is a gentle curve to the bottom. Place glass on the mould and do a slow, low temperature slump.

After first slump, empty the kiln wash back into your container (it can still be used as kiln wash). Fire again using the same slow low temperature schedule as for the first. 

It may also help to retain the rim on the shallow plate to cut your circle 12mm larger than the diameter of the mould.  This will allow a margin for the slight shrinking that even a low and slow temperature slump will cause.

Boron Nitride

What is boron nitride? What makes it a good separator?

Boron nitride is a heat resistant refractory compound of boron and nitrogen with the chemical formula BN. It is also chemically stable at elevated temperatures.  It exists in various crystalline forms that are similar to a structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN forms.  It is the form most useful in kiln forming as a smooth release separator, especially for steel.  It is also used as a high temperature lubricant, and has a wide use in cosmetic products.

There is a cubic form that is similar to diamond (called c-BN), but softer.  It has a superior thermal and chemical stability.  There is a harder form called wurtzite, but which is rare. Neither of these is of much use in kiln forming.

Hexagonal BN

Hexagonal BN (h-BN) is the most widely used form of boron nitride. It is a good lubricant at both low and high temperatures (up to 900C, even in an oxidizing atmosphere). Another advantage of h-BN over graphite is that its lubrication properties do not require water or gas trapped between the hexagonal sheet layers. So, h-BN lubricants can be used even in vacuum, e.g. in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads.  In kiln forming, the high temperature lubricating properties are made use of as separator between metal, ceramic and other supporting materials for the glass.

“Hexagonal BN was first used in cosmetics around 1940 in Japan. However, because of its high price, h-BN was soon abandoned for this application. Its use was revitalized in the late 1990s with the optimization h-BN production processes, and currently h-BN is used by nearly all leading producers of cosmetic products for foundations, make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products.”   
https://en.wikipedia.org/wiki/Boron_nitride

It has wide application in materials to give them self-lubricating properties.  Boron nitride has the properties of stabilisation of materials, reducing expansion and resistance to electrical conduction, making for wide use in plastics and electronics among a wide variety of other products.

Health and Safety

There are some health issues related to its use.  It is reported to have a weak association with the formation of fibrous material in the lungs and so result in pneumoconiosis when inhaled in quantity in particulate form.  It is best to wear a dust mask when applying and to do it outdoors, as simple ventilation will not prevent dust settlement indoors.

Firing Schedules for Wissmach 96

Petra Kaiser is reporting that there are people finding cracks in white W96, which she cannot be replicate.  However, they are using strange firing schedules.

The most popular one appears as follows, in Celsius, with my comments.

166°C per hour to 232°C and hold 20

166°C is relatively slow. It is a rate I would use for a fused 6mm piece.  An unfired two-layer piece I would fire at 200°C to the bubble squeeze.  There is no effect in soaking for 20 minutes at this temperature.  If there is a worry (often expressed) that there will be thermal shock unless you let the glass catch up, slow the rate of advance to 134°C.  This is of course excessively slow for a two-layer piece. 

If, however, you are tack fusing onto two un-fused layers, then 166°C may be appropriate, as you are shading parts of the base from the heat of the kiln. But the soak is not necessary.  It does not do anything useful.

166°C per hour to 538°C and hold 20

As the rate for this segment is the same as for the first, I repeat the soak is not necessary.  If the glass survived the first 200°C at this rate, it will survive the next 300°C too. 

This rate for two layer pieces could be increased to 200°C without damage.

The 20-minute soak at this temperature again does nothing useful.  If the glass survived to this point, you can continue the temperature rise to the bubble squeeze at the same rate as in this segment.

278°C per hour to 621°C and hold 30

Although this rate is not excessive, there is no real reason to speed the temperature rise.  If you use 200°C from the outset to the bottom of the bubble squeeze, no time will be lost in getting to the bottom of the bubble squeeze.

However, this schedule leaves out the important second part of the bubble squeeze.  This is a slow rise to about 50°C above the start of the bubble squeeze process. 

Insert an advance of 50°C per hour to 670°C with a 30-minute soak

278°C per hour to 788°C and hold 15

788°C is a temperature given in the Wissmach tutorial on firing schedules.  However, Petra Kaiser has found that 771°C with a 10-minute soak is sufficient for a full fuse (or 765°C with a 12-minute soak).

