Kiln Wash Sticking to Glass

Causes and avoidance

 

Photo credit: Immerman Glass

 In general, kiln wash for glass is made up of aluminium hydrate with kaolin (China clay) as a carrier. I do not know the exact chemical changes of kiln wash at fusing temperatures. But I do suspect it has to do with the kaolin. The aluminium hydrate is stable to much higher temperatures (melting point of 2,072°C/3,762°F). So, I don't believe that part of kiln wash is changing.

 Some reading has led me to learn that by 600°C/1113°F the kaolin begins to go through a non-reversable chemical change. Prior to that, water can rehydrate the kaolin. In the hydrated state kaolin forms hexagonal plates that can slip over one another. Once 600°C/1113°F has been exceeded crystallisation cannot be reversed. It does not become fully crystalline until 935°C - 950°C/1717°F - 1744°F. The crystallisation stops the lubricating effect. I suspect that on the second firing these crystals (which contain silicon dioxide) interact with the glass and stick, although not fully combining with the glass. Why this does not happen in the first firing, I do not know.

 The fact that the crystallisation cannot be reversed must be the key as to why kiln wash with kaolin cannot be re-used once fusing temperatures have been used in a previous firing. It also indicates that repeated tack fusing on kiln wash will ultimately fail as the crystallisation will gradually increase with each firing.

 However, at slumping temperatures, it appears the crystal formation is so slow as to have no effect on multiple firings.

 There are of course ways to avoid kaolin. There is a kiln wash, called Primo Primer that does not have kaolin in it. And you could make your own kiln wash from aluminium hydrate. This is known as slaked alumina in ceramics. It can be used on its own, although the incorporation of  binders makes the application easier. The grades used in ceramics are usually coarser than kilnformers want. But it can be made finer by putting it in a rock tumbler with some stainless steel ball bearings. You can run the result through a fine screen to remove the ball bearings. Mix with water to brush on, or sprinkle dry over the shelf. The aluminium hydrate can be re-used, if they are kept free of contaminants. Aluminium on its own does not provide as smooth results as when the kiln wash contains kaolin.

 Chalk, also known as whiting, is calcium carbonate. This is often used as a separator in vitreous paint firings and some forming operations. It has low solubility in water, so cannot be painted onto shelves or moulds. It needs to be used as a loose or compacted powder. It goes through chemical changes too, making renewal after firing advisable. Above 800°C/1473°F calcium carbonate changes to calcium oxide, or quicklime. This corrosive form is another reason it is disposed of after any higher temperature firings.

 Kiln wash and calcium carbonate can be fired many times at low temperatures, because their chemical composition remains relatively stable. Once higher temperatures are used, chemical changes occur. This seems to enable them to stick to the glass or form undesirable compositions. This phenomenon requires removal and re-coating of shelves and moulds after full fuse firings.

Kaolin provides significant advantages in the smooth application of kiln wash.  Caution needs to be exercised in using it after it has been fired to fusing temperatures, although it can be used at low temperatures for indefinite numbers of firings.

 Methods for removal of kiln wash are in this blog post.

Citric Acid Cleanser

Christopher Jeffree has kindly outlined the reasons for the effectiveness of citric acid as a cleaner for removing refractory mould residue and acting on kiln wash stuck to glass.  This is his work (with a few personal notes removed).

"Citric acid works well for removing the plaster scale that builds up in vessels used to mix plaster, and it helps to remove traces of investment plaster and kiln wash from glass.  Its metal-chelating properties probably help with dissolution of calcium deposits, but I am less clear why it is so good at removing kiln wash.  The constituents of kiln wash are kaolin and alumina hydrate, neither of which I would expect to be soluble in dilute acids.  Equally, the refractory materials in investment formulae I would expect to be insoluble.  However, kaolin forms layered structures in which flakes, molecular layers, of alumina hydrate and silica interact through hydrogen bonding. It is possible (I am guessing here) that citric acid can disrupt those hydrogen bonds, thereby disaggregating the clay.  All we can say is that empirically, it works.

