Using Calcium Phosphate to Heal Bone Defects

 

Using Calcium Phosphate to Heal Bone Defects

Posted Krista Grayson on Nov 15, 2018

Bone defects arising as a consequence of trauma or disease typically require surgical intervention to promote healing. The defect must be filled to provide a framework to support and encourage the growth of new, living bone.

The gold standard for filling bone defects is autologous bone, but the additional morbidity involved for the patient in harvesting bone for grafting has led to a growing preference for alternative methods. The need for further surgery to acquire the bone is eliminated by using donated bone, but such allografts carry the risk of an immune response preventing the graft from being accepted.

Learn more about calcium phosphate and other materials in our ebook, The Physician’s Guide to Synthetic Bone Grafting Biomaterials. Written by Mo-Sci CTO, Dr. Steve Jung.

Access the Guide »

Consequently, the use of synthetic bone graft materials is steadily gaining popularity.1 A variety of bone graft substitutes have been used in the search to find an alternative to bone that provides a rapid and strong repair. These include demineralized bone matrix, calcium phosphates, collagen- and hydroxyapatite-based substitutes, and bone morphogenetic proteins. This article will focus on calcium phosphate ceramics and bioactive glasses.

Calcium Phosphates for Synthetic Bone Grafts

Calcium phosphate ceramics closely resemble the mineral components naturally present in bone tissue, and so represent an attractive option for a synthetic bone filling material. Their biocompatibility and ready availability have led to calcium phosphate ceramics being widely used as an alternative to autografts and allografts.

Calcium phosphates have proven to result in good cell attachment when used as bone substitutes and tissue engineering scaffolds. They are also known to provide predictable outcomes and lower morbidity for the patient whilst being cost effective compared with traditional bone grafts.2,3

Initially, calcium phosphate ceramics lacked sufficient porosity to allow immediate bone ingrowth and rapid integration into the bone tissue. However, variations in the parameters used during the preparation of calcium phosphates has led to the production of products with more favorable chemical and physical characteristics, such as specific surface areas and porosity.

Careful selection of the precise combination of properties has enabled development of bone filling materials that improve the adhesion, proliferation and differentiation of cells, thereby allowing improved osteoconductivity.4

Bioactive Glasses

Bioactive glass is a particularly favorable form of synthetic bonegraft as it is osteoconductive, bioactive and antimicrobial. In addition, minerals, such as calcium, are released from the bioactive glass providing key substrates for the production of new bone.

Bioactive glass induces specific biological activity when implanted in the body that causes an amorphous calcium phosphate layer to develop on its surface. Over a few hours, this layer incorporates blood proteins and collagen and crystallizes into hydroxycarbonate apatite, which makes it very similar to natural bone mineral. Bioactive glass thus bonds readily to the recipient bone.

Furthermore, the properties of bioactive glass, such as particle size and rate of reabsorption, can be tailored by adjusting the exact composition to meet the requirements of a specific repair procedure.4,5

Bioactive Glasses Encourage Bone Repair

Bioactive glass has been successfully used in a range of tissue engineering procedures.3 With its versatility, achieved through the tailoring of properties, its intrinsic strength and biocompatibility, bioactive glass presents many of the features needed in a synthetic bone substitute.

Bioactive glass bone filler composites have also been loaded with drugs, proteins and growth factors to facilitate repair by delivering the therapeutic agents directly into the defect region.7

It has been shown that damaged bone regained its original strength more quickly when repaired using a composite bone filler material that included bioactive glass compared with bone repair using composite alone. Furthermore, when bioactive glass is added to the bone substitute, the efficacy achieved is comparable to that obtain with the gold standard—autologous bone grafting. A bioactive glass synthetic bone substitute was recently shown to be effective in the repair of cavitary bone defects in patients with chronic osteomyelitis.8

Bioactive glass has also shown great promise in a variety of other orthopedic applications including spinal fusion and the coating of implants and the strengthening of bone at the site of joint replacements, plates, or screws. Bioactive glass coatings on orthopedic implants did not induce any adverse effects or inflammatory response in the surrounding tissue.9 Furthermore, bioactive glass coatings accelerated cell attachment, spreading, proliferation, differentiation, and mineralization of the extracellular matrix and promoted rapid bone growth. Spine fusion performed in rabbits using a mineralized collagen bone substitute with and without added bioactive glass demonstrated that the addition of bioactive glass led to earlier fusion of the bone. In addition, the addition of bioactive glass achieved a repair very similar to that seen with autograft in terms of the amount and quality of the new bone.10