The speed at which you reach the top temperature affects what you need to use as the top temperature.  This rate of less than 300°C will not require more than 771 as a top temperature. However a faster rate will require a higher temperature, and with it potential bubble problems, over firing, needling, and inconsistent results.

afap to 527°C and hold 120

This seems to come from the old Spectrum 96 schedules where a temperature equalisation soak was established above the annealing point.  Even if it were necessary, two hours is excessive.

The temperature equalisation of the glass should occur at the annealing point. Therefore, this segment is unnecessary.  And should be replaced by an AFAP to 510°C

55°C per hour to 510°C and hold 120

If the previous segment is eliminated, the rate in this one should be AFAP to 510°C with a soak of 30 minutes for a full flat fuse of 6mm.  There is no need for a longer temperature equalisation soak, as this is enough time for all the glass to be within 5°C of each part.

If you were tack fusing, a soak of an hour would be sufficient for a single layer of tack on a 6mm base.

28°C per hour to 399°C and hold 1

This rate is appropriate for a piece of 19mm.  A 6mm piece could use a rate of 80°C per hour.  A tack fused piece as described above could have an annealing cool of 60°C per hour.

Depending on the natural cooling rate of your kiln, it is possible to turn the kiln off at this point.  If you kiln cools off faster than the cooling rates given above, then you do need to programme a second stage cool.

  

55°C per hour to 93°C and hold 1

This is excessively slow for a 6mm thick full fused piece – a possible rate would be 200°C per hour.

The one-minute holds in these two down rates are only required where your kiln controller will not accept “0” as the number.  If the controller will accept 0, then use that, as 1 minute will not do much of anything, except confuse.

Writing and evaluating  schedules

When you are writing or looking at others’ schedules, review what is happening to the glass at various temperatures.  This excellent guide tells you what is happening to fusing glass at various temperature ranges.  Float glass has some different characteristics.

Combine that knowledge with what you are trying to achieve in the firing.

Guidelines on temperatures

Time versus Rate 

Assessing pre-programmed schedules

Making your own schedules

Comparisons of "CoE" and Temperatures

This table shows the lack of correlation between CoE and temperature characteristics of the glasses.  See the previous post for the discussion.

	Nominal	Temperatures (celsius)
Manufacturer	CoE	anneal	slump	full fuse
Pilkington UK Float	83	540	720	835
USA Float	83	548	720
Australian Float	84	505-525
Wissmach 90	90	482	638	771
Bullseye	90	482	630-677	804
Uroboros FX90	90	525	649-677	771-788
Kokomo	93	507-477	565
Artista	94	535	565
Spectrum	96	510	663	796
Uroboros	96	510	664	767-774
Wissmach 96	96	510	638	771
	Sorted by annealing point, averaged as necessary               CoE      Anneal       Slump      Full fuse
Kokomo	93	482	565
Wissmach 90	90	482	638	77	1
Spectrum	96	510	663	796
Uroboros	96	510	664	771	(ave)
Wissmach 96	96	482	638	77	1
Australian Float	84	515
Bullseye	90	482	654	804	(ave)
Uroboros FX90	90	525	663	780	(ave)
Artista	94	535	565
Pilkington UK Float	83	540	720	835
USA Float	83	548	515
	Sorted by Slump point, averaged as necessary              CoE       Anneal   Slump Full fuse
USA Float	83	548	515
Artista	94	535	565
Kokomo	93	492	565		(ave)
Bullseye	90	482	654	804	(ave)
Spectrum	96	510	663	796
Uroboros FX90	90	525	663	780	(ave)
Uroboros	96	510	664	771	(ave)
Wissmach 90	90	482	638	77	1
Wissmach 96	96	482	638	771
Pilkington UK Float	83	540	720	835
Australian Float	84	515			(ave)
	Sorted by full fuse, averaged as necessary
Uroboros	96	510	664	771	(ave)
Wissmach 90	90	482	638	771
Wissmach 96	96	482	638	771
Uroboros FX90	90	525	663	780	(ave)
Spectrum	96	510	663	796
Bullseye	90	482	654	804	(ave)
Pilkington UK Float	83	540	720	835
Artista	94	535	565
USA Float	83	548	515
Australian Float	84	515			(ave)
Kokomo	93	492	565		(ave)

Broken Base Layers

Sometimes in fusing, the base layer can exhibit a crack or break without the upper layers being affected.  This kind of break almost always occurs on the heat up.  In a tack fuse, the top layers may still be horizontal and unaffected by the break beneath them.  If a full fuse, the upper layers will slump into the gap, or apparently seal a crack that is apparent on either side.

