Body Casting with Plaster of Paris Bandages?

 

The advantage of plaster of Paris bandages to create a body cast are:

· its light weight,

· rapid setting characteristics,

· the degree of detail picked up, and

· rigidity.

Its disadvantages are

· its easy deformation while wet,

· its fragility when dry, and

· it is a messy procedure.

The advantages over other methods of taking a cast from life are such that this is an almost ideal casting medium.

Preliminaries

There are some preliminaries that need to be completed before starting. You need to make sure your model knows what will be happening and is comfortable with it. You need to empower the model so she/he can call a halt to the casting if they are uncomfortable with anything in the process.

Make sure the area is warm for the model as the person will be standing without clothing for at least half an hour. To ensure the model is not chilled, use warm water to soak the plaster of Paris bandages. Top this up with hot water as necessary.

Explain the process to the model. If you have some pictures or examples of previous work this helps the model visualise what will happen and what the result will be. There needs to be some rapport between you two and the model needs to know what to expect because some parts of taking the casting are very intimate.

Provide supports as the model will have to maintain pose for up to 30 minutes. This can be as simple as straps to support the arms, or a table to lean on.

You should provide showering or bathing facilities for the model to enable the cleaning of the remaining oils and Vaseline, as well as the drips of plaster of Paris that will be on the skin outside the casting area. This will help warm the model up again, if the temperature has not been warm enough.

You should consider what clothing you wear too. The plaster of Paris does not come out of cloth at all well. So what you use should be of a nature that can be thrown away or reserved for future casting projects. I have found bare feet provides the most secure footing.

You need some waterproof floor covering to reduce the amount of clean up after casting. If the waterproof floor covering has some texture that is better than smooth plastic sheet. The floor covering becomes very slippery when wet.

Taking the Cast

You will of course have a good idea of the pose you want for the cast. But you must try it out with the model to ensure you really like the pose and make any alterations. You also need to determine if the model can hold the pose for the required amount of time. Then the various props or supports can be put in place for the model. Most of this can be done clothed.

The model needs a separator between the skin and and the plaster of Paris/modrock. Olive oil has staying properties that baby oil does not and this is what I use now. Ensure that it is applied liberally – so don’t use extra virgin.

Olive oil is fine for arms, and torso of female models. Male models may have significant amounts of hair on their arms and chests. This will be difficult to separate from the cast, so you really need to find male models with little hair. Where there are significant amounts of hair, Vaseline is the best I have found. It needs to be put on very liberally, providing the surface on which the plaster of Paris is placed. So, enormous amounts of Vaseline need to be placed on pubic, armpit, and chest hairs. It should coat the area providing a smooth surface with few hairs poking through.

I obtain plaster of Paris bandages from hospital suppliers as they are cheaper than from art stores, although you have to buy in larger amounts. Cut the bandages into ca. 300mm lengths for ease of application. Longer strips tend to fall off the skin due to the weight of the unattached piece. For detailed areas or places where there are creases, shorter pieces are needed. These are usually available as off cuts from the end of the roll when you are cutting the longer strips. If not, you need to cut some shorter strips in preparation, as once you start you need to work rapidly as you will not have time to pause to cut more strips or make them smaller.

Mark the limits of the cast in oil pastels on top of the oils. This provides a guide on the limits of where the plaster of Paris bandages need to go. This comes off the skin easily and additionally leaves a faint mark on the cast for future reference on where the cast should be filled to replicate the pose you decided upon.

Lay strips one at a time and overlapping from the top to the bottom of the area. Doing it in reverse makes it difficult to keep the bandages against the skin. Also place the beginning strips so as to form a hook or hanger to keep the cast on.

Ensure you are overlapping the strips by laying them in different directions, even after the area seems completely covered. Also make sure you have extra thickness at the edges of the cast to assist in taking it off. Three layers of plaster of Paris on the main areas is usually enough, but 5 or 6 at the edges is a good idea.

You need to work systematically as once the plaster of Paris has cured, you cannot attach further strips. It takes only about 5 – 8 minutes for the strips to cure and become stiff.  Work smoothly and rapidly to minimise the length of time the model has to maintain the pose.