"I prefer to use citric acid partly because it has a defined composition, but also because it is safe and pleasant to handle – no odour, and comes in the form of easily-dissolved dry crystals like granulated sugar.  Vinegar stinks, and glacial acetic acid is  an aggressive flammable, corrosive liquid with a chokingly acrid smell.

"Calcium sulfate has low solubility, but is not completely insoluble in water - gypsum (calcium sulfate dihydrate) has a solubility of about 2.5g per litre (0.25%)  from 30-100 C. Its solubility is retrograde, meaning that it decreases, rather than increasing, with temperature.  Natural gypsum is an evaporite, a type of rock that often forms by evaporation of lake water in a geological basin with little or no outflow. It can also be produced hydrothermally in hot springs, when water containing sulfuric acid passes through limestone.  

"Calcium citrate is not very soluble either, only in the order of about 0.85g per litre, but the important thing from our point of view is not to get the material into solution but to separate its crystals and make it detach from the glass.

"In other contexts, warm citric acid is used by jewellers and silversmiths as a pickle for dissolving copper oxide (firestain) from silver and gold alloys  after heating / soldering.  It is a safer alternative to the traditional jeweller's pickle of 10% H₂SO₄.

"Citric acid also dissolves rust from iron, without much etching the iron itself, so is good for cleaning rust off tools etc.

"These pictures show a plaster mixing bowl with (presumably) CaSO₄-rich deposit on the surface, cleaned by soaking with 5% citric acid for 4 hours,

and flash from the pate de verre castings with tightly adhering kiln wash, cleaned using 5% citric acid soaked for 4 hours, and vinegar (white wine) soaked for 24 hours.

"I'm not sure about reaction products - I was speculating a lot there, running through hypotheses that I can't support. We don't really have data on the composition of the layers that are stuck to the glass, or a clear idea of why they sometimes stick and sometimes don't (e.g. the differences between transparent and opal glasses in this respect). Maybe this would be a topic to discuss with technical people at Bullseye."

Hope this helps
Best wishes

Chris Jeffree

Subsequent to this work Christopher has done more work and found that Tri-sodium citrate is an even better chemical for cleaning glass of kiln wash and mould material.

The Purpose of Flux

The primary purpose of flux is to prevent oxidation of the base and filler materials in the short time between cleaning and soldering. Tin-lead solder, for example, attaches very well to copper, but poorly to the various oxides of copper that form quickly at soldering temperatures. This applies to lead and brass too.

Flux is a substance that is nearly inert at room temperature, but it becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Secondarily, flux acts as a wetting agent in soldering processes for lead, copper and brass.

Without flux the solder does not firmly attach to the lead or copper foil and often forms sharp peaks.


See also
Flux, an introduction
Fluxes, a description
The Purpose of flux
The action of fluxes

Soldering fluxes

The Action of Fluxes

All common untreated metals and metal alloys (including solders) are subject to an environmental attack in which their bare surfaces become covered with a non-metallic film, commonly referred to as tarnish. This tarnish layer consists of oxides, sulfides, carbonates, or other corrosion products and is an effective insulating barrier that will prevent any direct contact with the clean metal surface which lies beneath. When metal parts are joined together by soldering, a metallic continuity is established as a result of the interface between the solder and the surfaces of the two metals. As long as the tarnish layer remains, the solder and metal interface cannot take place, because without being able to make direct contact it is impossible to effectively wet the metals surface with solder.

The surface tarnishes that form on metal are generally not soluble in (and cannot be removed by) most conventional cleaning solvents. They must, therefore be acted upon chemically [or mechanically] in order to be removed. The required chemical reaction is most often accomplished by the use of soldering fluxes. These soldering fluxes will displace the atmospheric gas layer on the metal’s surface and upon heating will chemically react to remove the tarnish layer from the fluxed metals and maintain the clean metal surface throughout the soldering process.

Chemical reactions
The chemical reaction that is required will usually be one of two basic types. It can be a reaction where the tarnish and flux combine forming a third compound that is soluble in either the flux or its carrier.