Mo-Sci produce implant grade bioactive glass in a variety of forms suitable for a range of bone repair applications, and can tailor its composition to meet specific requirements.5 The composition and form of the bioactive glass can be adjusted to match the intrinsic conditions of the patient and the rate and pattern of bone formation required.4

References and Further Reading

1. Kinaci A, et al. Trends in Bone Graft Use in the United States. Orthopedics 2014;37(9):e783 e788.
2. Saffar JL, et al. Bone formation in tricalcium phosphate-filled periodontal intrabony lesions. Histological observations in humans. J Periodontol 1990;61(4):209–216.
3. Barrère F, et al. Bone regeneration: Molecular and cellular interactions with calcium phosphate ceramics. Int. J. Nanomed. 2006, 1, 317–332.
4. Lobo SE, et al. Biphasic Calcium Phosphate Ceramics for Bone Regeneration and Tissue .Engineering Applications. Materials 2010;3:815-826.
5. Mo Sci Corporation website. http://www.mo-sci.com/en/products
6. Jia W, et al. Bioactive Glass for Large Bone Repair. Adv Health Mater. 2015;4(18):2842 2848.
7. Schumacher M, et al. Calcium phosphate bone cement/mesoporous bioactive glass composites for controlled growth factor delivery. Biomater. Sci. 2017;5:578 588.
8. Ferrando A, et al. Treatment of Cavitary Bone Defects in Chronic Osteomyelitis: Biogactive glass S53P4 vs. Calcium Sulphate Antibiotic Beads. Bone Jt Infect. 2017;2(4):194 201.
9. Mehdikhani-Nahrkhalaji M, et al. Biodegradable nanocomposite coatings accelerate bone healing: In vivo evaluation. Dent Res J (Isfahan). 2015;12(1):89 99.
10. Pugely AJ, et al. Influence of 45S5 Bioactive Glass in A Standard Calcium Phosphate Collagen Bone Graft Substitute on the Posterolateral Fusion of Rabbit Spine. Iowa Orthop J. 2017; 37: 193–198.

Annealing

I have just come across this item which is scientific, but not highly technical. It may help understanding some of the difficulties of annealing

Glass 101: Annealing

The process of annealing glass relieves internal stresses, which could otherwise leave it susceptible to cracking or shattering in response to minor mechanical or thermal shock. This makes annealing a vital step in the production of strong, stable, and heat-resistant glass.

Why Anneal?

From a physical perspective, glass is a highly unusual material. Most solid materials are highly ‘ordered’ with their constituent molecules or atoms arranged in regular, repeating patterns called crystal lattices. But the molecules in glass obey a different set of rules: they are, by definition, disordered. In fact, the arrangement of molecules inside solid glass resembles that of a liquid, except that they are fixed in place and don’t move around. We say that glass is an amorphous solid.

Crystalline solids and amorphous solids (i.e., glasses) respond to heat in very different ways. When crystalline solids are heated, they undergo a spontaneous phase transition at their melting temperature. Take water, for example: at -1 °C it’s an ordered, crystalline solid (ice), at 1 °C it’s a completely disordered liquid. But when glass is heated, it doesn’t undergo a phase transition per se. Instead, it very gradually tends toward a more liquid state. This smooth decrease in viscosity as temperature increases is due to the glass transition (transformation), and it’s one of the defining features of glasses.2 The glass transition temperature can be practically considered as the temperature where the liquid converts to a solid on cooling or conversely of which the solid begins to behave as a viscoelastic solid on heating.

Glass is worked and formed at very high temperatures, where its viscosity is low. When it is allowed to cool rapidly (supercool), the glass becomes stiffer and stiffer as the disordered molecules simply become more fixed in their positions. As this happens, internal stresses can become trapped in the solidifying glass. The result is extremely brittle glass that can shatter easily.

The process of annealing enables the elimination of internal stresses, producing strong and durable glass suitable for widespread application.

What is Annealing?

Essentially, annealing is the process of cooling glass in a controlled manner to reduce internal stresses in the finished glass.1 Annealing commonly occurs at one of two points in the glass production process:

1. In many manufacturing processes (such as the float glass process), high-temperature glass is cooled gradually after it reaches its glass transition temperature. This is known as straight annealing.
2. In some glass forming processes, for example, glass blowing, glass cools spontaneously after forming. In these cases, annealing is performed by reheating the glass to its glass transition temperature and then allowing it to cool in a controlled, gradual way. This process is sometimes called reannealing to distinguish it from straight annealing.