Even though you need to work rapidly and systematically, you must not go so fast that you fail to eliminate bubbles, or adequately cover all the area with several layers. You should ensure that you smooth the plaster of Paris through the bandage weave to the skin. If you watch what you are doing you can catch the bubbles that often form under the bandage. Smooth the bubble to the edge of the bandage where the air can be gently forced out.

Another care needs to be taken to avoid pressing too hard on the wet plaster of Paris bandages, as you will leave dents that will not be pushed back out by the skin of the model. It always is a fine balance between too little and too much pressure.

As you cannot see whether you have enough layers on after you have laid down the first, you need to work in a systematic fashion, such as alternate diagonals or horizontally and vertically. This together with overlapping each bandage, helps to ensure you have enough layers.

When you have finished applying the strips over the area, you can begin the clean up while the lower part of the cast sets.

Taking the cast off is a delicate process. Even on an apparently hairless skin, there are many small fine hairs that stick to the cast. This resists the removal of the cast. The areas where Vaseline had to be applied will be even more “sticky”. First break the seal between the skin and the edge of the cast all around. Then lift the cast from the skin. If you are doing a front or back torso, lift the cast upwards as this is more comfortable for the model. If possible, get the model to help in this process.

Although the top of the cast sounds firm when tapping it, it still is delicate. You have to be careful when handling the cast while it is wet. You can place the cast on a soft surface with the hollow down, so the form is not crushed or you can hang it from a string round the cast.


These two pictures illustrate the finished and dried plaster cast, inside and outside.

Even when completely dry, the plaster of Paris cast is fragile. It needs some protection. A simple protection is oil based varnish – not the flexible ones like polyurethane. The oil based varnish provides some additional strength. It protects against the water based mould materials too.



Following these procedures will provide you with a one-use life cast with lots of detail captured.

Revised 28.11.25

What are the Effects of Firing on the Kiln Floor?

Credit: The Pottery Wheel

There will be differences when firing on the floor of a kiln - the two important ones are working temperature and annealing.

Differential temperatures 

When firing on the floor of the kiln, expect the effective temperature to be a little lower than when on an elevated shelf. The temperature is always lower at the floor of the kiln and hotter higher in the kiln. This effect is often experienced with glass nearer the elements than usual.

This differential temperature between the floor and the top of the kiln is alleviated to some extent by the infrared heating, if the glass is exposed to it. If the glass is shaded, there will be a distinct difference.

Annealing

Annealing and cooling will be affected most. On the floor the glass can only cool from the top surface, as the bottom of the glass can only cool as fast as the cooling of the kiln. The annealing soak needs to be longer and the cooling needs to be slower than with an elevated shelf.

I suggest that firing for one layer thicker than calculated for the profile will cope with both these conditions.

What Are Alternatives to Glue?

Credit: Glass Art by Margot

Small pieces, confetti and stringers which are glued often move out of place during firings. Glue, whether dry or wet, often boils at relatively low temperatures.  This can be enough to move, or displace, and even flip light pieces.

The alternative to glue is to use a thin film of clear powder.  First arrange the pieces and then place a fine film of powder at the edges of the pieces. At tack fuse and above, the powder will incorporate with the base glass and be invisible. 

One caution is that a thick layer of powder which obscures the base glass will produce a sandpaper finish at tack temperatures, and at worst produce a multiplicity of tiny bubbles making things look grey at the finish.

In low temperature tack fuses, use a few grains of fine clear frit on either side of anything that might be subject to rolling or sliding to avoid the rough or grey appearance.

But...

If it is necessary to use glue to transport it to a remote kiln, use the minimum amount, diluted with 50% or more water, or solvent if solvent-based. Add only a small drop of the glue to the edge or end of the pieces. This avoids the glue leaving large black spots under the pieces because the burnout products cannot escape. It also prevents large amounts of glue being used.

Most of the glues have boiling points below 270°C/518°F, so a slow ramp to that temperature will create an evaporation rather than boiling and displacing the glued pieces.

Carbon for moulds

Recently there has been a flurry of interest in “carbon” moulds. These are actually graphite moulds, a particular form of crystalline carbon. Whether these are as good materials for kilnforming as they are for glass blowing can be understood by considering some of the characteristics of graphite.

Graphite is crystaline carbon and it has a planar structure that allows slipping between planes just as kaolin and boron nitride do. Graphite and boron nitride share similar physical structures and continue to be slippery at high temperatures.

