Why do Ground Edges Appear on the Surface of the Glass?

"Why do the ground edges of my glass appear on top of the glass?"

The scum from grinding edges, which promotes devitrification, often appears in in fusing. But why does it appear on the surface? 

There is a lot of movement of glass edges during a firing. On the way up in temperature, the glass is relatively stiff and expands with a vertical edge during most of this phase.  At the top temperature the surface is expanding and pushes the edge flat to the separator.  Then, as the temperature falls the cooling glass surface contracts, pulling the edge to the surface together with anything it has collected from the separator.  Sometimes, cleaning the ground surface is not enough to prevent the rough surface from picking up some of the separator, and this is what is seen in the final product.

If you are going to grind to fit the glass pieces, you need to finish with a fine grinder bit. These are usually around 220 grit which might be fine enough if cleaned well, but run some tests to be sure. The safer grit to prevent the scum is 400, but the ground glass surface still needs to be scrubbed clean.

There are good reasons to avoid grinding, or when not avoidable, to fill gaps with powder or frit of the same colour.

What are the important elements for drilling holes in glass?

There are many aspects to drilling into glass.  This post reviews the major aspects.

Keeping Things Wet

When drilling glass it is important to keep the drill bit and glass wet always, otherwise the glass gets too hot and will break and cause the bonding of the diamonds on the bit to deteriorate. There are a variety of things you can do to achieve this:

Drill with the glass surface under water in a container.

Drill in a ring of clay, plasticine, etc., holding water. To do this, you need to make a ring about 50mm/2"  in diameter and press it around the drill site. Fill the ring with water to cool the drill site and glass. Diamond coolant is not necessary, but can extend the life of the bits.

Use a re-circulating water pump such as those made for indoor water features.

Direct the small flow of water (rapid drips) to the drilling site and catch the overflow in a separate bucket to the one in which the pump is submerged.  This extends the life of the pump and helps prevent clog ups in the water pipe.

Use a glass drill with hollow core bits and an internal water feed. This is the most expensive but it is the best equipment with which to drill holes of more than 4mm/0.158".

Drill Press

Drill presses vary from purpose-made through adaptations of industrial drill presses to hobbyist versions.  For light duty drilling that most glass workers do, a small press as set up for a dremel are suitable.

Example of a rotary tool press setup

This is an inexpensive solution to holding the dril steady while drilling.  It avoids the various contortions to stop the bit skittering across the glass when starting the hole.

Keeping the glass wet and cooling the drill bit for small pieces can be achieved by using a small temporary reservoir around the drill site to hold the water.  Alternatively, a small receptacle to submerge the glass can be used.

A plastic take-away container to hold a quantity of water

The water needs to be deep enough to cover the glass, but not so deep that it rises to the drill chuck, as that is likely to draw water into the rotary tool and short it out.  Notice also that the speed for the tool is at the minimum, because it is far too fast otherwise, and will overheat both the drill bit and the glass.

It is best to have an industrial drill press if you are doing a lot of drilling. It provides a stable drilling action and the pressure on the bit can be controlled. It is important to ensure the bit is running true without wobble. The drill press should have instructions to help correct any untrue running of the chuck. Make sure the drill bit is secured firmly. Core drill bits are easier to keep true, as they normally have a threaded fitting.

With a drill press, you can drill continually until the hole is completed, or until a white paste or dust begins to appear. This indicates the drilling is being done dry and will in a few moments heat up the glass too much. When the white paste appears, back out of the bottom of the hole a little to allow water to flush the glass out. Then continue.

Keep a firm grip on the glass being drilled.  If there is any chance of the glass spinning,  wear cut proof gloves.  Maintain the glass position, especially if you are intending to back out of the hole intermittently to allow water to the bottom of the hole. This enables you to get back into the hole without scratches.

