Nuclear Waste Vitrification

 

vitrification process

Image credit: radopactovotu/eu.com

Expanding Nuclear Waste Vitrification Strategies with Customizable Glass

by Rebecca Straw.

Nuclear power plays a key role in sustainable energy generation, offering a long-term solution with its extended operating life and low greenhouse gas emissions. As of 2025, nuclear energy supplies approximately 10% of the world’s electricity, with 440 power reactors operating across 31 countries.¹

Beyond power generation, nuclear technology is also starting to play a vital role in medical diagnostics, industrial processes, and space exploration. With growing demand for reliable, clean energy, the sector continues to expand, with new reactors under construction worldwide.² However, nuclear waste disposal remains a critical challenge, requiring innovative solutions to ensure long-term safety.

Vitrification and the Growing Importance of Nuclear Waste Management

High-level nuclear waste (HLW) is the most hazardous by-product of nuclear energy production, consisting primarily of spent nuclear fuel and its reprocessed liquids. Its long-lived isotopes and high radiation levels pose significant environmental and health risks, requiring robust containment strategies.^3,4

As unstable isotopes decay, HLW continuously emits heat and radiation, making secure storage essential to prevent environmental contamination.⁴ Certain radionuclides, such as the actinides plutonium and curium, have extremely long half-lives, necessitating immobilization techniques that ensure stability for thousands of years.³ Others, like Technetium-99 and Iodine-129, are highly soluble in water, increasing the risk of groundwater infiltration if not properly contained. These challenges demand a waste form that offers long-term durability, leach resistance, and mechanical stability under repository conditions.

Vitrification—the process of turning waste into glass—has emerged as one of the most effective containment methods for HLW.⁴ Unlike dilution or surface storage, vitrification permanently traps hazardous materials within a stable glass matrix, preventing their release into the environment. Additionally, vitrified waste is compact, insoluble, and well-suited for secure long-term storage and disposal.

Glass Selection in Nuclear Waste Immobilization

Selecting the right glass composition is critical for the success of vitrification. Borosilicate and phosphate-based glasses are the two primary materials used for HLW immobilization.

Borosilicate glass is favored for its high chemical durability, low thermal expansion, and capacity to incorporate a wide range of radionuclides.³ It has been the standard choice in countries like France, the UK, and the US, where large-scale vitrification facilities are in operation. Its success is largely due to its compatibility with various waste cations, well-characterized structure, and well-established processing technology.³

However, optimizing the waste loading—the percentage of waste incorporated per unit volume of glass—while ensuring the final product remains stable and processable remains a persistent challenge in vitrification processes. While increasing waste loading reduces overall storage costs and processing time, it requires precise control over the glass formulation to prevent crystallization or phase separation.

Optimized borosilicate glass is generally well suited for this purpose, but certain waste components present in HLW, such as molybdenum and noble metals, have low solubility in the borosilicate matrices, limiting how much can be incorporated, and its utility as a universal HLW matrix.

Expanding Vitrification Strategies Through Customizable and Alternative Glass Solutions

While borosilicate glass has long been the standard for HLW vitrification, its limitations in incorporating certain waste components have driven the exploration of alternative glass formulations. Phosphate-based glasses (e.g., iron phosphate, alumino phosphate) present a promising alternative to traditional borosilicate glass for the immobilization of HLW, particularly in the management of actinides, lanthanides, and other elements that are poorly soluble in borosilicate.

These phosphate glasses offer enhanced degradation resistance and superior tolerance to radiation, making them ideal for managing complex waste streams.³ In particular, phosphate-based formulations are highly relevant for next-generation reactor technologies, such as molten salt reactors. These reactors, which utilize liquid fuel salts, produce waste compositions that are rich in fluorine and differ significantly from those of conventional reactors, thus limiting the types of glasses that can effectively immobilize this type of waste.⁵

 Phosphate glasses also stand out for their ability to better accommodate halide-rich waste streams. These glasses can be processed at lower temperatures, reducing volatility, and can accommodate higher salt loading, making them an attractive solution for waste from advanced reactor designs. ⁵ For example, fast breeder reactors, which generate waste with high concentrations of plutonium and actinides, require non-silicate glass forms capable of accommodating these elements without compromising long-term integrity. 

Leading Innovation in Nuclear Waste Vitrification

MO SCI is pioneering the development of advanced glass formulations that improve waste loading and long-term performance while addressing the limitations of traditional glass forms. By focusing on customizing glass compositions to suit specific waste characteristics, MO SCI is helping to ensure that vitrification remains a scalable and effective solution for HLW disposal, supporting the transition to next-generation reactor technologies.

