Calibrating your new kiln

My new kiln fires differently than my existing one(s).

Each kiln will be different in minor or major ways.  Suggested schedules are only starting points, even though they worked in your previous kiln. You need to learn about your new kiln’s characteristics in the same way you did with your first kiln. There are a number of ways to do this.

Many people recommend making test tiles for the different levels of fusing that you use and at different temperatures to determine which is the best for your new kiln.  Additionally, you need to note the rate(s) at which you fired these samples to make the test tiles accurate representations of firings.  Yes, these will be good references.  And yes, they are valuable if you have the time for all these firings.

 Idle Creativity

Observation during the firing of test tiles is the best and quickest way to discover how your new kiln is performing at various temperatures.  In one firing you can note the temperatures at which the various tack levels occur, and the contour and full fuse temperatures.  You can even take pictures through the peep hole of your kiln (as long as you don’t put the camera too close to the kiln!).  This procedure will make knowing your kiln much quicker and accurate than unobserved multiple firings.

To make use of the notes of the temperatures where the results were achieved in the test firing, back off 10°C (or 20°F) from the observed temperature and add 10 minutes processing time.  You may find after a few uses of these temperatures you may want to adjust the temperature a bit more, but you will have done the major experimental work in one firing.

It is a good idea to set up other test tiles and run the experiment again at a slower rate of advance.  This will give you information about how the different rates of advance affect the processing temperature and the look and texture of the piece, especially on the bottom.  The texture imparted at different processing temperatures becomes more important in slumping and draping processes.

In another firing you can set up various moulds and observe when the slumps or drapes are complete. Recording this information lets you know slump temperatures for various styles and spans of moulds.  Make sure you record the temperatures that the pieces slump fully into the mould. You can then back off 20°C from those points and add 30 minutes as a starting point for you actual slumping firings.

New kilns require experience to know what the appropriate temperatures are.  Buy setting up test tiles you can observe in one firing the various levels of fuse from tack to full fuse, so saving lots of firing time.

Further information is available in the ebook: Low Temperature Kiln Forming.

Glass 101: An Introduction to Glass

 

Glass 101: An Introduction to Glass

Posted Krista Grayson on Jul 17, 2019

Everyone knows what glass is…or do they? We are all surrounded by glass in a myriad of forms and serving a diverse array of functions, from jars and glasses to windows and TV screens, but do we really know much about it? Glass has become so ubiquitous that it is widely accepted as an everyday commodity. Consequently, most of us take it for granted without considering what it is or how it shows such amazing properties and versatility.

Through the ages, glass has provided answers to many technological challenges and enabled unbelievable advances across most areas of our lives.1,2 However, despite being at the center of an ongoing success story, glass receives little attention from the majority.

What is glass?

Glass is commonly categorized as a type of ceramic, but it is not like any other ceramics. Ceramics generally have a crystalline structure and are opaque, whereas glass has a non-crystalline atomic structure and is transparent. Furthermore, glass exhibits a range of remarkable properties that set it apart from other ceramics. In a perfect state, glass is mechanically very strong, even when subjected to extreme changes in temperature, and has a hard surface that is resistant to abrasion and corrosion. Paradoxically, it is also elastic, being able to give under stress (up to a breaking point) and then rebound to its original shape. Glass also has extensive optical properties, is heat-absorbent and an electrical insulator.3

Glass is so unique that it cannot be simply defined. It is neither a crystalline solid nor a liquid; it is a disordered, amorphous solid. It is this amorphous structure that gives glass its unique properties. Neither can the composition of glass be described since there are infinite varieties of glass. A current database lists over 350,000 types of known glass and new workable glass compositions are being developed every day.

The production of glass

Glass is formed when the constituent parts are combined by intense heating and then rapid cooling. The rapid cooling immobilizes the atoms of the glass before they have a chance to assume a regular crystalline structure. This can occur naturally, as in the case of fulgurite that is formed by lightning striking sand, and obsidian that arises from the rapid cooling of volcanic lava.

