Hazards of Flux Fumes

Note:  These health risks are those associated with industrial exposure – frequent and for extended periods.  They do not apply directly to occasional and shorter periods of exposure.

Risks are assessed as acute and chronic.  Acute means immediate reaction.  Chronic means the effects are cumulative and may take years to appear.
 

Composition of Flux

The major components of commercial flux are varying combinations and proportions of zinc chloride (or ammonium chloride), hydrochloric acid, phosphoric acid, citric acid, and hydrobromic acid.  It comes in many forms and many brand names.  It is important to use water soluble flux in stained glass work to enable thorough cleaning.
 
 

 

Zinc Chloride Risks

Zinc chloride inhalation from smoke screen generators or smoke bombs may cause transient cough, sore throat, hoarseness, a metallic taste, and chest pain.  Exposure to high zinc chloride concentrations produces a chemical pneumonitis with marked dyspnoea, a productive cough, fever, chest pain and cyanosis. Pneumothorax and the adult respiratory distress syndrome (ARDS) have been reported. Fatalities have occurred….
http://www.inchem.org/documents/ukpids/ukpids/ukpid86.htm#:~:text=Toxicity%20Zinc%20chloride%20is%20corrosive,anorexia%2C%20fatigue%20and%20weight%20loss.
 

Ammonium Chloride Risks

Exposure to Ammonium Chloride is moderately hazardous, causing irritation, shortness of breath, cough, nausea, and headache. Most exposure is a result of contact with the fume form of this chemical (Ammonium Muriate Fume and Sal Ammoniac Fume), which is a finely divided particulate dispersed in the air. The fumes are capable of causing severe eye irritation. Consistent exposure can cause an asthma-like allergy or affect kidney function.
 
In the event of accidental contact, get immediate medical attention and follow these first aid measures:
·        Skin Contact: Immediately flush skin with water and disinfectant soap and use an emollient on irritated area.
·        Eye Contact: Rinse eye(s) with water for at least 15-20 minutes. Protect unexposed eye.
·        Ingestion: Rinse mouth thoroughly with water. Do NOT induce vomiting.
·        Inhalation: Move to fresh air and administer artificial respiration if needed.
https://www.msdsonline.com/2017/05/05/chemical-spotlight-ammonium-chloride/#:~:text=Exposure%20to%20Ammonium%20Chloride%20is,particulate%20dispersed%20in%20the%20air.
 
 

Hydrochloric Acid Risks

Hydrochloric acid is corrosive to the eyes, skin, and mucous membranes.  Acute (short-term) inhalation exposure may cause eye, nose, and respiratory tract irritation and inflammation and pulmonary edema in humans.  Acute oral exposure may cause corrosion of the mucous membranes, oesophagus, and stomach and dermal contact may produce severe burns, ulceration, and scarring in humans.
 

Acute Effects: 

Hydrochloric acid is corrosive to the eyes, skin, and mucous membranes.  Acute inhalation exposure may cause coughing, hoarseness, inflammation and ulceration of the respiratory tract, chest pain, and pulmonary edema in humans.  Acute oral exposure may cause corrosion of the mucous membranes, oesophagus, and stomach, with nausea, vomiting, and diarrhoea reported in humans.  [Skin] contact may produce severe burns, ulceration, and scarring…. Acute animal tests in rats, mice, and rabbits, have demonstrated hydrochloric acid to have moderate to high acute toxicity from inhalation and moderate acute toxicity from oral exposure.