Whether a glass is straight annealed or reannealed, the fundamentals remain the same: within a certain temperature range known as the annealing temperature (close to the glass transition temperature), glass is soft enough that internal stresses can relax through microscopic molecular shifts, but stiff enough that it doesn’t deform under gravity.

Lowering the temperature of glass from its annealing temperature very slowly means that there is sufficient time for heat to distribute itself evenly, and molecules have sufficient time to find their most stable positions within the cooling glass.3 Once the glass passes the so-called strain point, at which point microscopic flow effectively stops and molecules are fixed in place, the glass can be cooled more rapidly to room temperature. Gradually cooling the glass in this manner prevents the formation of stresses and ensures there are no “weak points” in the finished glass.

Properties of Annealed Glass

Residual stresses in un-annealed glass mean that it can generally be expected to crack or shatter during handling (or even spontaneously). Annealed glass is much stronger and more durable than un-annealed glass, rendering it suitable for cutting and drilling processes; and subsequent use in standard applications such as windows and structural elements.

Achieving a uniform stress distribution within the glass also renders it capable of resisting thermal shock. This means that annealing plays an important role in the production of labware and bottles used in food processing, for example.

Annealing plays a special role in optical glasses. Optical applications require especially low spatial variations in refractive index, which can only be achieved by a highly uniform structural state.

The residual stresses that can be tolerated in optical glass are many orders of magnitude lower than that of ordinary glassware, so optical glass must be “fine-annealed” over much longer time periods. For example, ordinary glassware can generally be annealed in hours; but optical annealing may last for weeks or even months to allow a much greater minimization of stresses. Fine-annealed glasses exhibit consistent and well-characterized refractive indices and can be ground and polished without introducing undesirable birefringence in the glass.

Custom Glass Solutions

At Mo-Sci, we are experts in all stages of glass production, from forming and annealing right through to milling, surface treatments, and glass analysis. We produce highly specialized custom glasses for unique applications in healthcare and industry. Get in touch with us to find out more about our custom glass development services or to request a quote.

References and Further Reading

1. Narayanaswamy, O. S. Annealing of Glass. in Glass Science and Technology vol. 3 275–318 (Elsevier, 1986).
2. Dyre, J. C. Colloquium : The glass transition and elastic models of glass-forming liquids. Rev. Mod. Phys. 78, 953–972 (2006).
3. Vogel, W. Glass Chemistry. (Springer Science & Business Media, 2012).

Link: https://mo-sci.com/glass-101-glass-annealing/

By the way, I hope you all have had a happy Christmas!

Glass Separators

Glass separators tend to be in three forms – powdered, liquid or fibre. These are applied to shelves, moulds and other surfaces that might come into contact with the hot glass.

What do they do?

Glass separators keep the glass from sticking to the shelves, kiln furniture and other supports during the higher temperature parts of the firing.  Glass as used for kilnforming reaches its softening point somewhere around 580°C. The glass will begin to stick to all surfaces as it gets warmer.  The separators are stable at high temperatures and do not stick to the glass or the materials used to separate the glass from its supports.

What are they?

       Liquid and powder separators are most often called kiln wash - or batt wash in the ceramics field.  Normally they are supplied in powder form that is mixed with water for painting onto shelves and moulds. 

They normally have a high content of alumina hydrate, some kaolin (also known as china clay) and sometimes a little silica, plus often a colouring agent that burns away on the first firing to indicate fired and unfired shelves.

       A high temperature lubricant, boron nitride, has come into use for kilnforming and has slightly different characteristics than the alumina hydrate-based kiln washes.

Sheet and blanket forms of glass separators are also widely used.  They have the general name of refractory mineral wool. They are often made from alkaline earth silicate (AES) wool, Alumino silicate wool (ASW) and Polycrystalline wool (PCW).  These have different temperature ranges and levels of health risk. The thin sheets are mainly used for covering shelves and other kiln furniture.  The blanket, which starts at about 12mm, is used mainly for insulation purposes.

Thin papers, similar in thickness to cartridge paper have been developed to give a finer texture than mineral wool separators.  These currently have the trade names Papyros and Thinfire, each with their own slightly different characteristics.