An example of tack fused elements on top of a previously fused base

Causes

This is more likely to be seen where there is a large difference between thicknesses.  If the base is a single or double layer and there are several layers of glass – especially opalescent – on top, there is a greater chance for this kind of break to occur.

The reason for this kind of break is that the upper layers insulate the part of the lower layers they are resting upon.  Glass is an insulator, and so a poor conductor of heat.  This means that the glass under the stack is cooler than the part(s) not covered.  A break occurs when the stress of this temperature differential is too great to be contained.

An example of  stacked glass in a tack fusing

This kind of break can also occur when there are strongly contrasting colours or glasses that absorb the heat of fusing at different rates.  One case would be where the dark lower layer(s) were insulated by a stack of white or pale opalescent glass.  The opalescent glass will absorb the heat more slowly than the dark base.  This increases the risk of too great a temperature differential in the base.

Reducing the risk of these breaks.

Even thicknesses

One way to reduce the risk of base layer breaks is to keep the glass nearly the same thickness over the whole of the piece.  Sometimes this will not give you the effect you wish to obtain.

Slow the firing rate

Another way is to slow down the temperature rise.  Some would add in soaks at intervals to allow the glass under the stack to catch up in temperature.  As we know from annealing, glass performs best when the temperature changes are gradual and steady.  Rapid or even moderate rates of advance with soaks, do not provide the steady input of heat.

This prompts the question of how fast the rate of advance should be, and to what temperature. 

Rate of advance

The rate of advance needs to take account of the thickness differential and the total thickness together.  A safe, but conservative, approach is to add the difference in thickness between the thinner and the thickest parts of the piece to the thickest.  Then program a rate of advance to accommodate that thickness.  E.g., a 6mm base with a 9mm stack has a total height of 15mm.  The difference is 9mm which added to 15mm means you want a rate of advance that will accommodate a 24mm piece.

The rate of advance can be estimated from the final annealing cool rate required for that thickness.  In the example above, the rate would be about 100°C per hour.  The more layers there are, the more you need to slow the heat up of the glass. The Bullseye table for Annealing Thick Slabs is the most useful guide here.

Firing already fused elements

If you were adding an already full fused piece of 9mm thick to a 6mm base, you could have a slightly more rapid heat up, bu not by a lot. This is because the heat will be transmitted more quickly through a single solid piece to the base glass.  It is safer to maintain the initial calculation. If your stack is tack fused, this will not apply, as the heat will move more slowly through the layers of the tack fusing much the same way as independent layers on the initial firing.

Conclusion

The general point is that you need to dramatically slow the speed of firing when you have stacked elements on a relatively thin base.  Even a two layer base can exhibit this kind of break when there is a lot of glass stacked on it.

Stretching Lead Came

Stretching lead came is so ingrained into the literature and general thought that it is difficult to regain the purpose of the practice.  But I will try.

The purpose is to straighten the came

Purpose

The purpose of putting the lead into a clamp and pulling on the other end is to straighten the lead came.  It is much easier to work with a straight came than one that is curved or kinked.  It gives visually straight lines, it provides smooth and sinuous curves without interruption in the line of the curve.

It is said that some came is “pre-stretched”.  This is really the result of alloys contained in some lead to make it stiffer.  It still needs to be straightened before use.  If the lead came is already straight, you do not need to do anything else before using it.  If you drop or otherwise accidentally bend the came, you need to straighten it before continuing.

Stretching can weaken the came 

Stretching

Pulling on the lead came is not to stretch it, it is to straighten it. Stretching the lead can make it weaker. Lead drawn beyond its structural limits will break.  But you can weaken it before the break. You can test for this weakening of the came by observation. If you see "alligator" marks on the surface, you have weakened the came by putting too much effort into the pull. Straightening the lead must avoid so much force as to weaken the structure of the material.