 

Chronic Effects: 

(Non cancer): Chronic occupational exposure to hydrochloric acid has been reported to cause gastritis, chronic bronchitis, dermatitis, and photosensitization in workers.  Prolonged exposure to low concentrations may also cause dental discoloration and erosion.  Chronic inhalation exposure caused hyperplasia of the nasal mucosa, larynx, and trachea and lesions in the nasal cavity in rats.  The Reference Concentration (RfC) for hydrochloric acid is 0.02 milligrams per cubic meter (mg/m 3) … The RfC is an estimate … of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risk of deleterious noncancer effects during a lifetime.  It is not a direct estimator of risk but rather a reference point to gauge the potential effects.  At exposures increasingly greater than the RfC, the potential for adverse health effects increases.  Lifetime exposure above the RfC does not imply that an adverse health effect would necessarily occur.
https://www.epa.gov/sites/production/files/2016-09/documents/hydrochloric-acid.pdf
 

 
Phosphoric Acid Risks

Phosphoric acid can be very hazardous in the case of skin contact, eye contact, and ingestion. It can also cause irritation if vapours are inhaled. This chemical can cause damage to the skin, eyes, mouth, and respiratory tract. Because of the potential hazards posed by this chemical, it is important to use care when handling it.
 
Repeated or prolonged exposure to phosphoric acid mist can lead to chronic eye irritation, severe skin irritation, or prolonged respiratory tract issues.  In case of accidental exposure to phosphoric acid, follow these first aid guidelines:

Inhalation — Seek fresh air and immediate medical attention.

Eye Contact — Remove contact lenses if present. Immediately flush eyes with plenty of water for at least 15 minutes and get medical attention.

Skin Contact — Wash skin with soap and water. Cover any irritated skin with an emollient. Seek medical attention. 

Ingestion — Do NOT induce vomiting. Never give anything by mouth to an unconscious person. Seek medical attention if any adverse health symptoms occur.
https://www.msdsonline.com/2015/06/17/phosphoric-acid-safety-tips/
 
  

Citric Acid

Citric acid can be a minor skin irritant, causing itchy skin and even minor burns to those that are sensitive to it. Hands should be washed immediately if citric acid comes into contact with bare skin. Protective gloves should be worn during handling to avoid any accidental contact. The acid can also irritate the walls of the throat if ingested or burn the lining of your stomach if ingested in large quantities.
 
Eye Irritation - Citric acid is a severe eye irritant. Accidental contact with the eyes can occur … by touching the eyes after the acid has contacted the fingertips. …  Protective eyewear should be worn when working with citric acid under laboratory conditions. Eyes should be flushed with water immediately if they happen to come in contact with the acid.
https://sciencing.com/hazards-citric-acid-8165149.html

Remember that this irritation is equivalent to squirting lemon juice into your eye.  It is not a chronic risk.
 

Hydrobromic Acid (HBr)

Hydrobromic acid and hydrogen bromide gas are highly corrosive substances that can cause severe burns upon contact with all body tissues. The aqueous acid and gas are strong eye irritants and [tear producers]. Contact of concentrated hydrobromic acid or concentrated HBr vapor with the eyes may cause severe injury, resulting in permanent impairment of vision and possible blindness. Skin contact with the acid or HBr gas can produce severe burns. Ingestion can lead to severe burns of the mouth, throat, and gastrointestinal system and can be fatal. Inhalation of HBr gas can cause extreme irritation and injury to the upper respiratory tract and lungs, and exposure to high concentrations may cause death. … Hydrogen bromide has not been found to be carcinogenic or to show reproductive or developmental toxicity in humans.
https://web.stanford.edu/dept/EHS/cgi-bin/lcst/lcss/lcss47.html#:~:text=The%20aqueous%20acid%20and%20gas,gas%20can%20produce%20severe%20burns.
 
 
 

Precautions to be taken by glass workers

The risks outlined above are related to dealing with concentrated amounts of the materials in industrial settings.  Risk levels are much reduced in the craft setting.  The risks are mainly centred on breathing and eye exposure. 
 
It is important to wear masks of the quality that will deal with inorganic fumes.  In Europe these are designated as FFP2.  In general masks rated at N95, P95, or R95 are the level required for filtering out 95% of particles that are larger than 3microns.  Dust masks are not sufficient protection. 
 