Submerge the piece if you are drilling without a core drill bit, if possible. But if that isn’t possible, just squeeze a little puddle of water on the surface and watch it swirl around. You can see if it is pulling ground glass out of the hole by watching the circulation. Placing a plasticine or clay dam around the drill area will keep the water confined.

Don't push down any harder than you comfortably can on the lever with the tips of your fingers. Keep it steady. Listen for the sound of diamond grinding glass

White core stuck in the drill bit

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Always remove each core right after drilling. They are very difficult to remove if there is more than one stuck.

Core pushed out with 16 gauge copper wire

 When using a Dremel for drilling glass, slow it down to the minimum with the speed control. Drill presses do tend to be on the slow side for glass drilling so it takes a bit longer, but there are big advantages in other respects.

Drilling with a Flushing Head

A Typical Drill Press Set Up

A flushing head with a re-circulating pump will deliver water to the drill site through the core of the drill. These are supplied complete or as a fitting for an existing drill press. This is suitable for holes of 4 mm and larger. Smaller core drills are impractical both because the glass is easily trapped in the drill and the wall thickness of the drill makes them almost solid anyway.  An additional requirement is to have a means to direct the water to the waste bucket.

            Pump (black) at the bottom and flushing head where the water enters (chrome) at the top

Avoiding Chipping

There are a number of methods to avoid chipping out the back of the glass when drilling:

Place a piece of scrap glass under your good glass to avoid break-outs on the backside. By pressing firmly but gently on the glass (not the bit) the bit will go through the upper piece of glass without major chipping the back. This can be a difficult process to keep stable when both the pieces of glass are wet.

Another method is to put duct tape under the glass to help minimise chip out.
 Although I find a smooth firm base is best - it could be wood, hard plastic, or any other thin firm material that will not dull the bit when it goes through at the end of drilling.

A further process, used in industry, is to drill from both sides to avoid chip out. Go slowly toward the bottom of the hole. When the hole is almost through, turn the glass over and drill back to front.  It is critical to centre the drilling on the back exactly with the hole on the other side. 

Sometimes the glass is curved and drilling from the back is not easy. This is when a drill press mechanism to stop the bit comes into its own. Before switching the drill on, lower it to the surface supporting the glass. You can adjust the mechanism to stop the press just as it reaches the support surface. Then place the glass under the press and the turn the drill on to begin the drilling.

Don't push hard as you come to the end. Don't push down any harder on the drill press levers than you comfortably can with the tips of your fingers throughout the process. Keep it steady. Listen for the sound of diamond grinding glass.

All these things will help to avoid chipping out the glass at the bottom of the hole.

Drilling holes with copper tube and grit

You can drill holes by using loose grit and a copper tube of the correct diameter. It can take quite a while. You will need to have a chuck big enough to take the tube, or have a means to reduce the tube diameter to the chuck size. Alternatively, use core drills that have had the diamonds worn away.  This is not a common process, now that diamond drill bits are more affordable.

Prepare the glass as for a drill press without a flushing head, so the water and grit are confined. The dam can be putty, plasticine, clay, or other mouldable material put around the area to be drilled.  The grit can be sandblast grit or other abrasive of about 100 to 200 grit.  Drill as normal.

Tools

There are a variety of tools that can be used to power glass drills.

Dremel and similar rotary motors

These are light duty high-speed drills. Those with variable speed controls are especially useful. They work best for small diameter holes. They must have the speed turned down for drilling, especially for larger holes.  These can be combined with a flexible drive shaft for lighter weight, but a drill press is much more stable.

Drill press

However, the most important thing to have when drilling glass is a drill press. Doing it by hand is very difficult and wears out diamond bits very fast. Dremel and others make drill presses for their tools, as illustrated earlier.

Drilling machines

A portable glass drilling machine 

Purpose-made glass drilling machines are important for larger holes and production work. The important thing about these is that they use hollow core drill bits, allowing the water to be fed through the drill bit directly to the glass-drilling site.