Man-made glasses are produced from varying mixtures of oxides. Although the precise chemical composition varies widely between different types of glass, it typically includes three components: a former, a flux and a stabilizer. Glass formers, such as silicon dioxide (silica), make up the largest proportion of the mixture and provide the transparency. Fluxes, such as sodium carbonate (soda) lower the temperature at which the formers will melt. Stabilizers, such as calcium carbonate (lime) provide the strength and make the glass water resistant. Without the inclusion of a stabilizer, water and humidity will attack and dissolve the glass.4

Immediately after glasses are batched and melted, they are slowly and evenly cooled. This process is known as annealing. This is an important step that enhances the strength of the glass by reducing internal stresses. It ensures that sections of varying thickness cool at the same rate. This avoids the development of steep temperature gradients that could cause the glass to crack.

Types of glass

The precise chemical composition of the mixture melted to produce glass determines the mechanical, electrical, chemical, optical, and thermal properties of the final product. Glass can thus be manufactured with broad-ranging characteristics. Through careful selection of the basic initial mixture and additives used in production, glass is produced with properties and structures to meet the requirements of specific applications.3

Although there are many thousands of different glass compositions, glass can be categorized as belonging to one of the six basic types, based on the chemical composition that endows it with specific properties.5

Soda-lime glass

Soda-lime glass is the most common, and least expensive, type of glass, accounting for 90% of all glass made. It usually contains 60–75% silica, 12–18% soda, and 5–12% lime. This is the type of glass used to make bottles and windows. It is mechanically strong but does not have good resistance to high temperatures, sudden changes in temperature, and corrosive chemicals.

Lead Glass

Lead glass (more commonly known as crystal) contains at least 20% lead oxide, which makes the glass brilliant, resonant, and heavy. Although lead glass, like soda-lime glass, will not withstand high temperatures or sudden changes in temperature, it exhibits excellent electrical insulating properties. Consequently, it is commonly used for electrical applications. It is also used for thermometer tubing and art glass.

Borosilicate glass

The addition of at least 5% of boric oxide to a silicate glass gives it high resistance to temperature change and chemical corrosion. Borosilicate glass is not as convenient to produce as either lime or lead glass, but is useful for pipelines, light bulbs, photochromic glasses, sealed-beam headlights, and vessels for laboratory or kitchen use.

Silica glass

Removal of almost all the non-silicate elements from borosilicate glass after normal melting and forming produces 96% silica glass. The resulting pores are sealed by reheating the glass to 1200° resulting in glass that is resistant to heat shock up to 900°C. 96% silica glass is used for the outer panes of the forward windshields of space shuttles to enable them to withstand the high temperatures reached during atmospheric re-entry.6

Aluminosilicate glass

Similar to borosilicate glass is aluminosilicate glass that includes aluminum oxide in its composition. Aluminosilicates are more difficult to manufacture than borosilicate glass, but have even greater chemical durability and can withstand higher operating temperatures. Aluminosilicate glass can also be used as a resistor in electronic circuits.3

Fused silica glass

Fused silica glass is the most difficult type of glass to produce, and so it is the most expensive of all glasses. Fused silica glass is pure silicon dioxide in the non-crystalline state and can withstand temperatures up to 1200°C for short periods. Fused silica is used to create astronomical telescopes, optical waveguides, and crucibles for growing crystals.6

Glass additives

Additives can be used to change the characteristics of glass. This may be done be for aesthetic purposes, for example, heavy metals, such as lead or manganese may be added to give the glass color. It may also be altered for functional purposes, for example, the addition of selenium makes the glass a light-sensitive conductor of electricity; a feature that forms the basis of photocopying.

Versatility of glass

Glass has an extensive range of potential forms and shapes. Its desirable properties can be manipulated during manufacturing, such as mechanical strength and chemical stability. This has led to the development of novel glass formats for use across an entirely new scope of applications.