Safety

As with all refractory materials, safety precautions are needed.  In the kilnforming world the risks are not those of the industrial environment because the quantities are less, and the time of exposure is much less.  Still, breathing protection should be used. Eye protection is advisable, as the particles are hard and can scratch the eye surface.  Long sleeves and gloves are advisable when handling refractory fibres.

 

Kiln Wash

This blog concentrates on liquid and powdered separators. It draws on information from the ceramics and kilnforming communities.

Basic Kiln Wash Materials

A lot of the kilnforming knowledge of glass separators comes from the ceramics field. A brief look at the development of kiln wash by ceramicists is instructive to kilnforming. 

In order to make a good kiln wash you need to select materials that have very high melting points and that, when combined, do not create a eutectic that causes melting. Knowing a bit about the properties of materials and the principles of kiln wash allows you to choose the ingredients that make the best wash for your specific situation and avoid costly problems. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

The basic materials started as:

EPK Kaolin (which includes alumina)      50%

Silica                                                50%

EPK Kaolin is a high quality, water washed kaolin which is white, has unusually good forming characteristics and high green strength. In mixtures, EPK offers excellent suspension capabilities.  The source of alumina in kiln wash was often kaolin, but now is most often alumina hydrate or alumina oxide.

Silicon dioxide has a melting point of 1710°C and aluminium oxide has a melting point of 2050°C.  A mixture of these two materials will not melt, and will protect the kiln shelves at high temperatures.

This is a good kiln wash for low and mid-range electric firings [for ceramics]. The only problem is that it contains silica, which is a glass-former. So, if a lot of glaze drips onto the shelf, it can melt the silica in the kiln wash and form a glaze on the shelf. Also, when you scrape your shelves to clean them, you create a lot of silica dust, which is a known carcinogen. So, using silica in your kiln wash is not … the best choice.

Another drawback of this recipe is that, if it is used in salt or soda firings, it will most certainly create a glaze on the shelf. This is because silica, as noted above, is a glass-former. When sodium oxide, which is a strong flux, is introduced atmospherically, it can easily melt the silica in the kiln wash into a glass. This is why silica should not be used in a kiln wash recipe for wood, salt or soda kilns. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

For glaze firings a kiln wash with more separator and less glass former is better:

Alumina hydrate            50%

EPK kaolin                    50%

Kaolin has a melting point of 1770°C and alumina oxide has a melting point of 2050°C, so it will not melt, even in a … firing [of 1250°C to 1350°C]. These ingredients are called refractory because they are resistant to high temperatures. … This recipe can be used at all temperatures and in all kiln atmospheres. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

Kiln washes with kaolin, especially if applied thickly, can flake off the shelf after repeated firing.  The cause of this is the shrinking of the drying kaolin - which is a clay – similar to dried out lake beds. Adding at least half the kaolin as calcined EPK kaolin reduces this shrinkage. Calcining involves drying the kaolin at about 1000°C for some time.  This reduces the physical property of shrinkage, but retains the chemical and refractory properties of a glass separator intact.

This gives a kiln wash consisting of:

Alumina hydrate            50%

Calcined EPK kaolin        25%

EPK kaolin                    25%

You can add more calcined kaolin – up to 35% – if you want. You need to keep enough un-calcined kaolin in the recipe to suspend the other materials so that the suspended materials can be applied smoothly.  One difficulty of increasing the kaolin content of the kiln wash is that it tends to stick to the glass - especially opalescent - on a second firing.

It is, of course, possible to do away with the kaolin entirely.  You can mix alumina hydrate with water into a full milk consistency and apply that to the shelf or other kiln furniture.  It is difficult to maintain the alumina hydrate in suspension, though. After the firing you can brush the dried separator from the shelf into a container for re-use.  You do need to ensure that the powder to be reused is free of contaminants.  It is also important to find very fine grades of the alumina hydrate to minimise the texture on the base of the glass.  Most ceramic grades are coarser than wanted for kiln forming.  You can put the powder in a rock tumbler to make what you find finer than as purchased.

There are many variations on these basic kiln wash recipes. To illustrate the wide variety, some potters just dust alumina hydrate on their shelves to protect them, while some wood firing potters use 100% silica and wall paper paste to make a very thick (1/2-inch) coating that protects their shelves from excessive ash deposits. Still others, who have the new advanced nitride-bonded silicon carbide shelves, don’t even use kiln wash at all because the glaze drips shiver off when the shelves cool. Other potters, who are very neat and don’t share their space with others, may not even use kiln wash so that they can flip the shelves after every firing to prevent warping.