An example of this type of reaction takes place between water-white rosin and copper oxides. Water-white rosin, when used as a flux is usually in an isopropyl alcohol carrier and consists mainly of abietic acid and other isomeric diterpene acids that are soluble in several organic solvents. When applied to an oxidized copper surface and heated, the copper oxides will combine with the abietic acid forming a copper abiet (which mixes easily with the un-reacted rosin) leaving a clean metallic surface for solder wetting. The hot molten solder displaces the rosin flux and the copper abiet, which can then be removed by conventional cleaning methods.

Another type of reaction is one that causes the tarnish film, or oxidized layer to return to its original metallic state restoring the metals clean surface.

An example of this type of reaction takes place when soldering under a blanket of heated hydrogen. At elevated temperatures (the temperature that is required for the intended reaction to take place is unique to each type of base metal) the hydrogen removes the oxides from the surface, forming water and restoring the metallic surface, which the solder will then wet. There are several other variations and combinations that are based on these two types of reactions.

Acids commonly in fluxes

Flux as a temporary protective coating
Once the desired chemical reaction has taken place (lifting or dissolving the tarnish layer) the fluxing agent must provide a protective coating on the cleaned metal surface until it is displaced by the molten solder. This is due to the elevated temperatures required for soldering causing the increased likelihood that the metal’s surface may rapidly re-oxidize if not properly coated. Any compound that can be used to create one of the required types of chemical reactions, under the operating conditions necessary for soldering, might be considered for use as a fluxing material. However, most organic and inorganic compounds will not hold up under the high temperature conditions that are required for proper soldering. That is why one of the more important considerations is a compound's thermal stability, or its ability to withstand the high temperatures that are required for soldering without burning, breaking down, or evaporating.

When evaluating all of the requirements necessary for a compound to be considered as a fluxing agent, it is important to consider the various soldering methods, techniques and processes available and the wide range of materials and temperatures they may require. A certain flux may perform well on a specific surface using one method of soldering and yet not be at all suitable for that same surface using a different soldering method. When in doubt it never hurts to check with the flux, or solder manufacturer for recommendations.

Courtesy of American Beauty Tools


See also:
Flux, an introduction
Fluxes, a description
The Purpose of flux
The action of fluxes
Soldering fluxes

Flux

Flux is a material that provides a “wetting” action between the metal (lead or copper in our case) and the solder.


There are various types of flux. Some are of more use in some circumstances than others. Among them are:

Tallow
This normally comes in a candle-like stick. It is made from rendered animal fat. Although this may put some vegetarians off, it is one of the best fluxes for leaded glass work and will work for copper foil, but is not generally preferred.  It is relatively natural, does not contain chemicals, and does not require re-application if left for a while. Over generous application does not produce any problems during the soldering. It just leaves more solidified tallow to clean after soldering. The cleaning normally requires a mild abrasive such as a brass or fibreglass brush to get the cooled tallow off the piece.

 




Oleic acid and other safety fluxes
Many of the safety fluxes are made of oleic acid (sometimes called stearin oil). These fluxes do not produce chemical fumes in the soldering process. They are easy to clean up with detergents and warm water. Safety fluxes require re-application if left to dry, as they are only effective while wet. Putting too much on leads to boiling off the liquid, making holes in the solder joint or line.

An example only.  There are many water soluble paste fluxes available

Chemical Paste fluxes
These fluxes come in a variety of compositions. You need to be careful about choosing, as some are very difficult to clean off the glass or solder line or joint. They do produce chemical fumes, so a fume mask is advisable while using this kind of flux. The paste does not require re-application if left, so the whole piece can be fluxed at once.

Acid fluxes
Acid fluxes such as the kind that is in the core of plumbers solder are intended to clean the joint at the same time as acting as the wetting agent. These are not recommended for stained glass work as they can affect the glass surfaces, especially irridised glass. They do produce fumes that require the user to have on a fume mask while soldering. The ease of cleaning relates to the particular composition of the flux, so testing samples is required before application.