Graphite's high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen-containing atmospheres graphite readily oxidises to form carbon dioxide at temperatures of 700 °C and above. https://en.wikipedia.org/wiki/Graphite

The consequence of this instability in the presence of heat an air is the creation of large bubbles from the decomposition of graphite into carbon dioxide when used above that decomposition temperature of 700°C/1292°F. Therefore, big bubbles are created in graphite texture moulds. They may not occur on the first firing because the exposure time was not long enough. By the second use, the graphite has already begun the decompose, and suffers large enough decomposition to produce the carbon dioxide.

Graphite might be useful as low temperature slumping moulds. The surface is easy to carve and shape to make smooth mould surfaces that do not require a separator. The disadvantages as a low temperature mould are the surface will erode quickly and mark easily, so detail will deteriorate and become damaged. Care is required to preserve it from marks and sharp materials. In addition, the slow, unnoticed decomposition will produce a rough surface over a number of uses.

The rapid deterioration of graphite moulds at low temperatures, make them unsuitable even for low temperature texture mould firings.

The expense of a thick slab of carbon thick enough to carve a decent depth and diameter of bowl needs to considered against a short life span. Small lamp working graphite moulds work well because the short times they are exposed to the heat do not give time for the decomposition to begin. And in any case, most often the moulds are open face, allowing any gases to escape without any distortion to the glass. 

What to do to Tweak Schedules?

What do you alter first – time or temperature – when tweaking schedules?

The choice is largely dependent on what the objective is and the process being used.

Since I am always trying to get the work done at the lowest temperature, I usually alter time first. This allows a greater amount of heat work which it the important element in getting the work done with the least risk.  I alter this general approach in relation to the process being conducted.

In casting, the objective is to get all the glass in the mould flowing and fully incorporated, so if past experience has shown too little flow at a relatively high temperature, my choice would be to extend the soak time. This avoids high temperature damage to the mould, and can help to avoid sucklers, or indents, in the casting.

In a full fuse, I attempt to get the work done in around 10 minutes. This implies that I would increase the temperature first and then the time if necessary.

At firings from contour to sharp tack and sinter fuses I tend to increase the soak time. The amount of time is determined by using peeks to know when the desired shape is achieved.

In general, peek at or near the top temperature. If the piece has not formed as desired, the time can be extended, with no extra firing needed.  This saves a lot of the extra firings that would be required.  And multiple firings still do not give you the time or temperature required for a single firing.

If, when you peek, the work has already been done, you can skip to the anneal. And on the next similar firing, use a slightly lower temperature, again peeking near the top temperature.

More detailed information about scheduling options can be found in the e-book Kilnforming Principles and Practices, available from Bullseye and Warm Glass

What are Commission Contracts?

 Initial Discussions

Once the clients decide they want something give them your time. Find out what they like about it, what they don't like, what space it's going in - is it on the wall, on a plinth? The time spent will pay dividends as the most important thing is your time and they'll remember it afterwards. It is part of the cost of doing business and forms the basis of the written contract.

Contract Details

A commission agreement protects the interest of the artist and the commissioner. The agreement is between the artist and the commissioning person or agent. It forms a contract and should contain all relevant details. Commissions need a written document so everyone knows what the terms are. The document can be as simple as a letter setting out the details, or as complex as the size of the commission requires. They all should include all these kinds of information.

• Write everything down in one place – name, address, and contact details of both parties.

• Include the information from the discussion about what the commission is to be.

• State the non-refundable design fee, which will be rolled into the final price if approved, and the number of design revisions allowed. The hourly cost of further revisions should also be stated.

• Upon approval of the design, agree a price exclusive of delivery, insurance and any installation.

• Estimates only are to be given for delivery, insurance and installation.

• The timescale must be realistic for the scale of the commission. Include terms to let them know of progress and if there's a delay or a problem.

• Detail the deposit terms. These typically are a non-refundable 20% to confirm the commission, a further 60% on completion and the final 20% on delivery. Smaller value commissions may have half the price to confirm the commission and half on completion. Invoices and receipts should be given for each of these stages.

• Include terms to terminate the contract by either party after a stated period of disagreement.