Drill bits

The other tool needed is drill bits. The recommended type depends on the size of hole to be drilled.

• Small diameter holes, up to and including 3 mm can use solid diamond-tipped bits.  A number of manufacturers make solid drill bits from 2-6 mm and some (especially lapidary suppliers) make the very small diameter bits less than 1 mm.
• Larger diameter holes are best drilled with hollow core bits, as less glass needs to be removed to achieve the hole. These can be used with a flushing head or simply by directing water to the drill bit, with a dam to hold the water around the site.
• The bits will last longer if you use a drill press. The press keeps the bit wobble to a minimum and maintains the vertical, both helping to reduce the wear on the bit.

A selection of hollow core drill bits, wire and punches to clear the drill bit of stuck cores, and dressing stones

Hollow core bits

Hollow core diamond bits are of two types:

• One -where a heating process attaches the diamond - is called sintered in Europe and a number of other countries. These are long lasting and more expensive than the alternative. They can be dressed with an aluminium oxide dressing stick to maintain their effectiveness.
• The second – where the diamond is bound to the metal with resins – is called bonded in Europe.  These are less expensive and are a good alternative for those drilling smaller quantities of holes.  Bits of this type of bonding wear more quickly and should not be "sharpened" with dressing stones.

A diamond core drill breaks out much less glass at the bottom of the hole than a solid drill bit.  So they are quicker and have a lower risk of creating failures.  Buying better (more expensive) bits is worthwhile as they work much better and last much longer than the cheaper ones.

Water pump and reservoirs

A further tool that is useful to have is a pump. This can be a small fountain pump with a valve to regulate the flow, and a flexible spout to aim the water on the drilling site.  A bucket is required to act as the catch basin for the water that comes off the drill and and another as the reservoir for the pump.

Drilling glass without a drill press

It is best to have a drill press for drilling holes in glass, but there are ways of doing it with a hand drill.  Make a ring of modelling clay, plasticine, putty or other mouldable material about 5cm/2" in diameter and press it around the drill site. Fill the ring with water and a little diamond coolant if you like. The liquid will cool the drill site and surrounding glass as well lubricate the drill bit.  Adding diamond coolant to your water can extend the life of the bits. 

Use a paint pen to mark the spot where the hole is to be. Without a drill press, starting at an angle with a slow drill speed will stop the bit from sliding around as you establish the drilling point. As the glass surface is roughened, bring the drill to vertical. Move the drill up and down a little as you drill to allow the water into the hole. If you are using a solid or spade drill, a little oscillation keeps the bit from jamming in the hole.  This process is suitable for solid drill bits.  Do not do this with a core drill, as it may damage the edge of the bit and wears diamonds further up the bit.

Drilling speeds for diamond bits in glass

Every diameter drill bit has an optimum drill speed. The smaller they are the faster the speed. Drill presses do tend to be on the slow side for glass drilling, but often have ways of altering the speed. So they take a bit longer, but there are big advantages in other respects, mainly less wear on the bits and fewer break outs.

Diameter -- Speed

3-4 mm -- 6000 rpm

5-8 mm -- 4500 rpm

9-12 mm -- 3000 rpm

13-16 mm -- 2500 rpm

17-25 mm -- 2000 rpm

26-28 mm -- 1800 rpm

29-44 mm 1500 rpm

45-64 mm -- 1200 rpm

65-89 mm -- 900 rpm

90-120 mm -- 800 rpm

[Based on CR Lawrence and Amazing Glazing recommendations]

As you can see the larger the diameter, the slower the speed. This is because you are attempting to keep the speed of the diamonds moving against the glass at approximately the same speed, regardless of the diameter. If you did not slow the speed as the diameter went up, the speed of the diamonds across the glass would increase, leading to overheating of the bit and reduction in its life.