Controlled-pore glass, which is porous glass with a sharply defined and adjustable pore size, can be used as a support for solid-phase oligonucleotide synthesis7 and as a stationary phase for a variety of chromatography techniques.8,9 Hollow glass biospheres have unique optical properties that have enabled the development of new research techniques, which hold huge potential for analytical devices of the future.10

Glass is also increasingly being adopted for a range of applications in medicine and dentistry. Bioactive glass is biocompatible and demonstrates antimicrobial activity. Furthermore, it can bond with both soft tissue and bone to promote healing. Bioactive glass has thus become an invaluable tool in tissue engineering and bone implants11 as well as in dental reconstruction procedures.12 It is also used in toothpaste and dental fillings to strengthen enamel and reduce bacterial colonisation.13

The future of glass

Glass has become the material of choice for solving a range of technological challenges. It is lightweight yet has the potential for strength, durability and optical clarity and its precise properties can be fine-tuned to meet a specific need. It can also be produced in a range of very different formats, including flat sheets, fine tubes, beads, and powder.

The versatility of glass has enabled incredible achievements, but the journey has by no means reached its end. With new production techniques and types of glass being continually developed, potential applications of glass products continue to expand and facilitate further remarkable advances. We are already benefitting from great interactive user experiences through the glass screens of mobile phones and tablets, but prototypes are now in development for touch-activated glass surfaces through which a range of digital devices can be accessed. Similarly, glass screens have been developed that provide a medium for virtual and augmented reality experiences.

Scientists continue to take advantage of the unique characteristics of glass, redefining what is possible. The latest projects include cleaning up nuclear waste by vitrification and using glass to develop safer batteries.

With a long and successful history, glass is still an active field of discovery and innovation with a future of exciting and ever-expanding capabilities.

Mo-Sci is a world leader in high-quality precision glass technology and produces a wide range of specialist glass products, the precise composition of which can be tailored to meet specific requirements.14

References

1. Rasmussen SC. Origins of Glass: Myth and Known History. In How Glass Changed the World. Springer 2012. Briefs in History of Chemistry, DOI: 10.1007/978-3-642-28183-9_2
2. Main D. Humankind’s Most Important Material. Object Lessons 2018. Available at https://www.theatlantic.com/technology/archive/2018/04/humankinds-most-important-material/557315/
3. What is Glass | Corning Museum of Glass. All about Glass. https://www.cmog.org/article/what-is-glass
4. Chemisty of Glass | Corning Museum of Glass. All about Glass. https://www.cmog.org/article/chemistry-glass
5. Types of Glass | Corning Museum of Glass. All about Glass.
   https://www.cmog.org/article/types-glass
6. Glass and The Space Orbiter | Corning Museum of Glass. All about Glass.
   https://www.cmog.org/article/glass-and-space-orbiter
7. Grajkowski A, et al. A High-Throughput Process for the Solid-Phase Purification of Synthetic DNA Sequences. Curr Protoc Nucleic Acid Chem. 2017 Jun 19;69:10.17.1-10.
8. Zucca P and Sanjust E. Inorganic Materials as Supports for Covalent Enzyme Immobilization: Methods and Mechanisms. Molecules 2014, 19, 14139—14194.
9. Igata Y, et al. A ‘catch and release’ strategy towards HPLC-free purification of synthetic oligonucleotides by a combination of the strain-promoted alkyne-azide cycloaddition and the photocleavage. Bioorg Med Chem. 2017 Nov 1;25(21):5962—5967.
10. Ward JM, Dhasmana N, Chormaic N. Hollow core, whispering gallery resonator sensors. The European Physical Journal Special Topics 2014;223(10):1917–1935.
11. Rahaman MN, et al. Bioactive glass in tissue engineering. Acta Biomaterialia 2011;7:2355—2373.
12. Sohrabi K, et al. An evaluation of bioactive glass in the treatment of periodontal defects: a meta-analysis of randomized controlled clinical trials. J Periodontol 2012; 83: 453—464.
13. Chatzistavrou X, et al. Fabrication and characterization of bioactive and antibacterial composites for dental applications. Acta Biomater. 2014;10:3723–3732. Available at https://www.ncbi.nlm.nih.gov/pubmed/24050766
14. Mo Sci Corporation website. http://www.mo-sci.com/en/products

Attending Craft Fairs is Important

Credit: Artefact Contemporary Craft Fair at Vessel Gallery 

It is easy to think up reasons for not attending craft events, e.g.:

·        Expense

·        Stocking costs

·        Accommodation and travel

·        Will I be able to sell to people?

·        Is my glass work good enough?