Straightening not Stretching 

Straightening

The amount of effort to be put into straightening the lead came is just enough to make it straight. This will vary depending on how straight the came is at the start.  The reason for drawing the lead toward yourself is that you can see as you look down the length when the lead came is straight. If you are pulling vertically, it is more difficult to see when the lead becomes straight. 

If the lead is badly kinked or twisted, it may be best to cut that section out. If you continue to pull to straighten a difficult section, you can weaken the whole length of came.  First, ease the kinks and twists out as much as you can by hand. Then do an initial straightening pull.  This initial straightening pull will show where the problem(s) lie.  You can cut that section out and straighten the remaining pieces without stretching the lead to the point of weakness.

Safety

Of course, you must employ some basic safety rules.  Make sure the lead is securely clamped.  In the cleat style lead vices, you can give the lever a thump with the pliers to ensure the teeth are set into the lead before pulling on the other end.  Other vices need to have other ways to ensure that the end is held securely.

The other basic safety rule is that you should brace yourself against any break of the lead, or slip from the vice.  One foot should be placed behind you so that in case of breaks or slips you will not overbalance and fall.  This has the added advantage of ensuring you cannot put your body weight into the straightening effort.

There are other common sense rules, such as gloves, removing obstructions behind you, etc.

Conclusion

Remember that the purpose is to straighten, not stretch the lead came. 

If you are putting your foot on the bench to add force to the puilling of the lead in a vice on the bench, you are putting too much effort into the job and risk falling when the came breaks or slips out of the vice.  If your whole body weight is being used to draw the lead toward you, you are using too much force. If you can see signs of a pattern developing on the surface of the lead, you are using too much force.

Straightening the came is not an exercise in a workout programme.  It is a steady firm drawing force until the came is straight.  If you have to use more than usual force, stop and figure out why.  Cut out the difficult section so you do not weaken the came.  Then straighten the remainder and continue leading.

Smooth Surfaces on the Bottom of Bowls

A frequently asked question is how to get a smooth shiny surface to the outside of slumped bowls. There are two certain ways – have the shape blown, or do a free drop. 

Avoid Moulds

In blown glass work the hot glass can be shaped in a cold mould, which means that the glass does not take up all the mould imperfections.  However, the glass must be put back into the glory hole to remove the chill marks from the cold mould.

A free drop is the process where the glass blank is placed over an opening which allows the glass to fall without touching any mould.  You need to observe periodically during the firing to arrest the drop when it is at the stage and shape you want.  You then need to remove and polish the rim that rested on the elevated ring that supported the glass during the drop.

Failing these techniques, you need to use a mould 

The surface of the glass that is in contact with the mould will take up the texture of the mould surface. When the glass is hot enough to take up the shape of the mould, it will be soft enough to take up some texture from the mould. The hotter you fire, the more texture will be imparted to the glass. 

You can minimise the texture of a mould 

Prepare the mould with the smoothest surface you can.  If the shape is simple enough, you can use very fine sandpaper - 6000 grit is useful.  This will give the smoothest possible mould surface.

Use the finest kiln wash you can find to coat the mould. The finer the powder is ground, the less texture is present.  You can also smooth the kiln washed surface with a balled-up piece of soft cloth or tights.  Do this very lightly, so that you do not rub off the kiln wash. Remove the excess powder before firing.

Minimise the temperature

A major way to reduce the texture is to fire at a slow rate to the lowest temperature you can, using a 30 to 90 minute soak. This will give you less texture than a fast rate to a higher temperature with a shorter soak. To determine how long is required at a low temperature , peek periodically to see if the slump is finished.

The principle is to fire as slowly and to as low a temperature as is practical.  This will reduce the chances of marking as long as the glass does not slip down steep sides.  

Cutting Hour Glass Shapes

Hour glass shapes, wasp waists, or those that are thinner along the length than the ends, should be avoided as much as possible.  They are difficult to break out from the score.  More importantly, they are an inherently weak shape. The longer the piece is with the narrow part along its length, the more likely it is to break; in cutting or in the long term, in the panel.  However, these shapes are sometimes unavoidable.