Kiln wash is such a ubiquitous material in the ceramics studio that we take it for granted. … There are many recipes to choose from and many solutions to common problems if we just take the time to learn about the materials we use. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

Variants on the traditional glass separators

There are variations in the use of alumina hydrate and kaolin, but there are also other glass separators available, although they tend to be expensive.

An example is zirconium. It is a glass separator with refractory properties, as in its zirconium oxide form it melts 2700°C.  In its zirconium silicate form it has a melting point of 2550°C.  These are available under a number of trade names. This can be added to the kiln wash mix in the knowledge that it will be stable throughout the firing.

But you must be careful in the amount you use, as zirconium silicate is used as an opacifier in glass and glazes.  Also, zirconium oxide is one of the hardest substances in the world.

Boron Nitride

Another very popular glass separator is boron nitride.  It has two forms. 

One is cubic boron nitride, a cubic structure similar to diamonds.

     

  

In the cubic form of boron nitride, alternately linked boron and nitrogen atoms form a tetrahedral bond network, exactly like carbon atoms do in diamond.  Cubic boron nitride is extremely hard and will even scratch diamond. It is the second hardest material known, second only to diamond.  Cubic boron nitride has very high thermal conductivity, excellent wear resistance and good chemical inertness, all very useful properties for a material subjected to extreme conditions. Because of its hardness, chemical inertness, high melting temperature (2973°C) cubic boron nitride is used as an abrasive and wear-resistant coating. Cubic boron nitride (CBN) is used for cutting tools and abrasive components for shaping/polishing with low carbon ferrous metals.  (http://www.docbrown.info/page03/nanochem06.htm)

Hexagonal Boron Nitride

The second form, useful in kilnforming is the hexagonal form of boron nitride.  It forms white plates of hexagons one layer thick like graphite.  These plates have weak bonds and so slide easily against one another.

https://www.substech.com/dokuwiki/doku.php?id=boron_nitride_as_solid_lubricant

It is a good insulator and chemically very inert.  It is stable to about 2700°C.

Hexagonal boron nitride (HBN) is used as a lubricant, since the weakly held layers can slide over each other.  Because of its 'soft' and 'slippery' crystalline nature, and its high temperature stability, HBN is used in lubricants in very hot mechanical working environments.  

(http://www.docbrown.info/page03/nanochem06.htm)

The slippery nature and high temperature stability characteristics make this material an excellent coating for moulds and other situations where the glass moves against its supports.

The coating of the moulds needs frequent re-coating because the layers slide from the mould. Boron nitride works very well on solid impermeable surfaces as it adheres easily to smooth surfaces. It can be used on porous surfaces, but does seal those surfaces, meaning that these surfaces cannot be returned to that porous state without significant abrasion.

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The next blog  has notes on refractory mineral wools as separators and health and safety in use.

Zinc Health and Safety

So much is said about the toxicity of zinc, I thought to look up some facts.

As there is significant concern about health issues, it is useful to look in detail at the health and safety issues around the use of zinc at elevated temperatures.  Zinc is absorbed into the body by inhalation of fumes and consumption of zinc containing materials.

Toxicity

Although zinc is an essential requirement for good health, excess zinc can be harmful. Excessive absorption of zinc suppresses copper and iron absorption … [which results in the symptoms of zinc intoxication].  Stomach acid contains hydrochloric acid, in which metallic zinc dissolves readily to give corrosive zinc chloride. … The U.S. Food and Drug Administration states that zinc damages nerve receptors in the nose, causing [loss of smell].

Evidence shows that people taking 100–300mg of zinc daily may suffer induced copper deficiency. … Levels of 100–300mg may interfere with the utilization of copper and iron or adversely affect cholesterol. … A condition called the zinc shakes or "zinc chills" can be induced by inhalation of zinc fumes while brazing or welding galvanized materials. 

https://en.wikipedia.org/wiki/Zinc

Poisoning

Consumption of zinc can result in death, but requires large amounts (over 1 kg in one case).  Smaller amounts result in lethargy and gross lack of coordination of muscle movements or apparent intoxication. https://en.wikipedia.org/wiki/Zinc

Research and W.H.O. Information

The Essential Toxin: Impact of Zinc on Human Health, by Laura M. Plum, Lothar Rink, and Hajo Haase^*

Compared to several other metal ions with similar chemical properties, zinc is relatively harmless. Only exposure to high doses has toxic effects, making acute zinc intoxication a rare event. In addition to acute intoxication, long-term, high-dose zinc supplementation interferes with the uptake of copper. Hence, many of its toxic effects are in fact due to copper deficiency. While systemic [balance] and efficient regulatory mechanisms on the cellular level generally prevent the uptake of [cell destructive] doses of [environmental] zinc, … zinc [within the body] plays a significant role in cytotoxic [death of individual cells] events in single cells. … One organ where zinc is prominently involved in cell death is the brain, and cytotoxicity in consequence of [inadequate blood supply] or trauma involves the accumulation of free zinc.