Managing Difficulties

If things go wrong at any of the stages during the commission it is perfectly allowable to end the contract if it becomes apparent the commission cannot be fulfilled due to disagreement. The non-refundable funds are to be retained by the artist.

As Mark Kristensen says “It's perfectly fine to walk away from a commission inquiry if, on mock up/design review, you cannot get on the same page. If, after putting an hour or so into making and adjusting design plans, client still cannot approve, it's probably best to pass.”

Establish at the beginning that the project includes a specified number of hours of planning/survey/meetings, of agency management, and of creative work. Other things may be added as the project requires. Also establish that exceeding those hours by 10% (or 5% if it’s really big) will incur additional charges.

Once into the project give weekly updates on how much time has been used and the progress achieved. That way, if the client starts adding things or changing their minds, they understand the consequences. If they think it’s worth paying for, great. If not, take it off the table. You can always be generous and waive the extra fees, but giving clients that kind of choice saves a lot of resentment (and time) on both sides.

All these must be included in the contract that is to be signed.

Stained Glass vs. Kilnformed Glass

There are minor differences between stained glass, kilnformed glass, and cast glass commissions, but the principles remain the same. There is a blog post on some of the nuances relating to commissions for kilnformed objects here.

Annealing dots

 

"No need to anneal dots assuming they’ll be used in projects."

Because dots are small they anneal in a short time, therefore no need for an anneal *hold*, but still the dots anneal. The larger the dots become – to cabochon size – the more important it is to have a temperature equalisation soak. This is what is usually called the anneal hold.

If the dots didn't anneal, they would break. Also because they are nearly spherical, they can withstand greater amounts of stress than flat glass. Industrial guidelines state that the easiest shape to anneal is a sphere. The next easiest is a cylinder. But the most difficult to anneal is flat slabs. It is even more difficult to anneal glass with uneven thicknesses.

So, annealing is always important, even though for small or thin items a soak is not necessary, because the annealing is achieved during the time the kiln cools. If there are a lot of dots in a small kiln, it may be a good precaution to use a 15minute soak at the annealing temperature.

What Formations are Made in Drapes?

The shape glass takes up during a drape seems to vary depending on whether the glass is a square or a circle at the start. The square often forms a taco shape before the “horizontal” ends soften and fall, pushing the glass into four drapes. The angle of these drapes will move more toward being straight down with additional time or heat. Circular draped glass tends to form three drapes, which also become more vertical with time and heat.

Early stages of a steep drape

One occasional problem with elevated moulds is that the draping glass is “longer” than the mould is tall. Understanding how the glass forms during a drape will help avoid this effect. The glass drape first forms a curve as it begins to drop, which continues to be in place until it touches the mould. The rest of the firing stretches the bow in the glass out.

If there is no mould at the bottom for it to touch because the mould is elevated, the curve goes beneath the bottom edge of the mould.

If the draping glass is longer than the elevated mould, the glass will drape to the shelf and can creep under the bottom edge, trapping the mould. 

 Alternatively if the mould is not elevated, the glass will flatten on the shelf. The flattened ends will spread outwards, but sometimes also creeping under the edge of the mould, trapping the mould.

The glass can be the correct length for the elevated mould, but the time and temperature combination scheduled allows the whole drape to stretch at the base, so the tips of the drape create one or other of the trapping effects.

The initial prevention of trapping the mould is to measure the length of the drape. The size of the blank should be no longer than double the length of the sides plus the width of the supporting top (the base of the mould when stood open end up). This length is the maximum diameter of a circle. 

 The diagonal of a square blank needs to be no longer than that either.  A simple visual method is to establish a square corner directly onto the glass.  Draw a 45° angle line and measure off the length on the diagonal. Draw a right angle line from the base to the point on the diagonal and you have the length of the sides ready to score.

It should be remembered that the glass will stretch to some degree during the drape. The length of that stretch is dependent on the temperature and time, so observation by frequent peeking will give the results wanted by either skipping to the next segment or extending the hold.

There is more information on scheduling for drapes in the e-book Low Temperature Kilnforming available from Bullseye  and Etsy.

Copper foil – to grind or not?

This is a question which always has two entrenched sides.  One for always doing it; one for grinding only when necessary for shape.

Some facts:

• The glue on foil is an impact adhesive.