Usually overlooked is eye protection.  The risks outlined here show that risks to eyes are equal to - or in some cases greater than – respiratory ones.  Eye protection is as important as breathing filters.  To fully protect the eyes, goggles of some sort are the minimum requirement.  Glasses will not be sufficient to prevent fumes reaching eyes.

 
For a “one stop solution” a full-face mask may be the simplest solution.  The filters on these are long lasting and replaceable.  They can be put on as one unit and are available in various face sizes.
 

At soldering temperatures, there are no lead or tin fumes created.  It is the fumes from the flux that are the risks in soldering.  These risks are small and can be dealt with by using adequate ventilation, masks, and goggles.

Choosing a Name for Your Business

Naming your business requires some thought.  Many people give their activity a name before it really is a business, often while it is a hobby.  At the point of deciding on creating a business, is the time to fully consider the name. It is important to choose the right name for your business. It can create a good first impression for customers and provide a great foundation for building your brand.  You can be creative when choosing the name. However, it needs to be appropriate and work on all types of marketing material, from signage to stationery. There are structured ways of choosing a name.

Personal Name

Many creative sole traders use their own name as their business name. It is easy and often creates the personal connection sought for by buyers.  Using your own name gives your business a more arts oriented and personal touch. Sometimes people are already ‘known’ under their own name prior to beginning the formal business, e.g., Amanda J Simmons.
 

Initials

Some designers use their initials or attach their discipline to their name or initials to add some clarity to what they do, e.g., MJM Ceramics. Others prefer a business name, so that it is less personal and not based around one individual. This can appear as more professional and allows the business to grow beyond the individual.
 
Some people use both - their surname for artistic one-off work, and a business name or brand name if they want to have a more general approach for multiples. You can be self-employed (e.g., using your own name) and run a limited company (with a business name) at the same time.
 
Even though associating your name in these two ways are attractive, more consideration is needed to ensure you are really on the right track.  Naming your business, or even a new collection, can be difficult. There are some practical tips to help you on your way:

 

Reflect on your vision, values, and passions

You have already pulled out these things in looking at your life goals and vision. Now you need to apply them to a name search.  A method that may help is to write down ten important descriptive words about these values, passions, and your vision. This is a starting point for thinking about the best words for the business.  These words will probably not come immediately.  You might want to involve others in a kind of brainstorming to develop a group of key words.  You need to consider how this name will look in the future as your business develops.

 

cREDIT: BUSINESS NAMES

The name needs to reflect you and your market.

“A good business name reflects who you are and which ideal clients you want to attract.”  It is the introduction to the business. It is the first thing they see, even before you. So, what you do and who you do it for is important to selecting the name. The name creates an image in customers’ heads.  Do you want to be factual or do you want to create a bit of mystery?  To help with this selection you need to remember your niche market and what you do and for whom as defined earlier. Review the business specialism - what you want to be known for - and your ideal clients
 

Inspiration sources

You do not have to be factual in the name and you can get inspiration from a number of sources.  Favourite songs, places, people, films, etc., can all be sources.  Brainstorm with others for names around your values, vision, passions, your specialism, your ideal clients.  You can also get ideas from the business names your competitors or role models use.  You need to record these ideas. This can be written, but an image or sketch can be even more useful. A mood board or mind map can be helpful too. A logo idea often comes with considering a name.  You should strive to have about five good, creative names for your business to make a choice. Not all the names will be available.  There are also online business name generators that can help focus your ideas.

 

Credit: Fiverr

Check the potential names

You need to make sure of several things before you settle on the name.
 
Is the name easy to spell? 
This is important in online searches as they are the most commonly used method of finding businesses. You need to avoid quirky expressions, and names with common variations (e.g., is it Mc, Mac or M’)
 
Is it easy to say? 
Almost as important as spelling is being able to pronounce the name in different dialects or languages. This is a worthwhile consideration as exemplified in car model names.
 
Is the name easy to remember? 
This goes together with the verbalisation of the name.  If it is easy to say, it probably is easy to remember and so be searched for without difficulty. 
 