·        Which show is the right one; there are so many?

These are the thoughts we use when trying to avoid something.  There are a number of reasons to make the effort to show your glass at events.  These are some of them.

Feedback

Attending a craft fair enables you to get personal feedback on your work and your prices.  The questions people ask will show the kind of marketing you need to do. By talking to people, you can refine the idea of your ideal client and what they like about your glass.  The things about your work that are not clear indicate what you need to consider about your glass or presentation.

“It’s much better to get feedback from strangers who are interested in design or crafts, rather than asking for feedback from your mum or best friend!”  Patricia Van den Akker

Raise your Profile

Attending the events that fit your glass work can build your profile in a way that on-line cannot.  An event where entrants are selected boosts your confidence, but more importantly, boosts the confidence of the buyer.  You can use this to build your reputation by posting on social media, and even help to get local business awards.  But be sure the event is one which contains your peers, as a poorly chosen show will not improve your profile.

Maintain Contacts

Participation in fairs and other events is an easy way to maintain existing contacts.  It shows how your glass is developing and maintains your profile within the buying community.  It is also a good excuse to connect with the people on your mailing list, and through social media.  An invitation is a welcome reminder to people that you value the client enough to welcome them to your event.

New Clients

While at an event, whether a trade fair, or another kind of event, the people you meet will be those who are interested and willing to buy.  These are your new customers coming to your door.  Get them to sign up to your newsletter online with Mail Chimp or other reputable email provider to ensure you comply with the GDPR legislation on privacy.  Give them information about your social media and website too.

Events Can Help Online Sales 

When promoting your participation in an event, you need to feature your website also.  This enables those who cannot attend the event to look at the site and some will purchase.  Also, when you are following up with people afterwards some will make online purchases although they did not buy at the event. 

Physical Presence 

You stand is a tangible presentation of your glass or collection.  A well-presented stand can have much more impact than an online display.  It is an occasion where people can touch your work and get a tactile impression as well as physical one. This engagement with your glass makes it more likely people will buy from you now or in the future.

Selling creative work is a process and your individual and trade clients go through various steps of getting to know you, like you and trust you before they purchase…   Events play a key role in this buying and commissioning process.  Patricia van den Akker.

Deadlines

Signing up to an event means you have to have things ready. You can‘t put things off, because you have to be there and be prepared.  This makes you direct all your efforts to all of your business needs at once – marketing, promotion, updating the website, pricing, preparing your stand and its display of glass work, etc.  It brings to the fore all the things you may have put on the back burner.

Attending craft fairs and other selling events can bring many benefits in addition to sales – profile, getting feedback, re-connecting with previous customers, finding new customers, and simply putting everything you need to promote your business in a one-time limited occasion.

Vitrification: The Workhorse of Nuclear Waste Management

 

Vitrification: The Workhorse of Nuclear Waste Management

Posted Krista Grayson on Jun 18, 2019

Nuclear power has been used to produce electricity for over 60 years, and is responsible for around 16% of the electrical energy produced worldwide. This percentage is likely to increase, as countries move away from fossil fuels and require a source of energy to make up the shortfall this will cause. 

With more reactors coming online, more fuel will be used, and this presents a problem: there is no conclusive method to handle the spent fuel. Vitrification (the transformation into glass) of nuclear waste is one established solution, but there is still plenty of room for improvement in this method.

The issue of nuclear waste

A key problem with nuclear power is what to do with the waste products generated when the fuel is completely spent. Although it is no longer useful in power generation, this waste is still very dangerous if not stored correctly. 

The most problematic waste, called ‘high level’ nuclear waste (HLW), is highly radioactive, has an extremely long half-life and requires cooling and containment due to the elemental decay, which gives off heat and radiation. 

Additionally, some radioactive isotopes such as Tc-99, Se-79, and I-129 are mobile in water, which requires further measures in order to reduce their ability to move into the groundwater. Secondary waste streams can also present issues as this waste can contain large amounts of molybdenum and noble metals.

Safely storing nuclear waste through vitrification

One method of long term storage and disposal involves the processing and transformation of the spent fuel into a glass, a technique known as vitrification. It has been used for HLW immobilization for over 40 years in most countries that have a nuclear power program, including France, Germany, Belgium, Russia, UK, Japan, and the USA.