• Impact adhesives stick most securely to smooth surfaces, and resins stick well to roughened surfaces.

• The adhesive is severely weakened by the heat of soldering.

It is a misconception that the adhesive on the back of copper foil tape is a structural element of panels. The adhesive is a temporary means of fixing the foil in place. The heat of soldering seriously degrades the adhesive. 

Therefore, the adhesive can only be a temporary measure to hold the foil in place while the came-like solder structure is created.

The structural element of copper foil panels is in the fin of solder connecting the beads on each side. This forms a came made of solder instead of lead. To be sound, there needs to be a domed bead of solder on each side with solder connecting them.

It doesn't matter in the long term whether the foil sticks well to the glass or not. Yes, it is easier to work if it does stick, but the strength remains in the came formed of solder.

Not every score and break is perfect, and often grinding is required to fit the pieces together. But when the score is exactly what is needed, there is no reason to grind it. The adhesive will stick better to the smooth than the roughened surface.

My conclusion is that it does not matter which side of the grinding issue you support. But why go to extra effort of grinding when there is no noticeable effect in the end? 

It is the came that is created by the fin of solder joining the beads on each side that provides the strength in copper foil panels.

Relieving Existing Stress - How?

Why is the stress not relieved after the strain point when slumping?

The answer relates to whether it is on the cool or on the heat up.

Cooling

The annealing occurs at a higher temperature than the strain point. The aim of the annealing soak is to even out the temperature within the glass to be equal to or less than 5°C/10°F (∆T=5C). When this small differential in temperature is achieved, there is little stress in the glass. In an adequate anneal, stress will be relieved during the soak. This differential needs to be maintained through the first cool, taking the glass temperature to below the strain point.

Relieving stress occurs between the glass transition point to just above the strain point. The viscosity of the glass is so high below the strain point (brittle phase) of the glass that no stress can be relieved.

The more rapid cooling during the brittle phase of the glass needs to be slow enough to avoid creating large contraction differentials within the glass. The reason for progressive cooling stages during the brittle phase of the glass is that it can withstand greater temperature differentials and so the cooling rates can be increased.

Heat up

Any stress on the way up for an already fused piece is induced by uneven heating. This can be across the piece, which is most evident in side fired kilns. The source of the infrared heating is nearest the edge of the glass, so it heats first leaving the centre cooler – sometimes the difference in expansion is great enough to break the glass.

In top fired kilns the differential is usually between top and bottom surfaces. Glass transmits heat slowly so the difference in temperature between the top and the bottom can be enough to cause a break from unequal expansions.

Both these conditions are caused by rapid ramp rates and short anneals on the cool.

Ramp Rates

Breaking of a flat piece on the kiln shelf is from too short a soak or too fast a cool, or both (unless there is an incompatibility). Breaking in a slump most often is a result of too rapid an initial ramp rate. A fused piece needs slower ramp up rates in a slump than in the initial fuse. It is now a single thicker piece, rather than multiple pieces as at the beginning of a fuse. While you might fire a flat 6mm/0.25” piece at 200°C/360°F for the fuse, the ramp rate for the slump needs to be no more than about 100°C/180°F. Tack fused pieces need much slower heat up rates during the slump, usually only half of the rate used to fuse the piece.

Tests have shown that even though the anneal soak for both firings can be the same, a more stress-free piece can be achieved by annealing as for one layer thicker. I do not know why, but I speculate that it is more difficult to achieve the ∆T=5C in the curved piece, than in a flat one.

More information is available in the ebook Annealing Concepts Principles and Practice.

The Relationship of Stress and Slumping

From time to time the assertion is that a break during the heat up of a blank while slumping is the result of residual stress remaining in an under-annealed blank.  Is there a relationship between inadequate annealing and slumping breaks?

It seems to be the general consensus that it is true.

It is clear that poorly annealed glass is more likely to break.  The assumption is that the additional bending stress added to the existing stress, causes a break.  Even if the fused piece is stressed, but not enough to break, a slow heat up would avoid the build up of stress to breaking level.  After all, the means of relieving the stress of toughened/tempered glass is by a slow heat up to allow the stress between the interior and surface to be equalised.  The same principle should apply to stressed glass in a slump.  My contention is that the ramp rates for the slump have been too fast to relieve any additional stress applied by the bending of the glass.