Hole Placement

The general rule on drilling holes in glass is that the edge of the hole should be further away from the edge than the thickness of the glass. This means that the edge of the hole on a 6 mm thick piece of glass must be more than 6 mm from the edge of the glass.

The calculations are simple arithmetic. You calculate the centre point of the hole by adding the radius of the hole to the thickness of the glass plus at least 1 mm. For example, to drill a 10 mm hole in 6 mm glass, you add 5 mm (radius of hole) to 6 mm (thickness of the glass) plus 1 mm = 12 mm as the minimum distance from the edge of the glass to the centre of the hole.  For methods of centring the drill see here.  Remember this is the minimum distance. For safety and durability in architectural or heavy circumstances, an additional margin must be added.

What Soldering Bit Size do I Need?

“What size of tip should I get for copper foil?”

The size of the tip is less important than the amount of solder applied. I can use my 200 watt iron with a 12mm tip in copper foiling. The iron power and large bit mean that there are no cool times to slow the progress of making the bead. I admit the 12mm bit is over large, but a 6 or 8mm bit will work well.

Do not go for very small bits intended for electronic soldering, because they cannot hold the heat as long as bigger ones, nor as long as is needed to run a long bead of solder. 

The amount of power and tip size are relevant to temperature recovery time. The soldering tip is the heat sink of the iron. With certain limits, the larger the tip is, the fewer times the iron has to heat back up to the appropriate temperature. The greater power also helps maintain the temperature with fewer drops below the right temperature.

Note that rheostats do not control the temperature, they control the amount of power reaching the iron, making the recovery time longer. 

General information on soldering irons.

Found Moulds

 I want to try thrifted [charity shop] dishes to slump into. What should I know and look out for?

Material

• Ensure they are ceramic or stainless steel.

• Plastic and glass will not work. Differences in weight will help determine the material, as plastic is light and ceramic is heavy.

• Cookware, almost certainly, is stainless, but not all woks or metal serving ware are.

• If the potential mould is not one of these heat resistant materials, a cast can be taken to make a refractory mould.

Shape

• The items should have a shallow draft. 

• The sides of the proposed mould should not be vertical. The more gentle the slope the better. 

• A rim is better than none, as the glass can rest on a rim, but not so easily on an edge. 

Testing

• The items need to be tested for their resistance to the heat of forming to determine how they will perform.

• Take the proposed moulds slowly to about 50 degrees above the working temperature that they will be subject to.

Preparation

• A separator will be required to keep the glass from sticking to the mould.

• The surface of the mould has to be altered to accept the separator. This varies according to material.

• Ceramics most often must have the glaze removed, although occasionally unglazed forms can be found. At the least, the glaze must be roughened to allow the liquid kiln wash adhere. There is a formulation of emulsion paint and kiln wash that can be painted onto the glazed surface, and sometimes boron nitride can be used with out altering the surface texture. On some shapes it is possible to lay shelf paper on the glazed mould without further preparation before slumping.

• Steel moulds need to have the protective oils removed, which can be done at the test firing. Boron nitride (a high temperature lubricant) is the popular choice of separator by many. It is possible to use standard kiln wash as the separator by heating the form to a little above boiling temperature and spray or brush the kiln wash onto the hot metal. This may need repeated applications, although the kiln wash needs to be only a film. This heating and coating method can be used for glazed ceramics too. On nearly flat steel forms, it is possible to place shelf paper over the steel to form the separator as in ceramic moulds.

• The interior of ceramic bowls is not likely to be perfectly flat unless it has a broad base like a soup bowl. A disc of fibre paper that has a smaller diameter than the interior base can be placed so the glass will form a flat base, or with a smaller diameter disc, a foot.

Use

• Ceramic forms are most often used as slumping forms because the glass contracts more than ceramic. Draping over ceramic shapes can be difficult because of the different amounts of contraction on cooling can cause the ceramic mould to become trapped. It is possible to wrap steep sided moulds with 3mm fibre paper to provide a cushion between the glass and the ceramic.