See also:
Flux, an introduction
Fluxes, a description
The Purpose of flux
The action of fluxes
Soldering fluxes

Flux, an Introduction

Flux is a key contributor to most soldering applications. It is a compound that is used to lift tarnish films from a metals surface, keep the surface clean during the soldering process, and aid in the wetting and spreading action of the solder. There are many different types and brands of flux available on the market; check with the manufacturer or reseller of your flux to ensure that it is appropriate for your application, taking into consideration both the solder being used and the two metals involved in the process. Although there are many types of flux available, each will include two basic parts, chemicals and solvents.

an example of paste flux

The chemical part includes the active portion, while the solvent is the carrying agent. The flux does not become a part of the soldered joint, but retains the captured oxides and lies inert on the joints finished surface until properly removed. It is usually the solvent that determines the cleaning method required to remove the remaining residue after the soldering is completed. 

It should be noted that while flux is used to remove the tarnish film from a metal's surface, it will not (and should not be expected to) remove paint, grease, varnish, dirt or other types of inert matter. A thorough cleaning of the metal's surface is necessary to remove these types of contaminates. This will greatly improve the fluxing efficiency and also aid in the soldering methods and techniques being used.

Courtesy of American Beauty Tools


See also:
Flux, an introduction
Fluxes, a description
The Purpose of flux
The action of fluxes
Soldering fluxes

Cleaning Materials and Solutions

You need to clean glass that is going into the kiln to avoid devitrification on the surfaces.  This can be a greater or lesser problem for different individuals.  It is probably related to your studio practice and the amount of oils in or on your fingers.

The first things to consider in cleaning glass for kilnforming are what you are trying to eliminate from the glass, the chemical nature of glass, and how to avoid putting further contaminants on the glass.

Cleaning is to remove surface deposits

The sensitivity of glass to minor contamination is shown by the fact that the small amount of oil from your finger tips can provide sources of devitrification.  This means the glass needs to be really clean and free from any deposits.  You need to remove oils and dusts and anything you may have added during assembly to leave nucleation points for devitrification. This includes any minerals in the water used to clean the glass.

Avoid soaking in acids

Glass is an alkaline (or basic) material.  This means that acids can affect the surface of the glass – at the microscopic level – enough to provide those nucleation points for devitrification to develop.  This means that you should avoid soaking in acids.  One popular acid is vinegar.  An odd thing about the way vinegar attacks glass is that the more dilute it is, the more etching it does of the glass.  This has to do with the greater amount of oxygen to transfer from the vinegar water to the glass, leaving microscopic etching as the minerals encased in silica are released from the glass surface.

If acids are used to clean the glass, rinse immediately in an alkaline solution such as baking soda.  You need then to get rid of the chemical reaction products formed by the neutralisation of the acid.  This should be done by immediately rinsing with running clear water. Follow this with a polish dry using unprinted paper towels.

Cleaning with spirits

My recommendation is to avoid spirits, especially those with additives such as rubbing alcohol. The amount of oil that is to be removed from the glass is small, so application of large amounts of spirits is not necessary.  It is reported that some aggressive spirits may affect the surface of the glass by combining with the minerals or the silica of the glass – this is not proven. If you do use spirits make sure they are thoroughly cleaned off and polished dry.  It is all too easy to leave residues.

What can I use to clean the glass?

The simplest cleaner is water.  A drop or two of dish washing liquid can provide a break to the surface tension, allowing the water to flow smoothly over the whole surface.  Then polish dry with clean unprinted paper towels.

In many areas, the public water supply is hard – i.e., has an appreciable level of minerals.  Calcium and iron are two common minerals in any water supply. Some water supplies have other additives such as chlorine, fluorine and other purifiers. Chlorine and fluorine react strongly with glass.  This means that air drying is not a good choice in cleaning glass in areas where there is an element of these chemicals in the water supply.  Iron is another strong reactor with glass.  In high iron areas this may prove difficult to use water as the cleaning element.

After using any of these solutions, rinse with clear running water and immediately polish dry.  Plain paper towels are better than cloths to scrub the glass to squeaky clean.

It is suggested that distilled water can be used instead of the public water supply.  Yes, it can.  But it is expensive and not necessary.  Instead there are a few commercial cleaning agents that work well.  In North America Spartan glass cleaner is recommended.  This can be used immediately after the water rinse and dry.  In Europe Bohle glass cleaner is recommended.  Except in the most severe contamination circumstances, I use only the Bohle glass cleaner (because I am in Europe) without any water at all. The same could be done with Spartan in North America.  I’m sorry that I have no recommendations for other parts of the world, unless collecting rainwater is an option.