What is it in other languages?
Try to ensure your name does not mean something offensive in another language.  Even if you are not operating internationally (just now), your name will be visible throughout the world.
 
Future proof your name
This is harder than it sounds, but you want the business name to be acceptable in a generation’s time.  This means that you avoid names that are “now”.
 
Check your name for legality 
Companies House in the UK gives a list of sensitive and prohibited words on its website.  This is most useful even if you do not intend to register with them. It helps you avoid current and future difficulties in a business name. There are equivalents in other countries.  Is the name the same - or nearly so - as a registered trademark? There is more information about registered trademarks on the Intellectual Property Office website (a UK resource).
 
Check to see if the name is already in use
You can use google to check on the existence of the name as a business one.  The Companies House website also has a free facility to check on name conflicts of registered companies. Check to see if your proposed business name is available as a web address. Also check on Facebook, Twitter, and other social media sites.  It is best to have the same name and address across all the web and social media sites.
 

You may, of course, revert to some form of your name for the business name, but running through these considerations can make sure you have considered all the options for an appealing and flexible business name

Accuracy in Following the Cartoon

A question arose on Facebook that resolved itself around the need (or not) to accurately follow the cartoon. The question itself was whether a pictured piece should be re-cut. It was a relatively complicated piece, which in its current state left two significant gaps between the glass and the cartoon line.

The vast majority of responses, was along the lines of don’t worry, just adjust the neighbouring pieces to fill the gaps. (“enjoy yourself” was another theme).

To follow the majority view - make adjustments in other pieces to allow the problem piece to fit - encourages bodge. It does not encourage development in the skills of the craft. It does not encourage long term enjoyment. It pushes improvement well into the future.

The cartoon is the design in line form. Its purpose is to control the construction of the piece. It is important to follow the cartoon to express the design. This means striving to cut and fit the glass to the cartoon as accurately as possible. Line is important in stained glass, as everyone knows. If the glass is not fitted accurately, the lines will not flow according the cartoon and the design.

This is not to say the cartoon cannot be modified. It is important to look at the cartoon in detail once you have been attracted by a design. You need to look at each piece and determine its difficulty. Can I cut it? Is the design overly complicated? How might I modify the cartoon to make easier to cut pieces, and still maintain the overall design? Ensure the cartoon is drawn with accurately thin lines for the form of the craft – copper foil or leaded glass, and fused glass.  Revise your cartoon to respond to the answers to this review of it.

Whatever method you use to score and break your glass, fit the piece to the cartoon as accurately as possible to avoid excess work later. The grinder is there to make fine adjustments to the cut piece so it will fit the cartoon.  If the glass fits snugly within the cartoon lines, there should be little, or no, alteration required to the cartoon during the rest of the construction.

This is not about the methods you use during the construction of a piece.  That is open to many approaches to achieve the same end. It is about an approach to the craft.  In particular, it is about how the cartoon is used to guide the work.  If the detail of the cartoon is optional, the final result may vary from the original conception significantly.

Craft development is about striving to perform the actions as accurately as possible throughout the work. The increased skill level that this develops, provides long-term enjoyment.  And each completed piece will give a feeling of accomplishment.

 

Hake brushes

Hake (ha-kay) brushes are made from goat's hair. Their advantage over other brushes for applying kiln wash is that they hold a lot of liquid. Proper ones made from joined bamboo work better than the ones with flat handles.

Traditional Japanese hake brush

People often note that these brushes tend to shed hairs. The solution to stray hairs (given to me in a Bullseye workshop) is to invert the new brush and apply super glue at the point where the hairs emerge from the handle.  This holds the hairs in place. It will work on flat handles too.

Inexpensive goat's hair brushes of the hake style.
As can be seen by comparison, there are fewer hairs in these.

Kiln wash application with a brush

Kiln wash is applied thinly in a 1:5 powder to water mix to shelves and moulds.  The object is to get a complete coverage with a smooth surface. 