Glass is desirable as a long-term storage form as it is a relatively insoluble, compact and solid. In this form it is easier to store and handle, saving space and reducing cost. Glass also possesses high chemical durability, allowing it to remain in a corrosive environment for thousands or even millions of years without failing. While glass is often thought of as a fragile material, a properly treated block of borosilicate glass is incredibly resilient.

How vitrification works

The process of vitrification is quite simple but can be difficult to execute. First, the waste is dried, then heated to convert the nitrates to oxides. Glass-forming additives are added to the waste material and heated again to around 1000 °C. The molten liquid is poured into a suitable containment vessel to cool and form the glass. Once solidified, the final vitreous product has incorporated the waste contaminants in its macro- and micro-structure, and the hazardous waste constituents are immobilized.

The two main types of glass currently used to immobilize nuclear waste are borosilicate and aluminophosphate glasses. Both of these materials allow high waste loadings and can immobilize large amounts of actinides. Borosilicate glasses can accommodate up to 7.2 mass pct of PuO2 for example. 

Advantages and limitations of nuclear waste vitrification

Although vitrification is often the preferred method of waste storage, there are some drawbacks to the current techniques, both with the necessary setup and materials used. Vitrification has a high initial investment cost, high operational cost and complex technology requiring qualified personnel. 

This makes it most economically viable in locations where relatively large volumes of radioactive waste with stable composition are available, such as HLW from nuclear power plants. 

Unfortunately, the current generation of glasses cannot handle large amounts of MoO3 and noble metals that are produced from secondary waste streams. These compounds are poorly soluble in borosilicate glasses and this limits the amount of waste that can be loaded into the material, increasing process time and material costs.

New materials to process molybdenum-rich nuclear waste

It is clear that vitrification is of vital importance for the long term success of nuclear power. Mo-Sci has been working on new types of glass that can immobilize a higher percentage of waste, as well as methods within the processing that can speed up the vitrification.

This includes an iron phosphate waste form able to contain 40 wt% of the simulated molybdenum-rich nuclear waste. This vitrified nuclear waste is prepared by melting the mixture of simulated waste components and iron phosphate glass additives in a commercial-scale cold crucible induction melter (CCIM). When the chemical durability of the waste form was measured, it was found to be as good as or better than that of borosilicate glass.

The CCIM melting technology can also process waste forms faster, safer and at a lower cost than other melting technologies as it removes the metal electrodes that directly contact the molten glass and refractory used to contain the melt. This novel CCIM-processed iron phosphate waste form could drive large savings in time and money in the industry’s need to remediate nuclear waste, and make nuclear power an even more attractive option for the future. 

See our article on iron phosphate waste forms for more information on these new developments.

References

1. Thompson, L. (2010) Vitrification of Nuclear Waste http://large.stanford.edu/courses/2010/ph240/thompson2/
2. Criscenti, L. et al. (2013). An international initiative on long-term behavior of high-level nuclear waste glass. Materials Today, 16 https://doi.org/10.1016/j.mattod.2013.06.008
3. Ojovan, M. I., & Lee, W. E. (2011). Glassy Wasteforms for Nuclear Waste Immobilization, 42 https://doi.org/10.1007/s11661-010-0525-7 
4. Ojovan, M. I. (2007). Glasses for Nuclear Waste Immobilization, WM’07 Conference. https://www.researchgate.net/publication/267700284_Glasses_for_Nuclear_Waste_Immobilization/download 
5. Cheol-Woon, K. (2018) Iron Phosphate Waste Forms for Nuclear Waste Disposal https://mo-sci.com/iron-phosphate-nuclear-waste-disposal

Selling online

Credit: 48HoursLogo.com

Once you have achieved a lot of visits to your website, you need to convert the visits to sales. 

The first thing you need to do is consider your products.  Craft products are more difficult to sell online than mass manufactured items that are completely standard and so have known quality.

Is your product suitable for online selling?

·        The general case is that lower priced gift items are easier to sell than expensive ones. 

·        Is delivery expensive relative to the cost of the item? 