After applying these glass cleaners, you still must polish to squeaky clean and dry.

Copper inclusions

Inclusions of metals can be achieved with care.  Copper is a very good metal, as it is soft, even though its expansion characteristics are very different from glass.  This note provides some things you might consider when planning to include copper in your fused pieces.

The copper sheet should be stiff, but not thick. If the metal can be incised with a scribe and maintain that through gentle burnishing, it is suitably thick. The usual problem is that the copper is too thick rather than too thin.  Copper leaf can be very faint if a single layer is used.  Placing several layers of leaf improves the colour, but often provides wrinkles.  In summary, the requirement is to get a thickness of copper that will retain its structure, but not be so thick and stiff as to hold the glass up during the fusing process.  

Do not use the copper foil as used for stained glass applications. The adhesive backing produces a black colour from the adhesive and many bubbles -  sometimes a single large one.

Copper can provide several colours.

Copper sheet normally turns burgundy colour when oxidised.  This means that there is enough air reaching the copper to oxidise it to deep copper red.  This most normally happens, because a lot of air can contact the metal during the extensive bubble squeeze usually given to inclusions.

To keep the copper colour, clean the metal well metal well with steel wool or a pot scrubber. If you use steel wool, wash and polish dry the metal before fusing.  Reduction of air contact with the metal helps to retain the copper colour.  There are two methods I have used.  Addition of a glass flux like borax or other devitrification spray will help prevent the air getting to the surface.  Another method of avoiding oxidisation, is to cover the copper with clear powdered frit, as well as the surrounding glass.

In certain circumstances you can get the blue green verdigris typical of copper in the environment.  This is an extent of oxidisation that is between the clean copper coloured metal and the burgundy colour of extensive oxidisation.  The key seems to be be a combination of restricted air supply, shorter bubble squeezes and lower temperatures.  Experimentation is required to achieve this consistently.


The spaces under and over the copper give the opportunity for bubbles to form. 

This means that the copper needs to be as flat as possible for one thing.  Burnishing the copper can have a good effect on reducing the undulations in the copper.  Thinner copper is easier to make flat than thicker.  If you can stamp a shape from the copper with a stamper designed for card making, it is a good indication that it will burnish flat.  Thicker copper sheet holds the glass up long enough in the temperature rise during the bubble squeeze to retain air around the metal.  This remains the case even after burnishing to be as flat as possible.

The second element that can help to reduce bubbles around the copper is to sprinkle clear powder over the copper sheet once in place on the glass.  The spread of the powder over the glass assists in giving places for the air between layers to escape.

These two things combined with a long slow squeeze can reduce the amount of bubbles you get.  It cannot totally eliminate them.

Of course, a longer bubble squeeze allows air to be in contact with the copper and promotes the change to a blue green or burgundy colour.

Composition of Glass

Glass can do most anything. From bottles to spacecraft windows, glass products include three types of materials:

• Formers are the basic ingredients. Any chemical compound that can be melted and cooled into a glass is a former. (With enough heat, 100% of the earth's crust could be made into glass.)
• Fluxes help formers to melt at lower temperatures.
• Stabilisers combine with formers and fluxes to keep the finished glass from dissolving, crumbling, or falling apart.

Chemical composition determines what a glass can do. There are many thousands of glass compositions and new ones are being developed every day.

Formers
Most commercial glass is made with sand that contains the most common former, Silica. Other formers include:

• Anhydrous Boric Acid
• Anhydrous Phosphoric Acid

Fluxes
But melting sand by itself is too expensive because of the high temperatures required (about 1850°C, or 3360°F). So fluxes are required. Fluxes let the former melt more readily and at lower temperatures (1300°C, or 2370°F). These include:

• Soda Ash
• Potash
• Lithium Carbonate

Stabilisers
Fluxes also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. So stabilizers need to be added. Stabilisers are added to make the glass uniform and keep its special structure intact. These include:

• Limestone
• Litharge
• Alumina
• Magnesia
• Barium Carbonate
• Strontium Carbonate
• Zinc Oxide
• Zirconia

Based on an article from the Corning Museum of Glass

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.

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.