To ensure full coverage painting four coats is sufficient for excellent coverage.  The kiln was should be applied in four directions – horizontal, vertical, and each diagonal.  This ensures any gaps in one coat will be covered by the others. A broad brush that holds a lot of liquid provides good coverage.  A hake brush is ideal.  The brush should be held almost vertical with the ends of the bristles only touching the surface.

A traditional Japanese hake brush

There is no need to dry each coat before applying the next. It is not like painting your wall. All coats can be applied one directly after the other. No drying between coats is required.  In fact, earlier dried coats tend to make the application clumpy and streaky.

Some people advocate a fifth coat.  I don’t know what the fifth coat is for. What direction other than the four cardinal ones can there be?  It maybe it is insurance that the surface is coated evenly.  This can be checked visually.  The kiln washes used for glass are routinely coloured.  If the shelf shows unevenly through the kiln wash, a little more needs to be brushed onto the more thinly coated area.

It is possible to smooth the kiln washed surface once the kiln wash has a dusty surface – it does not have to be completely dry – you can put a piece of paper between the shelf or mould and your hand.  Gently rub the surface to get a really smooth finish to your kiln washed shelf.

 

More information:

https://glasstips.blogspot.com/2009/08/applying-kiln-wash.html

https://glasstips.blogspot.com/2009/08/smooth-kiln-wash-on-shelves.html

Vitrigraph Pots from Refractories

Many people are now buying or having made stainless steel square pots for doing stringer and murinni pulls.  This may be the best material for the purpose, but square pots can be made from other refractory materials.

Vermiculite

One material that can be used multiple times is 25mm vermuculite board.  These can be cut to a convenient size and fastened together with stainless steel or brass screws.

If you want to make a pot 100mm square and 100mm high, cut four boards 125mm square, and one 100mm square for the bottom.  This last one will need to have a hole of the desired size drilled at its centre.  A 19mm diameter hole is a medium sized hole.  Keep in mind that you cannot make it smaller, but you can make it bigger. 

Place the four larger squares around the base.  Drill pilot holes for the screws (if you don’t you will split the boards).  Two or three holes along each edge should be as much as needed for long term security. 

The screws at the left side of the box are omitted in the drawing, but are required

Alternatively, you can make the base to fit onto the bottom rather than inside.  In this case, cut the base to 150mm square and fix it to the sides with stainless steel screws from the bottom.

The screws at the left side of the box are omitted in the drawing, but are required

A disadvantage of the vermiculite is that glass sticks to it.  You can overcome this by lining the bottom and inside of the square with 2mm or 3mm fibre paper.  You cannot hammer out the residual glass without destroying the whole box. This lining will protect the surfaces, and the fibre can be removed after a firing, leaving clean sides.  This will not be as long lasting as the stainless steel pot is, but it is economical and adaptable to your specific requirements.

Refractory fibre

Another refractory material that can be used to construct square pots is 25mm refractory fibre board.  The sizes of the components are the same as for vermiculite.  This time rather than screws, you need stainless steel pins – 50mm long sewing pins are suitable.  You can also use high temperature wire, about 50mm long with a small right-angle hook at one end.

Assemble the four sides around the bottom as previously, and push the pins into the board to secure them.  You will need more pins than you did screws for the vermiculite.

As an additional securing measure, wrap the box horizontally with two bands of 0.5mm kanthal or high temperature wire and twist the ends together.  Then on two sides wrap more of the high temperature wire under the bottom and twist the ends together on the top of the two sides.  These wires do not have to be really tight.  They are there to prevent any failure of the pins.

This refractory fibre box is light weight although it will not last as long as the vermiculite one, but it is quick and easy to put together with a minimum of tools – knife, wire cutter, straight edge.  You can line this with fibre paper as for the vermiculite.

Safety note:

When working with vermiculite or refractory fibre, you should wear breathing protection and dust your clothing outside or change after the box is complete and put those clothes in the washing machine.

You can make a vitrigraph box from refractory materials rather than buying a stainless steel one.  Information on making one is given.