For further information on nuclear waste management solutions, please contact us today. 

Rebecca Straw

References and Further Reading

1. World Nuclear Association. Nuclear Power in the World Today [Updated 6 Jan 2025]. Available from: https://world-nuclear.org/information-library/current-and-future-generation/nuclear-power-in-the-world-today#:~:text=Nuclear%20energy%20now%20provides%20about,in%20about%20220%20research%20reactors.; (Accessed on 3 Mar).

2. Hyatt NC, Ojovan MI. (2019) Special Issue: Materials for Nuclear Waste Immobilization. Materials (Basel);12(21).

3. Bohre A, Avasthi K, Pet’kov VI. (2017) Vitreous and crystalline phosphate high level waste matrices: Present status and future challenges. Journal of Industrial and Engineering Chemistry;50:1-14.

4. Sanito RC, Bernuy-Zumaeta M, You SJ, Wang YF. (2022) A review on vitrification technologies of hazardous waste. J Environ Manage;316:115243.

5. Riley BJ, McFarlane J, DelCul GD, Vienna JD, Contescu CI, Forsberg CW. (2019) Molten salt reactor waste and effluent management strategies: A review. Nuclear Engineering and Design;345:94-109.

The Effect of Glass Temperature on Cutting

There are many opinions on how glass cuts when cold.  Some report cutting outdoors in sub-freezing temperatures, others that only warm glass cuts well.  I decided to see what scientific information there may be on this idea.

The Science

The scientific literature mostly concentrates on the effects at higher temperatures than we are concerned with.  However, there are some things that are applicable, and some of these effects of temperature are outlined below.

·         High humidity results in loss of strength. 

·         The strength of glass is reduced by 25% at 100°C compared to 0°C.

·         Glass needs several days to be at an even temperature throughout.

·         Variance in temperature across the glass causes unwanted breakages.

·         Colder glass becomes more brittle due to loss of elasticity.

·         Hardness of glass increases with decreasing temperature.

The terms of strength, hardness and brittleness have scientific definitions that are hard to apply to the everyday glass cutting that we do.  Strength may or may not have applicability to glass cutting.  Elasticity may or may not be an important factor in cutting.  Surface hardness may play a part in cutting while cold.

Applicability of the Science

However some things seem to apply. 

High humidity results in loss of strength.  This may be a factor in low temperature cutting.  The humidity in a relatively closed environment increases with the reduction in temperature.  Breaking glass is about the creation of a weakness in the glass along the score line.  In so far as strength is a factor in the break running along the score line, this may be an element in cold glass cutting.  If the whole glass is weaker, the difference in strength at the score line is less and so promotes unwanted breaks.

Variance of the temperature of the glass throughout the substance of the glass promotes unwanted breakages.  Perhaps the cold glass that is difficult to cut is not equally cold throughout.  Certainly a number of people report that they store their large glass outdoors and can still score and break the glass during the winter perfectly well before bringing it into the studio. 

Glass becomes more brittle with decreasing temperature, and it also becomes harder.  Perhaps these two elements are a factor in controlling breakages.  If the glass is both harder and more brittle, a different scoring method is required. 

The way in which glass at any temperature breaks is related to the force of the score, the speed of the score and the angle of the cutting wheel.  If the glass is both harder (at the surface) and more brittle it requires less scoring force or a blunter wheel angle.  The more blunt the wheel on a thicker (i.e. stronger) glass, the more vertical the stress lines are created in the glass.  So in a cold and harder glass, a blunter wheel angle seems appropriate, even though the glass is not thicker.

It is not usual for people to have cutting wheels of different angles, so an easier, although more skilled, approach is to reduce the scoring force in cold conditions.  Reducing the force in scoring a hard and brittle glass causes the stress lines to be more vertical than increased forces do.  Increased forces cause lateral lines of stress to be created, leading to unwanted breakages.

Secondly, the glass being more brittle, less force in breaking stress is required.  As the glass becomes colder, the less elastic it is.  This elasticity is an important element in breaking the glass at room temperatures. The score needs to be run gently to counteract the loss of elasticity and the consequent increase in the brittle strength of the glass.

Conclusions

My conclusion, after the reading I’ve done, is that cold glass becomes slightly stronger and more brittle than room temperature glass, and so requires a slightly different method of cutting. This difference is to reduce the pressure of scoring and the force of breaking (applying stress to the glass).  

An alternative conclusion by Dennis Brady is that his tests showed that the difference between cold glass and warming for 5 minutes in an electric blanket produced the following effects:

"Float glass no difference.