I hear of very few people testing for stress after any firing.  I read of people asserting the existing stress is amplified on the slump firing.  I do not read of any experience of people testing their flat piece, discovering excess stress, and  re-firing to relieve the stress before slumping, but the assertion of excess stress from the first firing continues as a cause.

Without testing there is no way to know whether the first firing had excessive stress.  The use of polarising filters is such a simple, easy and quick way to determine if there are stress problems in the fired and cooled piece.  It should be the business of practitioners and teachers to assert the need for stress testing as the next task when the glass has cooled. Unless people asserting this possibility do the testing of their proposition, it can only remain among the untested elements of kilnforming.

If there were to be a lot of stress in the flat blank, it needs to be fired again, annealed longer and cooled more slowly than previously to relieve the stress before any other process is conducted.  The firing to relieve the stress needs to be only to the lower portion of the slumping range at maximum.  

Each piece that is intended for further firing, needs to be tested for stress before the next firing, and not just at the end of the firing sequence.  To get an accurate reading of the stress, the piece must be allowed to cool until the internal temperature equals the surface temperature.  This may take overnight, but at least as long as the combined anneal soak and the associated cool. The delay caused by waiting for the complete cool may encourage people to skip the stress testing.  But it is risky to avoid testing for stress because of impatience.  Another firing can be conducted while waiting for the first piece to completely cool.

Annealing sufficiently on every firing is the way to ensure that any slumping break is not the consequence of stress from the previous firing.  The Bullseye sheet Annealing Thick Slabs (Celsius and Fahrenheit) gives the annealing times and cool rates.  This document applies to all fusing glass, except the annealing temperature used. Study the table, and follow it closely.  Keep in mind that the effective thickness for other than full fuse, is between 1.5 to 2.5 times the thickest part.  After that first firing, test the piece for stress when it has cooled sufficiently.  It is also important to test the successfully slumped piece for stress before using, gifting, or selling it.

What Range of Glass Cutter Styles are Available?

Is there any cutter that fits all people?

Probably not, given the number varieties there are on the market. Cutters seem to fall into two groups – the straight or pencil cutter and those with grips.

Some examples of cutters

The pencil cutter gives the most freedom of movement. It is very versatile, and direct in use. However, many do not have the grip strength to use the pencil cutter for long periods. It can often be because the pressure that is being applied is excessive. If that is the case, reducing the scoring pressure may make the pencil cutter suitable.

Whatever the case, many move on to variants of the pistol grip cutter to ease the stress on their hands. This does relieve the pressure on the fingers, but it puts the wrist into an awkward position that transfers the effort into the shoulder. This creates tension in the shoulder and neck, which is also uncomfortable after a time.

 

 

 These photos show two of the many variations on the standard pistol grip cutter.

If excessive pressure is not the cause of changing to a pistol grip cutter, there are alternatives which will relieve both kinds of pressure. These are an apparent compromise between the pencil cutter and the pistol grip cutter. What is important is that it allows the hand to have the same orientation as for the pencil cutter, which is good for smooth flexible score lines. The user has the pressure transferred to the palm of the hand. thus relieving both the strain on fingers and the stress on the shoulder and neck. It is not a compromise, but an improvement in function.

Assembled Toyo cutter

The neatest version of this compromise is the Toyo Custom Grip cutter. When in use the fingers are along the side of the barrel, but the pressure is between the “shelf” and the thumb pad. In addition the length can be adjusted to the length of the user’s fingers by shifting the parts of the barrel, allowing four lengths to be chosen.

Disassembled cutter

Assembled for small hands

    

                                         Assembled for larger hands

Assembled for the largest hand

The other major version of this is the Silberschnitt cutter handle. This can be attached at any height that is desired. It shifts the pressure to the base of the thumb just as the Toyo does. Additionally, the Silberschnitt fits almost any pencil cutter, so there is no necessity to buy a special pencil style cutter.

A cutter with the Silberscnitt universal handle

Although I prefer a pencil cutter, the Toyo Custom and the Silberschnitt cutter handle are my recommendation to those who have difficulty with the pencil cutter. It changes the whole outlook on scoring and breaking glass.