Softening the Tack Profile

Often people want a particular profile not provided by the schedules in the controller or the ones they normally use for tack fusing. The question arises as to whether to increase the temperature or extend the soak on a previously fired piece.

You can do either.

You can extend the time or increase the temperature. There are benefits and drawbacks with each.

Increasing the temperature is the choice for a quicker firing. But you have less control.  By increasing the temperature, you will certainly get a softer edge to the glass. You do not know until the firing is finished how much the glass has changed.

Extending the time means that you know a softer profile will be created simply by more heat work being put into the glass.  If you combine the extended soak time with peeking at intervals, you have much more control over the exact profile achieved.  Observation at 5- or 10-minute intervals after the target temperature is achieved, will enable you to get exactly the profile you want. Just advance to the next segment when that profile is achieved. 

The drawback is that the firing takes a little longer and you have to be present at the time the working temperature is reached.  You can schedule that by using the delay feature on your controller.

Note that on any re-firing of a piece you need to be aware that you are firing a single thicker piece rather than the original multiple layers.  This will require a more cautious rate of advance up to the softening point of the glass – generally around 540°C.  After that, the original rate(s), soaks and annealing can be used.

Of course, the considerations of temperature versus time can be applied to an initial firing as much as to a re-friing of a piece.

Observation is the best way to have precise control over the profile of your tack fusing.

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

Phase Separation and Crystallization in Glass

 

From the Mo-Sci Corporation Blog:

Posted by Krista Grayson 

While historically a source of problems for glass producers, the phenomenon of phase separation is now known to offer advantages in the production of certain materials such as glass ceramics and porous glasses. Whether desirable or undesirable, understanding and controlling phase separation during the glass manufacturing process is crucial. In this article, we explore the basics of phase separation and how it can be manipulated to create advanced materials for various applications.

What is phase separation?

In physics and chemistry, the word “phase” refers to a region of a material that is chemically uniform and physically distinct. Phase separation, which typically occurs in liquids, is where a homogeneous mixture separates into two or more of these phases. For example, a mixture of water and oil at room temperature will naturally “phase separate” into a distinct phase consisting of pure oil, and another consisting of pure water. We can say that such a mixture is “immiscible.”

The morphology of this phase separation can vary depending on the relative concentration of both components. If the mixture is predominantly water, the oil phase will take the form of distinct (or “discontinuous”) droplets dispersed throughout an interconnected (or “continuous”) water phase. If the mixture is predominantly oil, the opposite will take place. At roughly equal proportions of oil and water, each phase will tend to be continuous.

Phase Separation in Glass

Phase separation commonly occurs in glass melts. Borosilicate glass – which contains both silica and borate as network formers – is a well-studied example.1,2

Unlike our water/oil example, phases in glass melts are not necessarily chemically pure. Borosilicate glass, for example, will typically undergo phase separation into a “borate-rich” phase and a “silica-rich” phase, with both phases containing different proportions of each network former. In addition, the morphology of separated phases in glass can vary. While it is possible for droplet-like phases to form via classical nucleation and growth, spontaneous “spinodal” phase separation can result in the formation of intertwined tendril-like continuous phases.3

On the left, spinodal decomposition produces “tendrils” of different phases. On the right, nucleation produces droplets of the darker phase within the lighter phase. (Gebauer et al., 2014)4

This phase separation, which occurs at high temperatures in the molten glass, persists and “freezes in” when the glass is cooled into a solid. If both phases are vitrifiable, they may form glasses after cooling (this is called a glass-glass phase separation). However, if one phase is prone to crystallization, the mixture can cool into a glass-crystal phase-separated solid.5

Phase separation in glasses was long seen as undesirable – and for many applications, it still is.6 The existence of different phases modifies the physico-chemical properties of glass melts, making it difficult to mold and reduce the quality of the final glass.

The physics of phase separation in glass-forming materials is complex, and even today the specifics are subject to intense debate.7 However, glass manufacturers nonetheless determined ways of avoiding or minimizing phase separation during glass manufacturing.