The principle to use in scoring and breaking out the glass is to remove less glass than that you are retaining at each stage of the process.

This has consequences: 

• ·         breaking the first score is the easiest
• ·         only a rough outline of the final piece should be scored and broken from the sheet
• ·         Relieving scores and breaks will be necessary.  The number will depend on the relative thickness of the thin and thick parts.
• 
  

You can make the first score and break of one side of the shape from the main piece of glass – usually with little difficulty or need for relieving scores. (1)

You then should score and break off the piece to be retained from the larger sheet.  Be sure to give a margin for the final piece. (2)

Now score the other part of the hour glass shape.  Do not tap the score. Begin gently to run of the score from each end.  Don’t worry if the runs do not meet up.  Do not tap to make them meet up. (3)

If running the score from both ends is not enough to make the run complete, you will need to use relieving scores.  These scores can be like onion rings – generally concentric curves running in the same sort of shape as the curve to be broken out.  

Or you can use the fish scale approach – overlapping crescents.  These are most useful for deeper inside curves.

Either way, each score needs to be planned.  Each relieving score should be smaller than the width of the piece to be retained.  In general, this means the outer relieving scores can be wider apart.  As you approach the final shape, the distance between the scores will need to be less and less. (4,5,6)

More information on scoring and breaking out concave curves can be found here:  http://glasstips.blogspot.co.uk/2008/08/cutting-concave-curves.html

Lead Free Solders

Lead free solders have been created in response to concerns about lead, especially in the electronics industry. The following tables present a selection of available solder compositions.  The characteristics of these lead free solders can be compared to the common lead bearing solders in the last table.

Abbreviations for the metals of the compositions:

Ag=Silver; Bi=Bismuth; Cu=Copper; Ge=Germanium; In=Indium;

Sb=Antimony; Sn=Tin; Zn=Zinc

Melting Temperatures of Lead-Free Solders

Alloy  %                     Melting Temperature    Comments

Range (ºC)

Sn 65, Ag 25                         233           High strength; patented by Motorola (“Alloy J”)

Sn 99.3, Cu 0.7                     227           Eutectic

Sn 96.5, Ag 3.5                     221           Eutectic. Excellent strength and wetting

Sn 98, Ag 2                          221 – 226

Sn 77.2, Ag 2.8, In 20           175 – 186

Sn 95, Sb5                           232 – 240 Good high-temperature shear strength

Sn 42, Bi 58                         138           Well established; expensive

Sn 91, Zn 9                          199   Eutectic. Corrodes easily; high dross

Sn 95.5, Ag 0.5, Cu 4            217 – 350 Lead-free plumbing solder

Sn 97.25, Ag 2, Cu 0.75        217 – 219

Sn 91.8 Ag 3.2, Cu 0.5          217 – 218

Sn 95.5, Ag 3.8, Cu .07         217 – 220

Sn 95.5, Ag 4, Cu 0.5            217 – 225

Sn 95, Ag 4, Cu 1                 217 – 220

Sn 94.6, Ag 4.7, Cu 1.7         217 – 244

Sn 89, Zn 8, Bi 3                   192 – 197

Sn 97, Ag 0.2, Cu 2, Sb 0.8    287 – 318  High melting range; “Aquabond”

Sn 96.2, Ag 2.5, Cu 0.8, Sb 0.5      217 – 225

Sn 90.5, Ag 2, Bi 7.5             190 – 216

Sn-91.8, Ag 3.4, Bi 4.8          201 – 205

Sn 93.5, Ag 3.5, Bi 3             208 – 217

Sn 94.25, Ag 2, Bi 3, Cu 0.75   205 – 217

Sn90.7, Ag3.5, Bi 5, Cu 0.7     198 – 213

Sn 93.4, Ag 2, Bi 4, Cu 0.5, Ge 0.1         202 – 217

Sn 42.9, Bi 57, Ag 0.1           138 – 140

Sn 48, In 52                         118           Eutectic. Lowest melting point. Expensive

Source:

http://www.msed.nist.gov/solder/NIST_LeadfreeSolder_v4.pdf

Liquidus Temperatures (°C) of Candidate Lead-Free Solder Alloys for Replacing Eutectic Tin-Lead Solder