Rather than being a toxic metal ion, zinc is an essential trace element. Whereas intoxication by excessive exposure is rare, zinc deficiency is widespread and has a detrimental impact on growth, neuronal development, and immunity, and in severe cases its consequences are lethal. Zinc deficiency caused by malnutrition and foods with low bioavailability, aging, certain diseases, or deregulated homeostasis [equilibrium] is a far more common risk to human health than intoxication.

Conclusions

Zinc is an essential trace element, and the human body has efficient mechanisms, both on systemic and cellular levels, to maintain [balance] over a broad exposure range. Consequently, zinc has a rather low toxicity, and a severe impact on human health by intoxication with zinc is a relatively rare event.

Nevertheless, on the cellular level zinc impacts survival and may be a crucial regulator of [the death of cells occurring as a normal and controlled part of an organism's growth or development]  as well as neuronal death following brain injury. Although these effects seem to be unresponsive to nutritional supplementation with zinc, future research may allow influencing these processes via substances that alter zinc [balance] instead of directly giving zinc.

Whereas there are only anecdotal reports of severe zinc intoxication, zinc deficiency is a condition with broad occurrence and potentially profound impact. Here, the application of “negative zinc”, i.e., substances or conditions that deplete the body of zinc, constitute a major health risk. The impact ranges from mild zinc deficiency, which can aggravate infections by impairing the immune defence, up to severe cases, in which the symptoms are obvious and cause reduced life expectancy.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2872358/

Zinc came
Credit: leadandlight.co.uk

World Health Organisation Document

10.2.2 Occupational exposure

Occupational exposure to dusts and fumes of zinc and zinc compounds can occur in a variety of settings in which zinc is produced, or in which zinc and zinc-containing materials are used. Typical airborne exposures observed include 0.19–0.29 mg/m3 during the smelting of zinc-containing iron scrap, 0.90–6.2 mg/m3 at non-ferrous foundries and 0.076–0.101 mg/m3 in hot-dip galvanizing facilities. Far higher exposures are possible during particular job activities, such as welding of zinc-coated steels in the absence of appropriate respiratory protection and/or fume extraction engineering controls.

Occupational exposure to high levels of zinc oxide and/or nonferrous metals is associated with metal-fume fever. [a condition in which the sufferer has influenza type symptoms - a raised temperature, chills, aches and pains, nausea and dizziness. It is caused by exposure to the fume of certain metals - commonly zinc].  This is usually a short-term, self-limiting syndrome…. Induction of metal-fume fever is most common with ultra-fine particles capable of deep lung penetration under conditions of exposure. Studies on volunteers conducted under short-term exposure conditions (77–153 mg/m3 for 15–30 min) have detected pulmonary inflammation responses (including [inflammation] induction) which are consistent with manifestations of metal-fume fever and support an immunological [cause] for this acute reversible syndrome.

Evaluation

Based on the available information, it is not possible to define a no-effect level for pulmonary inflammation from exposure to zinc oxide fume.

10.2.4 Risks of zinc excess

Toxic effects in humans are most obvious from accidental or occupational inhalation exposure to high concentrations of zinc compounds, such as from smoke bombs, or metal-fume fever. Modern occupational health and safety measures can significantly reduce potential exposure. Intentional or accidental ingestion of large amounts of zinc leads to gastrointestinal effects, such as abdominal pain, vomiting and diarrhoea.

In the case of long-term intakes of large amounts of zinc at pharmacological doses (150–2000 mg/day), the effects (sideroblastic anaemia [inability to make haemoglobin], leukopenia [low white cell quantities] and hypochromic microcytic anaemia [iron deficiency]) are reversible upon discontinuation of zinc therapy and/or repletion of copper status, and are largely attributed to zinc-induced copper deficiency.