Glass Bonding

 

Enhancing Glass Bonding Characteristic with Silanes and Polymer Coatings

Although glasses are often valued for their chemical inertness, this property also presents challenges when attempting to form strong chemical bonds with other materials. Silanes and polymer coatings offer effective solutions by enhancing the bond between a glass and other materials in a composite.

The Challenge

The challenge of bonding glass to polymers spans across several industries, including industrial, automotive, and healthcare. In biomaterial applications, polymer carriers are often used to deliver glass or ceramic particles to specific treatment sites. However, bioactive glasses, commonly used in treatment delivery and bone regeneration, face a similar issue: the mismatch in critical surface tension and adhesion properties between the materials in the composite.¹ This mismatch is primarily driven by differences in hydrophobicity and hydrophilicity, making it difficult to create strong, stable bonds.

The key question, then, is how to improve and strengthen the bonds between glass and polymer materials to create stable composites that benefit from the properties of both material types.

The Solution

Fortunately, there are several approaches to improving the bonding characteristics of glass. One such solution is the use of silane or polymer materials as adhesive treatments. These materials help bypass the challenges of forming direct chemical bonds between the surface oxide groups on the glass and the substrate of interest.

Silanes

Silanes are particularly effective in improving the bonding between glass and other materials due to their ability to form highly stable siloxane bonds. These strong covalent bonds enhance the compatibility between glass and various organic or inorganic surfaces, creating a stronger interface than unmodified glass. Silanes are an excellent choice for composites, retaining the properties of glass while significantly improving surface chemistry and wettability.

One challenge in forming glass-bonded materials is the introduction of a new surface type, which can create potential regions of weakness and shear. Silanes act as effective coupling agents by forming strong covalent bonds with both the glass and the substrate, reducing these weak points and enhancing the overall stability and durability of the material.²

While physical abrasion and etching with hydrofluoric acid can improve adhesion by roughening the surface, chemical modification using silanes is often preferable. Chemical bonds offer superior grafting properties compared to physical methods, resulting in stronger, more durable connections between glass and other substrates.

Polymers

Polymer materials are widely used for bonding to glass due to their flexibility, which is highly advantageous in adhesive applications. While silicone-based materials can bond to glass, they are generally not as strong as many polymer adhesive options.

One such polymer adhesive, polyurethane, is a popular choice for bonding glass in various industries. This popularity is due to its flexibility, which helps absorb and mitigate vibrations induced by movement, enhancing the durability and integrity of the bonded structure. Similarly, acrylic adhesives are the ideal choice for oily or corrosive environments or for use in high-temperature applications. Epoxy adhesives also offer similar benefits, with excellent chemical and electrical resistance.

While many polymers show good adhesion to glass surfaces, their bonding interactions tend to be weaker than the covalent bonds formed with silanes, relying instead on intermolecular forces.³

Polymer adhesion is sufficient for many applications, especially where motion or substrate deformation is likely, as flexibility in the bonding is beneficial. However, for applications that demand the highest levels of adhesion, combining silane treatment with polymer bonding provides a superior solution. This approach significantly enhances bond strength, making it ideal for situations requiring both flexibility and durability.

MO SCI Solutions

MO SCI has a long history of developing custom glass solutions for even the most challenging applications. We can help you find innovative and effective ways to overcome the challenges of glass bonding and adherence to create devices that not only have the properties for peak application performance but are also stable and resistant to environmental degradation.

Contact us today to discuss your application.

Rebecca Straw

References and Further Reading

1. Brauer, D. S. (2015). Bioactive Glasses — Structure and Properties Angewandte. Angewandte Chemie – International Edition, 54, 4160–4181. https://doi.org/10.1002/anie.201405310

2. Yavuz, T., & Eraslan, O. (2016). The effect of silane applied to glass ceramics on surface structure and bonding strength at different temperatures. Journal of Advced Prosthodontics, 75–84. https://doi.org/10.4047%2Fjap.2016.8.2.75

3. Park, H., & Lee, S. H. (2021). Review on Interfacial Bonding Mechanism of Functional Polymer Coating on Glass in Atomistic Modeling Perspective. Polymers, 13, 2244. https://doi.org/10.3390/polym13142244

A low-cost Strip Cutting Method

 Cutting strips is repetitive, but requires accuracy. This can be achieved with expensive tools that do the job very well. It can also be done with only a few tools – most of which you already have.