Alloy Composition%     Liquidus             Melting Range

98Sn-2Ag                                             221-226

96.5Sn-3.5Ag              221                    221

99.3Sn-0.7Cu              227                    227

96.3Sn-3.2Ag-0.5Cu     218                   217-218

95.5Sn-3.8Ag-0.7Cu     210                   217-210

95.5Sn-4.0Ag-0.5Cu                             217-219

95Sn-5Sb                                            232-240

42Sn-58Bi                   138                   138

89Sn-3Bi-8Zn                                      189-199

Where there is a single temperature in the melting range column, the solder is eutectic.

Based on:

V. Solberg, “No-Lead Solder for CSP: The Impact of Higher Temperature SMT Assembly Processing,” Proc. NEPCON West 2000 Conf. (Feb. 28 - Mar. 2, 2000) Anaheim, CA (Source: Indium Corp.) # N.-C. Lee, “Lead-Free Chip-Scale Soldering of Packages,” Chip Scale Review, March-April 2000.

Source:

http://www.msed.nist.gov/solder/NIST_LeadfreeSolder_v4.pdf

Solidus and Liquidus Temperatures of Some Leadfree Alloys on Copper

Alloy  %                             Solidus (°C)        Liquidus (°C)

98Sn-1Ag-1Sb                      222                   232 

89Sn-4Ag-7Sb                      230                   230

91.2Sn-2Ag-0.8Cu-6Zn          217                   217

89.2Sn-2Ag-0.8Cu-8Zn          215                   215

89.2Sn-10Bi-0.8Cu               185                    217

85Sn-10Bi-5Sb                     193                   232

52Sn-45Bi-3Sb                     145                   178

42Sn-58Bi                            138                   138

Based on:

M.E. Loomans, S. Vaynman, G.Ghosh and M.E. Fine, “Investigation of Multi-component Lead-free Solders,” J. Elect. Matls. 23(8), 741 (1994)

Source:

http://www.msed.nist.gov/solder/NIST_LeadfreeSolder_v4.pdf

Eutectic Composition of Solders

Most solders and especially tin-lead alloys have a melting (or pasty) range between which the metal has moved from a proper solid (solidus) to a completely liquid (liquidus) state.  Wide melting ranges are ideal for plumbers, they are not for electronics, or stained glass.  It is much easier to run a nice bead with a narrow range of melting (pasty) temperatures.

Some alloys of solder have what is known as an eutectic characteristic.  This is where the liquidus and solidus states occur at the same temperature.  A composition of 61.9% tin and 38.1% lead is both eutectic and the melting occurs at a minimum temperature.

For comparison with lead free solder characteristics the following % compositions of Tin (Sn), Lead (Pb) and Silver (Ag) solders are given.

Element % of solders  Melting point        Comment

Sn 62, Pb 36, Ag 2       179                    Eutectic; traces of antimony

Sn 63, Pb 37               183                    Eutectic; traces of antimony

Sn 60, Pb 40               183-191             Traces of antimony

Sn 96.3, Ag 3.7           221                    High melting point. Eutectic

Sn 10, Pb 90               275-302

Sn 3, Pb 97                275-320

Sn 5, Pb 93.5, Ag 1.5   296-301

Source:

http://en.wikipedia.org/wiki/Solder#Lead-free_solder

Conclusions

Most of the alternative solders contain tin as it assists in the formation of bonds with a wide variety of metals.  These solders are also mechanically weaker than tin-lead solders.  Lastly, they are much more expensive than tin-lead solders.  Even within the tin-lead solders there is a variation in price, as tin is much more expensive than lead. If high temperatures were not a problem, you could use a high lead content solder.  However, that also raises the liquidus temperature and increases the pasty range.

The choice in lead free solders is between the high liquidus temperatures of the less expensive compositions and the high price of the eutectic, or nearly so, ones.  The lowest eutectic composition is the Tin-Bismuth solder, but it is also among the most expensive to buy.  You should also note that the inclusion of copper in the composition prolongs the life of the solder bit, as low lead content of the solder leads to the incorporation of small amounts of copper from the tip into the solder joint.