High levels of zinc may disrupt the [balance] of other essential elements. For example, in adults, subtle effects of zinc on copper utilization may occur at doses of zinc near the recommended level of intake of 15 mg/day and up to about 50 mg/day. Copper requirements may be increased, and copper utilization may be impaired with changes in clinical chemistry parameters, but these effects are not consistent and depend largely upon the dietary intake of copper. Distortion of lipoprotein metabolism and concentrations associated with large doses of zinc are inferred to be a result of impaired copper utilization. In groups with adequate copper intake, no adverse effects, with the exception of reduced copper retention, have been seen at daily zinc intakes of [less than] 50 mg/day. There is no convincing evidence that excess zinc plays a [casual] role in human carcinogenesis. The weight of evidence supports the conclusion that zinc is not genotoxic [damaging of genetic information in cells] or teratogenic [affecting the development of embryos]. At high concentrations zinc can be cytotoxic [toxic to cells].   https://www.who.int/ipcs/publications/ehc/221_Zinc_Part_3.pdf?ua=1

zinc sheet 
Credit: Belmont Metals

Use and Risks of Zinc in Kilnforming

Zinc melts at 420°C and boils at 907°C, so any fumes will be emitted only around and above the full fusing temperature of glass.

The main problem in kilnforming is that the metal melts at such a low temperature that it is not useful for containing the glass.

There is anecdotal evidence to indicate that firing zinc contaminates the kiln, leading to subsequent devitrification issues.  This can be cleared by firing bentonite at high temperature in the kiln to absorb the zinc.

It is not a high-risk metal, even if it were to vaporise (above 900°C).

Research papers show zinc poisoning to be extremely rare. It is usually associated with taking too large daily doses of zinc as a dietary supplement, or swallowing USA pennies - made largely of zinc - which dissolves in stomach acid and creates large problems for the digestive system.  Where zinc intoxication occurs, it is largely reversible.

Conclusion

The idea that zinc will poison you in kilnforming conditions is simply not correct.

Bubbles in texture moulds

People often assert that moisture is a cause of large bubbles on texture moulds.

 Let's think about this.

• Water evaporates by 100°C/212°F.
• Glass is not sticky until around 540°C/1000°F.
• Glass does not begin to slump until about 610°C/1130°F. 

Therefore, with a reasonable schedule the mould, shelf, etc., is dry before air can be sealed in.

 

Damp moulds are not the cause of bubbles in texture mould work.

This observation means you must seek other causes. Three related to glass are:

•        Air pockets: Texture moulds have lots of possible air pockets. Excellent, slow bubble squeezes are required to avoid creating bubbles. These bubble squeezes start about 50°C/90°F below slumping temperatures. The ramp rate for these can be as low as 25°C/45°F per hour, but more commonly are 50°C/90°F per hour. This rate applies to the slump temperature with a soak of 30 or more minutes. The deeper the texture, the longer the soak needs to be.

•        High temperatures: To resist bubble formation low temperatures are required.  The higher the temperature, the lower the viscosity. Low viscosity is less able to resist air pressure from below than glass at a lower temperature with greater viscosity. My testing and research show that 740°C/1365°F is hot enough to form the glass to the mould. Of course, low temperatures require long soaks. The soaks might be as much as 2.5 hours for a single sheet, or as little as 1.5 hours for three layers.

•        Thin glass: there is little weight or mass in a single sheet to resist bubble formation. Using low temperatures becomes even more important. The stiffer (higher viscosity) the glass is the more resistant it is to the pressure of trapped air. Using even lower temperatures will help resist bubbles, but much longer soaks must be used.

 

The moulds can be a cause of course. There may be boundary walls on the mould. There may be vertical sides to the relief. There may recesses that are deeper than the rest of the mould.

In extreme cases, pin sized holes may have to be drilled where a lot of air is trapped. Only experience will tell you where these low spots are holding sufficient air to create bubbles.

Most bubbles created in firing texture moulds come from the scheduling, the thickness, and the nature of the mould.

 

More information is available in the e-book Low Temperature Kilnforming, an Evidence-Based Approachto Scheduling.

Quoting for Fused Glass Commissions

When quoting on a fused glass commission, what are all the factors to consider?

Commission for Glasgow University

Quote the same way as for leaded or copper foil.  But if you don’t work in those forms, that statement will not be much help.

The elements to consider are:

·        Design time and value (making sure you retain the copyright of the design).

·        Amount of time to assemble. You need to think clearly about how long it really takes.  You need to be charging a reasonable amount for your time. Think about skilled trades people’s charges and that you have additional artistic skills.