Typically, this is achieved by tailoring the composition of glass melts, with phase separation only occurring for specific compositions. In a Na2O–B2O3–SiO2 glass system, for example, the following ternary phase diagram shows the immiscibility region in which phase separation will occur.

Simplified ternary phase diagram for the Na2O–B2O3–SiO2 system. (Bartl et al., 2001)8

Phase separation (and subsequent crystallization) can also be controlled by the addition of glass modifiers, and by varying heat treatment and cooling rates.9

Controlling and Exploiting Phase Separation in Glass

Note that within the immiscibility region in the diagram above, two common commercial glass compositions are labeled. Indeed, it’s now understood that phase separation ­offers advantages in certain applications. Today, heterogeneous phase-separated glasses cover a broad range of commercial applications, including Pyrex®, Vycor® opal glass, porous glass, and glass ceramics.

Glass-ceramics are a class of polycrystalline materials that share many properties with both glasses and ceramics, ideally providing the moldability of glasses with various special properties (such as high strength) of ceramics. Glass-ceramics are produced by the formation of crystal phases within an amorphous base glass (i.e., crystal-glass phase separation). Engineering glass-ceramics depends on controlling crystallization within the base material.10

Another application of controlled phase separation is in the production of porous glasses. Porous glasses are typically high-silica glasses that contain pores with a specific size distribution, ranging from angstrom to millimeter scales. Porous glasses are commonly produced from phase separation of alkali borosilicate glass, in which the mixture undergoes spinodal phase separation following heat treatment to yield two continuous phases.11 Following phase separation, the alkali-rich borate phase can be dissolved in acid and removed from the solid. This leaves a highly pure and porous silica glass “skeleton.”

Schematic showing the formation of porous glass from a phase-separated alkali (sodium) borosilicate mixture. (Hasanuzzaman et al 2016)11

Porous glass exhibits improved mechanical and thermal stability compared to ordinary bulk glass, making it a popular alternative to fused quartz which is comparatively difficult to form. Other applications make use of the pores themselves: such as filitering materials, catalyst supports, and targeted drug delivery.12–16 Mo-Sci is a world-leading provider of advanced glasses for healthcare, electronics and engineering applications. We offer a range of glass-ceramic seals and porous glass solutions, as well as providing custom solutions for virtually any glass application. Contact us for more information.

References and Further Reading

1. Charles, R. J. Phase Separation in Borosilicate Glasses. Journal of the American Ceramic Society 47, 559–563 (1964).
2. Möncke, D., Ehrt, D. & Kamitsos, E. Spectroscopic study of manganese-containing borate and borosilicate glasses: Cluster formation and phase separation. Physics and Chemistry of Glasses – European Journal of Glass Science and Technology Part B 54, 42–51 (2013).
3. Bergeron, C. G. & Risbud, S. H. Introduction to Phase Equilibria in Ceramics. (American Ceramic Society, 1984).
4. Gebauer, D., Kellermeier, M., Gale, J., Bergström, L. & Cölfen, H. Pre-nucleation clusters as solute precursors in crystallisation. Chemical Society reviews 43, 2348–2371 (2014).
5. Schuller, S. Phase separation in glass. (2018).
6. Morey, G. W. The Properties of Glass. (Books on Demand, 1954).
7. Da Vela, S. et al. Interplay between Glass Formation and Liquid–Liquid Phase Separation Revealed by the Scattering Invariant. J. Phys. Chem. Lett. 11, 7273–7278 (2020).
8. Bartl, M. H., Gatterer, K., Fritzer, H. P. & Arafa, S. Investigation of phase separation in Nd3+ doped ternary sodium borosilicate glasses by optical spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 57, 1991–1999 (2001).
9. Liu, S., Zhang, Y. & Yue, Y. Effect of cooling rate on crystallization in an aluminophosphosilicate melt. Physics and Chemistry of Glasses – European Journal of Glass Science and Technology Part B 52, (2011).
10. Control of nucleation in glass ceramics | Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences. https://royalsocietypublishing.org/doi/10.1098/rsta.2002.1152.
11. Hasanuzzaman, M., Rafferty, A., Sajjia, M. & Olabi, A.-G. Production and Treatment of Porous Glass Materials for Advanced Usage. in Reference Module in Materials Science and Materials Engineering (Elsevier, 2016). doi:10.1016/b978-0-12-803581-8.03999-0.
12. Hammel, J. J. & Allersma, T. United States Patent | Thermally stable and crush resistant microporous glass catalyst supports and methods of making. 341–341 (1975).
13. Jungbauer, A. Chromatographic media for bioseparation. Journal of Chromatography A 1065, 3–12 (2005).
14. Sotomayor, P. T. et al. Construction and evaluation of an optical pH sensor based on polyaniline-porous Vycor glass nanocomposite. in Sensors and Actuators, B: Chemical vol. 74 157–162 (2001).
15. Takahashi, T., Yanagimoto, Y., Matsuoka, T. & Kai, T. Hydrogenation activity of benzenes on nickel catalysts supported on porous glass prepared from borosilicate glass with small amounts of metal oxides. Microporous Materials 6, 189–194 (1996).
16. Using Porous Glass Microspheres for Targeted Drug Delivery Mo-Sci Corporation. https://mo-sci.com/porous-glass-microsphers-targeted-drug-delivery/.