·        Do the items have to be sized, e.g., rings?

·        Are the items easy to post safely without breakage?

·        How much packaging will be necessary?

If the answers are that what you sell is expensive to buy or deliver, must made to a size, or are difficult to post, you may have difficulties in generating sales.  If you have items that are likely to sell less well online, consider the other ways you can sell them – trade events, galleries, shops, wholesale.  Also think about making items that are easier to sell online, but still fall within your style.  This approach will help support your more difficult to deliver or more expensive items that won’t sell well online.

Then

Online selling techniques are not so different from in-person selling, except that you have to rely on text and images to do the selling.  This puts more emphasis on words and images and getting your personality into those two things.

The basics are:

Get the viewer’s attention

Stimulate their interest

Develop their desire for the object

Convert these elements into the purchase.

There are many things that can create these three pre-requisites for a purchase.  

Images

The quality of images is extremely important.  Photographs must be sharp, focused, and with lots of light.  They must be taken to show the quality of craftsmanship.  Multiple pictures of the item help to give a better feeling of the object.  They should be taken from various angles, including the unseen backs of items to show the craftsmanship and help promote the assurance of quality. Lifestyle images bring items to life, but have to be carefully arranged.  This is often done best by a professional photographer.

Get and maintain interest

What you do must be apparent immediately.  Do you have recognisable work or style? Is your business name memorable?  An explanation of what you do and why it is unique is important to maintain interest.  Links from these explanations to relevant individual items or product groups are appropriate to keeping people engaged.

It is important to maintain interest after the initial contact.  Make it easy to find other relevant items. Use links, buttons, suggestions, etc., liberally.

Keep the site alive with case studies. These can be the background to your workday, or events in your business life.   Inside views of the development of new lines shows how you progress from idea to finished work.  You have interesting ways of working, that many people are interested in knowing about. Show your working practices, tell them the story of making.

You need to keep in touch with potential and existing clients.  Direct posts to those you have contact details for, with information on developments keeps you and what you do in customers’ minds.  These must not be direct sales pitches.  You can ask questions of these people to keep them engaged. They may also tell others about you and your work.  General posts to targeted audiences can help spread the word too.  Some paid promotion on social media can help, if targeted to the right people.

Provide information

Explain the potential questions about each item that client may have.  Think about the kinds of questions you ask about non-glass craft products. Use those approaches in stating and answering these questions.

Make the explanations personal and consistent with your site and the glass products you are offering.  In many cases, it is desirable to establish a FAQ section, including terms and conditions.  This can help maintain confidence of the buyer in your ability to make and supply the work.

Purchase

What’s for sale

You need to overcome any difficulties that the client might face in coming to the buying decision.  The website should be immediately clear about what you do and for whom.  Price levels need to be clear, possibly by grouping or sorting. Images need to connect with client desires.  This is where lifestyle images are useful.  Do remember that first impressions are all important.

Develop trust

Development of, or appearance of trustworthiness is essential to buying.  People buy from those they know, like and trust.  Development of this is essential for consistent online sales, as well as anywhere else.  This can be promoted by your presence on a group of platforms that you link between. Good descriptions of products and about yourself are important to maintaining the trust of the client.  Testimonials, if you have them, are useful. 

The website must appear professional.  Knowledge of your location is important to developing confidence in your work. Knowing where else your work is available is also important in validating confidence in your business.  Knowledge of where else your work can be purchased gives creditability to your standing within the craft  buying community. This can include your attendance at craft and trade fairs, as well as any awards or press mentions.

Buying and delivery

Make it easy to purchase.  One-click links can help ease the customer into buying. Use of a known payment provider increases confidence that the purchased item will be delivered and that there is a mechanism to get money back if not.  Make sure you explain about postage and packaging, unless you have included it in the in price.  If P&P is included, make sure that you are clear in the text accompanying the image and description.  If you don’t do that, the price comparisons with those that don’t include P&P are skewed against you.  Include plain English terms and conditions, to engender trust if something were to go wrong.

But

Don’t rely exclusively on online sales. There is enormous competition online, even though it is a means to get your work known to a wide range of people Importantly, it is a way to get year-round sales rather than the summer and autumn craft circuit.  Other sources for consistent sales - without you being present all the time - are galleries, shops and wholesalers.