• Steel forms are most often used for draping, because the steel contracts more than the glass on cooling. It is possible to slump in shallow forms or those with a shallow draft. It is also possible to place glass into a steep sided bowl below the height at which the sides become too steep. Careful levelling of of both mould and glass is required to be successful.

Glass Microspheres in Medicine

 

Using Embolic Glass Microspheres to Target Chronic Disease

By Sierra Kucko

Glass Microspheres: The Tiny Superheroes of Glass Form Factors

Two main factors contribute to the properties of glass: composition and form factor. While emphasis is often placed on glass composition, the form factor is arguably equal in importance. Glass in the form of microspheres has permeated various industries, ranging from aerospace to medical sectors. MO SCI specializes in the production of precision glass microspheres that have become invaluable to these industries, and their usage is ever-evolving. Whether it be controlling gaps for adhesive bondline spacing, improving the visibility of road markings, or drug delivery devices, glass microspheres fit the bill.

Targeted Treatment with Embolic Glass Microspheres

Biocompatible (and in some cases biodegradable) microspheres are especially appreciated for medical applications, such as transarterial embolization (TEA) or musculoskeletal (MSK) embolization.^1,2

TEA refers to the blockage of blood supply, which may sound like a bad thing, but in many cases, these are lifesaving procedures. For example, a substantial driver for this technology is cancer treatment. One way to combat a tumor or abnormal tissue growth is to cut off its blood supply, which can be achieved through the precise application of appropriately sized microspheres to occlude the fine vasculature ‘feeding’ it.¹

Similarly, MSK embolic microspheres are sought after to prevent the abnormal overgrowth of blood vessels, a consequence of chronic inflammation. This kind of inflammation is part of a pathological loop, whereby the inflammation promotes the formation of new blood vessels that in turn, can feed nerve growth and contribute to chronic, debilitating pain.² Microsphere embolization can therefore be used as a pain management tool, as well. For applications with this level of weightiness, the microsphere size is a chief feature.

Together with the form factor, glass microspheres can be tailored through their composition. First and foremost, any implantable glass must be compatible with the body. Ancillary to this, the composition can be altered to offer additional functionality. Using TAE as an example to put this concept into context, the composition of glass used in this type of application is unique and important.

TAE is a procedure utilized by interventional radiologists. Interventional radiology (IR) is the diagnosing and/or treatment of cancer and other conditions while avoiding major surgery. To achieve this, small tools such as needles, catheters, or wires are utilized in conjunction with radiation like MRI, ultrasound, etc. to apply treatment precisely to the tissue site.^1–3 Personalization and optimization of outcomes is a clinical challenge of any medical intervention, making the accurate delivery and distribution of the embolic particles in real-time indispensable.

Due to the use of radiation to guide the placement, the embolic particle should be radiopaque (opaque to radiation) to ensure that guided delivery to the site can be realized. Compared with glass, this radiopacity is lacking or more difficult to achieve in microspheres derived from other material types.

Partner with MO SCI for Precision Glass Microspheres

Each application of glass is unique and therefore may require unique chemistries and form factors. Glass microspheres are becoming increasingly popular, since their form factor alone may improve the function of the glass (depending on the application) when compared to their powder or frit counterparts.

For applications that require precise microsphere size and composition, it is important to turn to trusted experts. MO SCI produces a wide range of glass microspheres in a variety of chemistries to suit nearly any need. Contact us today to learn how glass microspheres may be beneficial for your application.

References

1. Pérez-López A., et al. (2022). Embolization therapy with microspheres for the treatment of liver cancer: state-of-the-art of clinical translation. Acta Biomaterialia. https://doi.org/10.1016/j.actbio.2022.07.019

2. Gremen E., et al. (2022). Safety and efficacy of embolization with microspheres in chronic refractory inflammatory shoulder pain: a pilot monocentric study on 15 patients. Biomedicines. https://doi.org/10.3390/biomedicines10040744

3. Kishore S, et al, (2021). Transarterial embolization for the treatment of chronic musculoskeletal pain: a systematic review of indications, safety and efficacy. ACR Open Rheumatology. https://doi.org/10.1002/acr2.11383

Can I Get Texture on Front and Back?