Transparent art glass no difference.

Opal or streaked glass definite difference.

Machine rolled less difference than hand rolled."

His conclusion was that the "lower viscosity of opalescent glass made it less likely to follow a score when cold."

Of course you can warm the glass up before scoring it, but the research seems to indicate that only opalescent glass will benefit from heating.

Revised 25.12.25

Glass Coatings for Batteries

 

Using Glass to Optomize Composite Materials for Battery Applications

by Rebecca Straw

Polymer-based resins, such as epoxy, polyurethane, and silicone, are commonly used for sealing and potting applications in batteries. By filling voids within a battery enclosure, these materials help to promote heat dissipation and electrical insulation while offering protection against mechanical stress, chemical exposure, and moisture.¹

Although resins are valued for these properties, they can exhibit inherent limitations when used alone.² For this reason, achieving optimal performance in potting adhesives has been a significant challenge over the past decade. This includes meeting requirements like low dielectric loss, enhanced shock resistance, high thermal conductivity, and stable weight.³

Enter composite materials: By integrating glass components—such as glass fibers, and hollow or solid glass microspheres, —it is possible to create a composite material with superior thermomechanical, structural, and electrical performance.⁴

Glass Fibers: Reinforcing Strength and Sustainability

Glass fibers are made from extremely fine strands of non-crystalline glass. They are highly valued for their excellent surface-area-to-weight ratio, and the amorphous nature of glass ensures that its properties remain consistent throughout the fiber.^4,5 When mixed with epoxy, these composites are widely utilized in structural applications and valued for their outstanding mechanical properties –⁶

In battery applications, these attributes bolster the composite’s capacity to endure mechanical shocks and vibrations, thereby improving overall durability and reliability. Additionally, the option to use recycled glass fibers contributes to sustainability by reducing waste, aligning with the industry’s push toward circular economy practices.⁴

Hollow and Solid Glass Microspheres: Lightweight Innovation with Multifunctional Benefits

Hollow glass microspheres, also known as microballoons, are thin-walled glass bubbles containing a void in the center. The balloon sizes generally range from 10 to 90 microns, and they have low density, high crush strength, and chemical durability.  Their shape and smooth surface minimize stress concentrations at the interface between the fillers and the matrix, enhancing structural integrity. When used in resins, they create polymer composites with augmented thermal and sound insulation, reduced weight, and improved fracture toughness, making them ideal for advanced applications.^7,8

Solid glass microspheres are spherical particles made from glass, These particulates are valued as fillers in polymer composites due to their unique properties, including customized thermal conductivity, isotropy, smooth spherical surfaces, and minimal stress concentration at the filler-matrix interface. Their uniform shape guarantees consistent shrinkage and enhances the processability of filled materials, while also minimizing orientation effects during molding.^11,12

In battery potting applications, engineered solid glass microspheres can boost insulation performance, increase thermal conductivity, and ensure dependable material properties, making them particularly suitable for protecting sensitive components.

Silane Coatings: Seamlessly Bonding Glass and Polymer for Superior Composites

Silanization is a surface modification technique used to increase the interfacial adhesion between glass particulates, such as microspheres and fibers, and polymer resins in composite materials. By applying silane coupling agents, a strong chemical bond is formed between the glass and the polymer, significantly improving the mechanical properties, loading capabilities, and surface adhesion of the composite.¹³

Silanization has also been shown to protect glass fibers from mechanical damage during processing and environmental degradation, making it particularly beneficial for battery potting applications.⁴

Tailored Glass Solutions from MO SCI

By integrating glass materials into polymer resins, researchers can create composites that meet the stringent demands of modern battery technology and contribute to greater sustainability and safety.

MO SCI’s expertise in producing high-performance glass fibers and microspheres, coupled with advanced silane coating technologies, ensures that customers have access to tailored solutions for optimizing battery potting and sealing materials.^14,15

Contact us today to discuss your material performance requirements.

References and Further Reading

1. Epec. [Online] Battery Potting & Encapsulation. Available at: https://www.epectec.com/batteries/potting-and-encapsulation.html (Accessed on 21 November 2024).