Think about combining online sales with craft fairs and other selling events.  These face to face events give you the opportunity of getting direct feedback on your work, which will help develop what you do.  Promote your attendance at events on the website and tell about your website at events.  Blog about the events before and after their occurrence on your website and social media.  Tell stories from the events on your social media and in the website, too.

Selling online requires getting attention, stimulating interest and promoting desire to buy.  Some of the things you can do are noted.  But do not put all your effort into online.  You can gain a lot of information by attending face to face selling events.

Are Electric Furnaces the Future of Glass Manufacturing?

 

Are Electric Furnaces the Future of Glass Manufacturing?

Posted Krista Grayson on May 16, 2019

Overview of glass production 

Glass production is typically energy-intensive. Glass furnaces may reach 1300-1550 ºC for the melting and refinement of the raw materials, depending on the formulation required. 

Natural gas and electricity are the main energy sources, however historically, the glass industry has favored gas because it is an established technology, with low price, high purity, ease of control and the fact that there is no requirement for storage facilities. Gas furnaces have long life-times, on average over 12 years and sometimes up to 20 years.

Until recently electric glass melting furnaces have been used for specialty glasses, and particularly glasses with significant volatile constituents such as fluoride opal glasses, borosilicates and lead crystal. Interest is growing in extending its use through the industry.

Electric glass furnace production

The most effective method of electric glass production is to use electrodes immersed in the glass either as electric boosting (providing 5-20 % of total energy input) or all-electric melting. The immersed electrodes are connected to a power supply and transformer, to pass an electric current through the glass. 

In all-electric furnaces, the melting energy comes from the electrodes (joule heat), with a gas burner being used for the initial start-up, or as an emergency heat source. These furnaces mainly operate ‘cold top’, where the raw material is distributed evenly over the melting surface of the glass, forming an insulating ‘batch blanket’. Melting and refining take place in one vertical process, with glass being drawn through a throat at the bottom of a deep melting tank.

Advantages of electric melting

Electric furnaces offer several advantages over gas furnaces. For example, they have very low direct emissions of CO2, thermal NOx or SOx emissions. With pressure to reduce emissions coming from both customers and legislation, this is a significant benefit. While it is possible to improve conventional gas furnaces to reduce emissions, this can result in more complex technology that results in additional maintenance, the use of non-environmentally friendly chemicals, and limitations to equipment lifespan. 

Another benefit is that heat losses from electric furnaces are much lower. The thermal efficiency of gas furnaces peaks at around 45%. This means more energy is lost as heat than is used to convert the raw materials to molten glass. Heat losses occur from the superstructure of the furnace and in the residual waste gases, even if heat recovery systems are used. In contrast, the electrical approach means that the melting energy is transferred directly into the glass. Thermal efficiency can be over 70% even in a small electric furnace and can reach 85% in a large electric furnace.

All-electric furnaces are also more energy efficient than gas-fired furnaces; they use around 35% less energy. The difference in energy efficiency is particularly important for small furnaces. As furnace size decreases, the energy efficiency of electric furnaces remains very high, whereas the efficiency of gas furnaces drops dramatically and can be less than 20%. 

Electric boosting can be a highly effective way to reduce overall energy consumption. It also means that energy release can be highly focused, helping to determine conditions in the glass bath. In some cases, a well-designed boost system can improve glass quality homogeneity, seed and stone losses. In contrast, in gas furnaces, where focused energy release is not possible, imprecise temperature profiles can be created in the glass. 

A key advantage of the cold-top electric furnace is that everything that goes into the batch stays in the glass, aside from the gases released from the melting process, which permeate out through the batch blanket. Losses of batch constituents such as fluorine, boron, lead, various volatile refining agents and other constituents are almost eliminated. 

Disadvantages of all-electric melting

While electric furnaces have lower capital costs, they have shorter life-times (2-7 years compared to 10-20 years for conventional furnaces) and higher energy costs. The economic viability of electric furnaces is closely related to the cost of electricity compared with gas. Higher thermal and energy efficiencies can offset this cost for smaller furnaces, but this might not be the case for larger furnaces. 

