Glass 101: Fused Silica vs. Quartz

 

Glass 101: Fused Silica vs. Quartz

Posted Krista Grayson on Jul 27, 2021

Quartz, fused quartz, quartz glass, silica, fused silica… the list of terms used to describe various silica-based materials is long, confusing, and often misunderstood. In this article, we take a close look at the unique properties of quartz and fused silica (and a few related materials), and clear up the confusion surrounding these terms.

Quartz vs. Silica

The first important thing to know about quartz and fused silica is that they both primarily consist of the same ingredient: silica, also known as silicon dioxide. Silica has the chemical formula SiO2 and is the primary constituent of most types of glass. The main form in which silica is found in nature is the mineral quartz: a hard, transparent crystalline material that makes up an appreciable fraction of the Earth’s crust. While quartz primarily consists of silica, it also contains naturally occurring impurities in various proportions depending on its geological origin.

So, silica is a specific chemical compound, silicon dioxide, with the chemical formula SiO2. On the other hand, quartz is a naturally occurring crystalline mineral, which consists primarily of silica but contains some impurities.

Crystalline and Amorphous Solids

To fully understand the differences between different silica-based materials, we first need to review the fundamental differences between crystalline solids and amorphous solids.

The distinction comes down to how atoms are arranged inside the solids. In a crystalline solid, the constituent atoms are arranged in regular, repeating patterns known as crystal lattices. Quartz is an example of a crystalline silica-based material: silicon and oxygen atoms are arranged in a well-defined ordered structure.

However, in an amorphous solid, the atoms have no long-range order. The seemingly random arrangement of molecules in an amorphous solid resembles that of a liquid, except that they are fixed in place and don’t move around. Most materials that we think of as “glass” are amorphous solids: in fact, any material with an amorphous atomic structure can be described as “glassy”.  

Whether atoms are arranged in an orderly manner or oriented randomly can profoundly influence a material’s characteristics. One of the most striking examples is the glass transition effect exhibited by amorphous solids. Outside the world of silica or other oxide-based materials, disordered “glassy” metals are often used for their unusual mechanical characteristics compared to conventional metals.1

Silica-based materials – like quartz – can be characterized both in terms of their chemical composition and whether they are crystalline or amorphous.

Defining Silica-Based Materials

Now that we’ve reviewed some important fundamentals, we can define the differences between quartz, fused silica, and other silica-based materials.

Quartz

As mentioned previously, quartz is the main form in which silica occurs in nature. Quartz is a crystalline solid; so, while it can resemble glass both in terms of its appearance and its chemical makeup, it has very distinct properties from glass.

Industrial applications of quartz (that is, the crystalline mineral) are limited, but include quartz crystal oscillators in electronic systems – most familiarly in wristwatches.

Perhaps confusingly, “synthetic quartz” can be manufactured for industrial quartz applications. This would perhaps be better referred to as crystalline silica, but is often referred to simply as “quartz.”

Fused Silica and Fused Quartz

Here, the word “fused” refers to a processing step: fused silica is nominally pure silica that has been melted and cooled to form a glassy, amorphous solid. Fused silica resembles other glasses in many ways; but it does not contain any additives. Fused silica is a specialty material with a number of high-performance applications.

The terms “fused silica” and “fused quartz” are often used interchangeably. More accurately, “fused quartz” refers to an amorphous solid formed by melting naturally-occurring quartz. So, while fused silica is ostensibly pure SiO2, fused quartz contains impurities depending on the quartz that was used.

Silica Glass and Quartz Glass

These terms are typically used in a more generic sense, and can usually be considered interchangeable. Both of these terms could refer either to fused silica or fused quartz.

Applications of Fused Silica

While fused silica is chemically similar to quartz, its amorphous structure gives it a number of distinct and highly desirable thermal, mechanical and electrical properties.

Glasses commonly contain additives such as alkali, alkaline earth, or other oxides to lower the glass processing (melting) temperature and to improve chemical and physical properties – but fused silica is very pure. Consequently, it has higher working temperatures but offers different characteristics from other glasses.

Fused silica has a very low coefficient of thermal expansion, meaning it does not expand or contract much when heated or cooled. As a result, fused silica is highly resistant to thermal shock and can withstand very rapid heating or cooling without cracking. The thermal characteristics of fused silica make it highly valuable for high-temperature industrial components such as crucibles, trays, and boats for steelmaking and glass manufacture.2

Fused silica is transparent to a very wide spectrum of light, extending from deep ultraviolet to far-infrared. This makes it a key component in optical fibers, as well as in a range of lenses, mirrors, and other UV- or IR-transmitting optics.3,4

Fused silica is also extremely chemically inert and resistant to most acids (with the notable exception of hydrofluoric acid). This chemical inertness lends fused silica to biomedical applications, often taking the form of porous silica.

The combination of thermal stability, transparency, and strength makes fused silica a strong candidate for new and developing applications such as photolithography substrates, etched microwave circuits, and as a protective layer in semiconductor devices.