·        Amount of glass to be purchased (rather than used) to make the piece, even if much is from stock – you must replace it after all.

·        Number and cost of kiln firings.  Be clear about how many firings might be required, if something does not work out first time.  Be clear about how much each firing costs including depreciation on the kiln.

·        Incidental supplies.  All the little things that are necessary to supply your practice, such as art materials, kiln supplies, etc.

·        Overheads. This is the cost to run your practice.  If the studio is part of the home premises, add a proportion of the running expenses of the house to the cost.  The cost of business - advertising, promotion, printing, etc., all need to be included.

·        Profit. You do need to make a profit to stay in business. Decide what that is and add that percentage to the cost.

·        Allowance for contingencies (20% of the price already determined is usual).

·        Delivery/installation costs (normally in addition to the cost of design and making).

It is advisable to find out what the client’s budget for the commission is before starting any designing.  If it is too small for their specification, decline the commission.  Otherwise, you can design to the budget.  A large budget allows expansive or highly detailed works.  A small budget restricts the size or detail possible.

Some people charge more for a commission. Some, like me charge less, as I am getting most of the money up front, rather than maybe sometime in the future.  Cash is important.

Some artists take 1/3 to make the design, 1/3 on approval of design, and final 1/3 on completion. This is widely used in the interior design field. You may want to consider requiring a non-refundable deposit of one third to make a start and the remaining two thirds on completion as an alternative. 

A contract of some sort is essential.  It needs to cover the expectations of both parties.  Cost, of course.  When is it to be completed? Requests for colours, shapes, location, style, etc.  If the client wants approval at various stages, you need to either state what these stages are, or more sensibly, decline the commission. 

The contract does not have to be legalistic.  It can be a letter stating the terms of the commission that is sent by you to the client and acknowledged by them.

Determining the price for a commission requires consideration of the costs of time and materials, and the values of what you do.  A contract of some sort is required. It can be a simple letter with a statement of the agreed conditions.

Cleaning masses of pieces

Are there any easy tips on how to clean off the cutting oil without having to wipe each of 168 pieces individually?

There are a variety of approaches. Some put multiples into a basin of water as they are cut.  Some with soap added, some with window cleaner or vinegar.  When all are cut, the pieces are swirled around in the water/additives solution and laid out on kitchen towels to drain while each is polished with clean towels.  Some put the glass in a bag into the dish washer.  This leaves the glass with the residue of a number of corrosive chemicals on the surface.

If you must put additives into the soaking water, I suggest you use a combination of 1 part water, 1 part isopropyl alcohol, and 1 part 5% citric acid.  Avoid the use of vinegar. There is a significant risk of etching the glass, leaving a dull surface. Citric acid will not affect the glass, nor leave residues after rinsing.  The alcohol will speed the drying.  But see this post on another better chemical than Citric acid. You can leave glass soaking in tri-sodium citrate for up to 48 hours without etching.

Essentially these practices are to soak the pieces until all are cut to have a mass cleaning and drying session.

But I don’t use oil in my cutter and so I can follow this procedure:

clean the glass sheet first,

score with no oil in the cutter,

break,

set aside to assemble.

Prior to assembly I clean each piece with isopropyl alcohol and a polish with paper towel.

Cleaning glass for fusing is much simpler if you do not use oil in the cutter.  There is no absolute necessity to do so.  The glass will score and break very well without oil.

Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the softening and strain points. The higher one is the point at which glass begins to bend.  It is also the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 55C above and below the annealing point. The ideal point to anneal is thought to be at the annealing temperature, as annealing occurs most rapidly at this temperature.

Annealing Range

However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only the air temperature within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. A soak within the annealing range is required to ensure the glass temperature is equalised. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to 515°C during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before  the annealing cool begins.

Strain Point and Below

No further annealing will take place below the strain point. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken.  The cool below the anneal soak needs to be at a slow controlled rate that is related to the length of the required anneal soak. Too great a differential in contraction rates within the glass can cause what are most often referred to as thermal shock.  The control of the cooling rate reduces the chance of these breaks.

Softening Point

The glass is brittle below the softening point temperature, although it is less and less likely to be subject to thermal shock as it nears the softening point.  It is after the softening point on the increase in temperature that you can advance the temperature rapidly without breaking the glass.  So, if you have a glass that gives its annealing temperature as 515C, you can safely advance the temperature quickly after 570C (being 55C above the annealing point).