2. Yang, L., et al. (2024). Characterization of Potting Epoxy Resins Performance Parameters Based on a Viscoelastic Constitutive Model. doi.org/10.3390/polym16070930

3. Hu, J.B. (2020). High-performance ceramic/epoxy composite adhesives enabled by rational ceramic bandgaps. Scientific Reports. doi.org/10.1038/s41598-019-57074-7

4. Săftoiu, G-V., et al. (2024). Glass Fibre-Reinforced Composite Materials Used in the Aeronautical Transport Sector: A Critical Circular Economy Point of View. Sustainability. doi.org/10.3390/su16114632

5. Park, S-J., et al. (2011). Chapter 6 – Element and Processing. Interface Science and Technology. doi.org/10.1016/B978-0-12-375049-5.00006-2

6. Safi, S., et al. (2016). Evaluation of interfacial properties of the silane blend sized glass fiber–epoxy composite by the microdroplet test. Journal of Composite Materials. doi.org/10.1177/0021998316661620

7. Liang, J-Z., et al. (2014). Estimation of thermal conductivity for polypropylene/hollow glass bead composites. Composites Part B: Engineering. doi.org/10.1016/j.compositesb.2013.08.072

8. Wouterson, E.M., et al. (2004). Fracture and Impact Toughness of Syntactic Foam*. Journal of Cellular Plastics. doi.org10.1177/0021955×04041960

9. Mo Sci. [Online] Porous Silica. Available at: https://mo-sci.com/products/porous-silica/ (Accessed on 21 November 2024). 

10. Li, M., et al. (2024). Preparation of DOPO-KH550 modified hollow glass microspheres/PVA composite aerogel for thermal insulation and flame retardancy. Journal of Colloid and Interface Science. doi.org/10.1016/j.jcis.2023.10.073

11. Mallick, P.K. (2000). 2.09 – Particulate and Short Fiber Reinforced Polymer Composites. Comprehensive Composite Materials. doi.org/10.1016/B0-08-042993-9/00085-1

12. Mishra, D., et al. (2016). An experimental investigation on the effect of particle size on the thermal properties and void content of Solid Glass Microsphere filled epoxy Composites. IOP Conf. Series: Materials Science and Engineering. doi.org/10.1088/1757-899X/115/1/012011

13. Perdum, A-I., et al. (2022). HOLLOW GLASS MICROSPHERES TREATED WITH SILANE COUPLING AGENT. University Politehnica of Bucharest Scientific Bulletin, Series B. ISSN 1454-2331.

14. Mo Sci. [Online] Glass Microspheres. Available at: https://mo-sci.com/products/glass-microspheres/ (Accessed on 21 November 2024).

15. Mo Sci. [Online] Custom Development Services. Available at: https://mo-sci.com/services/custom-development-services/ (Accessed on 21 November 2024).

Why do Bubbles Appear in a Circle?

Example used with the maker’s permission

Piece in kiln ready to fire

Description of the piece.

A commissioned piece made up of Bullseye glass, 38cm/15” diameter, 3mm base with 2 and 3mm strips laid on top to a 6mm maximum depth, fired on a “standard” full fuse at 795°C/1463°F, annealed for 3.5 hours. The piece took up the whole of a newly primed shelf.

The fired piece 

The fired piece developed an off centre bubble, and when the piece was cooled it rocked. The shelf was checked and it was level.  

Previous firings of similar projects were mostly successful, but one was not, although felt to be interesting:

The piece bubbled and various sized holes were randomly drilled to determine the effect.

Other successful pieces were only slightly smaller:

The question was what to do to recover the piece, and why did it happen. Previous bowl blanks on the same program were ok, although this one was larger.

My Response:

The layup is the problem. A thinner (single?) layer in centre surrounded by radiating strips will trap air in the interior and so create bubbles.  This kind of layup needs to have 1 - 3 mm fibre paper topped with shelf paper, under this kind of piece to allow air out.

This is a commission, so a repair is not acceptable. A new one needs to be created, because there would be reputational damage by passing off a repair that will inevitably show evidence of the fault. In general, repairs are unsuccessful.  Repurposing the glass is a better solution.

Other possible causes in addition to the lay up are:

When a piece rocks on a flat surface it has become bowed, and it is evidence of stress developing during the annealing cool.

The annealing and cooling were likely to be inadequate – too short a soak, too fast a cool, or both.

In this case, the anneal soak was certainly long enough, so too fast a cool is the likely cause of the bowing.

The nearness of the glass to the kiln walls is often a cause of uneven heating, although in this case it did not become a problem.

It is also possible that the additional diameter was enough to push a barely adequate bubble squeeze beyond the possibility of full elimination of air from under the glass.

As a result of the experience of the “moonscape”, variously sized holes were randomly randomly around the centre of the bubbled piece to determine if that would have been an acceptable fix. The thicker ring around the burst large bubble remained in the attempt at a fix.

Holes drilled randomly and extra clear dots were added.

The client was contacted to explain the accident and to confirm the making of another.

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.