There are occasions when it may be desired to have a double bas relief, or texture on both sides of the glass. There is one way, among others, to do this.

• Fire the glass on the texture mould (6mm thick is best).  Clean it well.
• Put a layer of kiln wash or whiting powder that is thicker than the profile depth of the texture you want to preserve. 
•  Smooth it over the shelf, but do not compact it. 
• Then firmly press the textured side into the powdered kiln wash, to maintain the existing profile.
• Then add the embellishments to the top and tack fuse with a slow ramp up to a low temperature.

Plan carefully, because the side fired down will not be as shiny as the top, unless low temperatures and long soaks are used.

There are, of course, other ways to achieve this.  The main one is to cast the piece.  But this procedure avoids the mould making process.

Narrow Grozing Pliers - Use?

 

Top pliers are used but undamaged.  The bottom pliers have rounded tips and will not grasp thin pieces of glass anymore.

Narrow grozing pliers are very useful in many circumstances, but their use is different from the standard 10mm grozing pliers.

Use narrow pliers to grasp small pieces and pull away from score. A firm but not hard grasp of the glass is required to pull the small pieces off. The square tips are important to the function of the pliers. If the tips are rounded, it is not possible to grasp the small pieces of glass to pull them away from the scored part without slipping off.

Squeezing hard crushes the glass and wears away the jaws quickly, which rounds the ends of the pliers, making them unsuitable for their intended purpose.

They are not meant for grozing. And in any case grozing is done with the serrations further away from the tip of grozing pliers. Misuse of the narrow pliers causes rapid wear and greatly reduces the useful life of the tool.

Possibility of Stopping Chipping Glass while Sawing?

Frequently there are chips on the bottom surface or breakouts at the end of cuts while sawing glass. There are several methods to reduce these effects.

Saw blade depth

The blade on an adjustable depth overhead saw should be set to just below the saw table depth. This reduces the break outs on the bottom surface. It helps to make the angle at which the saw blade meets the glass more acute, helping to reduce the chipping of the surface.

Of course, on an adjustable overhead saw blade could be set to just mark the surface to reduce chipping on the top. Then the table drawn back to adjust the blade to the full cutting depth. However, that is a lot of adjustment to reduce minor chipping that will be remedied in further cold work or fire polishing.

For saws that do not have adjustable depth, bottom surface chipping can be reduced by placing sacrificial glass below the main piece. This raises the main glass and creates a more acute angle between the glass and the blade, also reducing chipping on the upper surface.

Ends of Cuts

Break outs often occur at the ends of the cuts. Placement of a sacrificial piece of glass vertically at the exit of the cut helps to give a clean cut at the end. This will apply whether using a fixed or adjustable saw blade.

Of course, the two can be combined:

Is White a Difficult Glass?

Description of the Project

A white 3mm base with 3mm and 6mm decorations made up of mosaic pieces from previously fused glass (all the same CoE). At the end of the firing three corners had broken and their edges rounded. The fourth corner had sharp edges. The tentative conclusion was that there was incompatibility between the white and the previously fired pieces. There were no other cracks visible on the white or between the mosaic pieces. The author did not indicate what the schedule was for either firing, nor what the profile of the last firing was, but asserts white is a particularly difficult glass which does not work well with a wide variety of colours.