This photo shows some of them.

The adjustable try square is really useful, as once it is set, you can be sure all the distances will be the same. The distance should be the width you require plus half the width of your cutter head – usually 2mm.

You put the nails into the bench or a board along a straight edge as measured by the try square. Check them both after hammering them in.

Place the glass edge along the edge of the board/bench. Use the normal cutting square to place against the nails. Then draw the cutter along the straight edge.

Move the glass out to the edge of the bench and break the strip off.

The accuracy of this method is dependent on the way you line up the glass along the edge of the cutting surface.

What is the Annealing and Cooling Relationship?

 Annealing Includes Cooling

Often people recommend a long anneal soak for potentially difficult pieces followed by an arbitrary 55C/100F cool rate to 371C/700F or 319C/600F. It is arbitrary because the same rate is frequently recommended regardless of the length of the anneal soak.

It does not have to be guesswork. Bullseye has provided us with the science of the anneal/cool in an accessible form: Annealing Thick Slabs (which covers thicknesses of 6mm/.025” to 200mm/8”). This document provides the annealing time for the chosen thickness and the directly linked cooling rates based on scientific principles.

The anneal soak is determined by the profile and thickness of the piece. Work for the e-book Low Temperature Kilnforming showed a relationship between the profile and the annealing time. Annealing for the profiles of sintering, tack, contour and full fuses requires calculation of the thickness to be applied to various profiles:

• Sinter or lamination – 2.5 times the thickest part
• Tack fuse – 2 times the thickest part
• Contour fuse – 1.5 times the thickest part
• Full/flat fuse – 1 times the thickest part

The cool stages are not random either. They have an intimate but inverse relationship to the anneal soak. They are to keep temperature differentials within the glass to acceptable levels. The anneal soak determined by the profile and thickness is to attain and keep the internal temperature within a range of 5°C throughout the glass. This is often referred to as ∆T=5°C.

The cool rates are to maintain acceptable temperature differences within the glass. The first cool rate is to maintain that temperature differential of ∆T=5°C. The second cool rate allows a wider range of temperature differential of 10°C, or ∆T=10°C. This is possible because the glass has become viscous enough to withstand this greater range of different temperature. The final cool needs to maintained at a differential of 20°C, or ∆T=20°C. Again, this is possible because the viscosity is high enough to withstand this amount of differential.

This information about cool rates and an allowable ∆T spread also indicate that turning off the kiln at 371°C/700°F is not always safe. It is almost always safe to do this for anything calculated to be annealed as for 12mm thick, and it may be safe for a piece up to 15mm thick, but remember that a tack fused piece of two base layers and a further decorative layer needs annealing and cooling as for 19mm/0.75”. The Bullseye research shows that the cooling rate for this is less than the unpowered cooling rate of many kilns. If there are additional complicating factors such as strongly contrasting colours, the annealing and cooling needs to be longer and slower than a simple multiplication of thickness.

There seems to be a practice of a single annealing rate to 371°C/700°F or 319C/600F. So the question will arise “Why is it necessary to have multiple cooling stages.” The response is that it will use unnecessary time and power. Attempting to maintain the ∆T=5°C over extended temperature ranges will not provide extra sound annealing. As the glass can withstand a ∆T=10°C from 427°C/800°F to 371°C/700°F, there is less power required at the faster rate than the slow one. This is even more so for the cool to 319C/600F and lower temperatures.

Knowing the safe temperature to turn the kiln off, requires knowing the cooling rate of the unpowered kiln. This blog shows how to determine the natural cooling rate of your kiln.  Knowing this is as important as knowing what effect different fusing temperatures have on the glass in your kiln.

The object of this blog post is to demonstrate that cooling is part of annealing. Just as much attention must be paid to the cooling rates as the length of the annealing soak. They are inseparable for sound kilnforming practices.

Some work that may be of assistance in understanding the importance of knowing the relationship between the annealing soak and the annealing cool are:

Annealing Concepts, Principles, and Practice

Available from: Bullseye and Etsy

Kilnforming Principles and Practice

Available from: Bullseye  and Warm Glass

Low Temperature Kilnforming

Available from: Bullseye and Etsy