The low environmental impact is only maintained if the furnace can receive power from renewable energy sources and requires a power grid that is reliable and stable. 

There are also operational considerations. For example, the maintenance of electrodes to limit higher resistance caused by wear. It is not possible to melt higher temperature glasses (>1500C) and there is concern of corrosion/erosion of electrode material from certain glass compositions. Further, recycled glass may be an issue that requires new handling methods. 

Conclusion

In most places, it is still environmentally cleaner to burn fossil fuels in a furnace than to use them to generate electricity for electric melting. However, as renewables increase their contribution to electricity production, this situation will change. It also appears that improvements in energy efficiency of fossil fuel combustion technologies have leveled off. As emissions legislation kicks in and consumers increasingly demand materials and technologies that are environmentally friendly, there may be well a swing in glass manufacture from gas to electric energy. The other advantages of electric melting, such as better thermal efficiency and energy consumption, will also count in its favor.

References

1. https://www.eurotherm.com/efficient-future-for-the-glass-industry-is-all-electric 
2. https://www.glassmanevents.com/content-images/speakers/Andy-Reynolds-Fives.pdf 
3. http://www.electroglass.co.uk/articles/2010-09%20Electric%20Melting%20&%20Boosting%20for%20Glass%20Quality%20Improvement.pdf
4. http://ietd.iipnetwork.org/content/electric-melting

Metal inclusions

Two difficulties with metal inclusions in glass are common: stress and bubbles.

Stress

Metal inclusions always create stress in the glass. Different metals have different expansions and different strengths.  They also have different melting points - some so low that they liquify during the fusing process.

The trick in using metals as inclusions is to minimise the amount of stress. Small amounts of stress can be contained within the glass. The thicker or more mass inside the glass, the greater risk of stress breaks. The stronger or more rigid the metal is, the more stress will be generated.

Minimising stress is most easily achieved by using small amounts of the metal.  Thinning the metal as much as possible also reduces stress.  Flattening wire also helps reduce the amount of stress as well as keeping it in the place you want it without rolling away from its placement.

Bubbles

Bubbles often form around inclusions, especially of metals.  Metals that do not melt at fusing temperatures are stiffer than the surrounding glass.  You can see from the table noted above those metals which melt at higher temperatures than fusing.  These metals will create bubbles around their perimeter and elsewhere over the metal wherever there are wrinkles or undulations as the metal holds air in those places.

Thin metals

One possibility to reduce the bubbles is to thin the metal by hammering flat or use foil thicknesses of the metal.  Many specialist metal suppliers have very thin metals, often called shims.  They are increasingly available in online shops.

Weight

Another is to use enough glass on top to flatten the metal.  You should flatten the metal in the cold state as much as you can.  Then the weight of the glass presses down on the metal both in the cold and heated states. With a good long bubble squeeze, you can force more air out to the sides than with less covering glass.

Placing

A third possibility is placement. The further the metal inclusion is from the edges the more air is likely to be trapped to form bubbles.  If the air has less distance to travel, more is likely to escape.

Pressing

Supporting the edges or corners allows the centre to drop before the edges are sealed.  The weight of glass helps to press the air out to the sides.  Thicker glass (6mm/0.25") on top of the metal inclusion can help push the air away from the metal. You can also provide - within the design - paths for the air to escape. This can be elements such as powder, stringers and other glass accessories that can hold the glass up during the bubble squeeze process, but become invisible at fusing temperatures.

Fire in stages

A fifth possibility is to fire differently.  You can place the metal on a kiln shelf which is covered with fibre paper and put the glass on top of the metal and fire to a rounded tack fuse at the minimum.  To avoid dog-boning, you should cut the capping piece several centimetres larger than the final piece, so you can cut off the distorted edges. Clean the bottom and dry very well after firing and put the base under the top piece that has the metal attached.  Fire the combined piece slowly with a good bubble squeeze.  This can be applied to included vegetable matter too. 

Further information is available in the ebook Low Temperature Kiln Forming.

Inclusions often produce stress and bubbles.  There are some things that can reduce both when encasing metals or vegetation.