My observations are: 

• Compatibility is not an issue on the heat up. It is only a problem at annealing and cooling.
• Breaks on the ramp up (showing rounded edges at the conclusion of the firing) are normally the results of too fast rates.
• Breaks during cooling (showing sharp edges) are due to annealing, compatibility, cooling rates, or some combination of these.
• Previously fired glass can show some shift in compatibility and so needs slower up ramp rates than normal for the profile and thickness.
• Incompatibility between the base and the mosaic pieces would show up as breaks in the white glass under each top mosaic piece.
• Not all glass of the same CoE from different manufacturers is compatible.

Could this have been from incompatibility?

On the way to top temperature the pieces have not yet combined. The incompatibility will only show up during the cooling, as it is the imbalance of  viscosity and contraction between the fused pieces that cause the breaks.

Only one of the broken corners has those sharp edges, making incompatibility an improbable cause of the breaks. Further, incompatibility between the base and upper layers present either a crazed appearance at the connections, or simple breaks around the base of each decorative piece. Incompatibility would have multiple breaks all over the base, if not the top too. Finally, if the fired mosaic pieces were incompatible with the white glass, there would have been breaks throughout the whole piece, not just at the corners.

A further possibility is that the corners were very close to the sides of the kiln, because only the corners broke away from the piece,. If it was side fired, much slower rates are required. And all kilns tend to be cooler near the sides on the heat up than toward the centre, even if top fired.

My guess, based on the description, is that the up ramps were too fast, and the anneal was too short and the cool too fast. Unless the previously fused pieces were tested for stress it is not possible to know whether those were stressed before the final firing, which could have caused the break off of the three of the corners. The fourth corner break was on the cool down and is most likely to be too short an anneal and/or too quick a cool.

Is white glass especially difficult?

There is nothing in this piece to identify white glass as an extraordinarily difficult glass, or that a multiplicity of colours added to white would provoke breaks. The problems exhibited are most likely related to fast heat up ramp rates, and inadequate annealing and cooling.

How to get a Strong Sintered Snowflake?

Credit: Janet Wager

"My snowflakes are fragile. Will more glass make them stronger?”

My experiments for the e-book Low Temperature Kilnforming showed more frit does not make these sintered pieces stronger. Sintered pieces made from frit are made stronger with a combination of packing, time and temperature.

Packing

The amount of packing makes the whole more dense by linking more particles together. The most dense packing is done by putting down thin layers and packing each layer into the previous one. This allows more connections to be made between the pieces, which is important to the resulting strength.

Temperature

The temperature at which the frit is fired is important too. Firing at 650°C/1202°F will produce a weaker piece than one fired at 690°C/1274°F, if both are soaked for the same amount of time.

Time

Generally, the soak at the top temperature will be about two hours. The interplay of time and temperature are particularly important when sintering frit. The connections at the atomic level take more time to form at low temperatures than at high ones. The piece fired at 650°C/1202°F can be strengthened by extending the time to 4 hours instead of 2 hours, which will give it about two thirds of the strength of firing at 690°C/1274°F, for 2 hours.

Rates

The ramp rates need to be slow to achieve maximum strength. For example. A rate of 150°C/270°F to 690°C/1274°F needs a 2 hours soak, but a rate of 600°C/1080°F needs 6 hours to achieve the same strength. An alternative is to use a fast ramp to the strain point and then 50°C/90°F to the top temperature for a 3 hour soak.

Texture

The texture of the sintered piece will vary according to time and temperature. A firing at 690°C/1274°F for two hours will give a shiny top surface, but will be more textured underneath. A two hour soak at 650°C/1202°F will give a sugar grain appearance, but be weak. Extending the soak time to 4 hours will approximately double the strength. If you want to retain the sugar texture and have a strong piece, a long soak at a top temperature of about 650°C/1202°F for at least 3 hours will be needed.

Annealing

Sintered pieces need to be annealed for about 2.5 – 3 times longer than their thickness to be strong and the cool rates need to be related to the soak time to be as stress free as possible.

The e-book Low Temperature Kilnforming is available from:

Bullseye

Etsy

and me:  Stephen.Richard43@gmail.com