 Share

 Tweet

 Share

Slump Shrinkage

Glass on rectangular moulds often does not maintain a straight edge.  It pulls in and tends toward the “dog boning” of fused single layer glass even if not so dramatic.

Explanation

The reasons for the pull-in on rectangular moulds are similar to those for dog boning. You should note that squares are special cases of the general class of rectangles. The discussion here applies squares just as much as to rectangles.

If you grid the rectangular glass, it illustrates that the glass in the corners is moving in two directions.  It is moving and slightly stretching into the mould.  At the same time, it is trying to compress into the corner of the mould.  The glass along the sides are moving in only one direction – stretching only slightly and moving toward the bottom of the mould.

There is more compression than stretching in the corners. The sides have only to move in one direction and experience no compression and so move toward the bottom more easily.

Such is my explanation of the experience. 

Avoidance

The real question then is how to prevent this pull-in that is so commonly experienced on rectangular moulds with no rims.  One way would be to avoid such moulds altogether.  This of course, is not practical, so some approaches to compensate or avoid the problem are needed.

It is possible to compensate for this pull-in by slumping a rectangle with slightly bulging sides.  Rather than a regular rectangle, you create one with slightly outwardly curved sides.  Getting the exact amount of curve will be difficult and achieved only after a number of experiments.

The opposite compensation would be to round the corners of the glass, so there will not be so much glass to fit into the corners of the mould.  This again will require experimentation to achieve a predictable result.  And it often would interfere with the appearance of the final piece.

The easiest, but not always successful, way to prevent the pull-in is to alter the scheduling for slumps on such moulds.  It is a well-known property of glass that it does not have a single softening point, but progressively softens with temperature and time.  You can take advantage of this by using four elements in combination. 

·        Use a slow rate of advance to the slump temperature, to allow the glass to evenly absorb a lot of heat on the way to slumping. 

·        Use a low slumping temperature  This may be as much as 30°C less than your usual temperature.

·        Use a long soak at the slumping temperature.  This may be hours.  You need to allow the glass to slump into the mould without stretching.  To avoid stretching, you need a low temperature.  At low temperatures, the glass requires a lot of time to conform to the mould.

·        Observe at 10- to 15-minute intervals once the slumping temperature is achieved.

These processes are outlined in a blog post on dog boning.  Further information is available in the ebook: Low Temperature Kiln Forming.

Avoidance of pull-in of the glass on rectangular moulds is related to scheduling and observance.  There are some compensations that can be tried, but require considerable experimentation to be successful.