Custom Glass Solutions from Mo-Sci

Mo-Sci develops and manufactures a range of high-performance glasses for technical applications. To find out more about our fused silica and porous silica products, or to discuss a custom glass application, get in touch with a member of our team today.

References and Further Reading

1. Glassy metal set to rival steel : Nature News. https://www.nature.com/news/2011/110109/full/news.2011.4.html.
2. Vert, T. Refractory Material Selection for Steelmaking. (John Wiley & Sons, 2016).
3. Khalaf, A. L., Shabaneh, A. A. A. & Yaacob, M. H. Carbon Nanotubes and Graphene Oxide Applications in Optochemical Sensors. in Synthesis, Technology and Applications of Carbon Nanomaterials 223–246 (Elsevier, 2019). doi:10.1016/B978-0-12-815757-2.00010-3.
4. Wang, S., Zhou, C., Zhang, Y. & Ru, H. Deep-etched high-density fused-silica transmission gratings with high efficiency at a wavelength of 1550 nm. Appl. Opt. 45, 2567 (2006).

Citric Acid Cleanser

Christopher Jeffree has kindly outlined the reasons for the effectiveness of citric acid as a cleaner for removing refractory mould residue and acting on kiln wash stuck to glass.  This is his work (with a few personal notes removed).

"Citric acid works well for removing the plaster scale that builds up in vessels used to mix plaster, and it helps to remove traces of investment plaster and kiln wash from glass.  Its metal-chelating properties probably help with dissolution of calcium deposits, but I am less clear why it is so good at removing kiln wash.  The constituents of kiln wash are kaolin and alumina hydrate, neither of which I would expect to be soluble in dilute acids.  Equally, the refractory materials in investment formulae I would expect to be insoluble.  However, kaolin forms layered structures in which flakes, molecular layers, of alumina hydrate and silica interact through hydrogen bonding. It is possible (I am guessing here) that citric acid can disrupt those hydrogen bonds, thereby disaggregating the clay.  All we can say is that empirically, it works.

"I prefer to use citric acid partly because it has a defined composition, but also because it is safe and pleasant to handle – no odour, and comes in the form of easily-dissolved dry crystals like granulated sugar.  Vinegar stinks, and glacial acetic acid is  an aggressive flammable, corrosive liquid with a chokingly acrid smell.

"Calcium sulfate has low solubility, but is not completely insoluble in water - gypsum (calcium sulfate dihydrate) has a solubility of about 2.5g per litre (0.25%)  from 30-100 C. Its solubility is retrograde, meaning that it decreases, rather than increasing, with temperature.  Natural gypsum is an evaporite, a type of rock that often forms by evaporation of lake water in a geological basin with little or no outflow. It can also be produced hydrothermally in hot springs, when water containing sulfuric acid passes through limestone.  

"Calcium citrate is not very soluble either, only in the order of about 0.85g per litre, but the important thing from our point of view is not to get the material into solution but to separate its crystals and make it detach from the glass.

"In other contexts, warm citric acid is used by jewellers and silversmiths as a pickle for dissolving copper oxide (firestain) from silver and gold alloys  after heating / soldering.  It is a safer alternative to the traditional jeweller's pickle of 10% H₂SO₄.

"Citric acid also dissolves rust from iron, without much etching the iron itself, so is good for cleaning rust off tools etc.

"These pictures show a plaster mixing bowl with (presumably) CaSO₄-rich deposit on the surface, cleaned by soaking with 5% citric acid for 4 hours,

and flash from the pate de verre castings with tightly adhering kiln wash, cleaned using 5% citric acid soaked for 4 hours, and vinegar (white wine) soaked for 24 hours.

"I'm not sure about reaction products - I was speculating a lot there, running through hypotheses that I can't support. We don't really have data on the composition of the layers that are stuck to the glass, or a clear idea of why they sometimes stick and sometimes don't (e.g. the differences between transparent and opal glasses in this respect). Maybe this would be a topic to discuss with technical people at Bullseye."

Hope this helps
Best wishes

Chris Jeffree

Subsequent to this work Christopher has done more work and found that Tri-sodium citrate is an even better chemical for cleaning glass of kiln wash and mould material.

Kiln Elements - Aging

As elements age, they generally increase in their resistance. This increase in resistance decreases the amount of amperage and, so, the amount of heat given off by the elements. This explains  why older kilns sometimes go so slowly and may not reach their maximum temperature.

There are several factors which affect the longevity of elements and so have implications for firing practices.

• ·        Contaminants such as silica which is contained in kiln wash and some glazes attack the aluminium oxide coating of the wire.
• ·        Allowing the wires to become tightly wound increases overheating of sections of the element.
• ·        Powders, paints and kiln wash accidentally touching the elements cause rapid corrosion of the elements if not cleaned off before firing.
• ·        Firing close to the elements allows fumes to contact the elements.
• ·        Subjecting elements to reducing atmospheres will age the elements quickly.  This would be done by introducing organics or oils into the kiln without venting.  Among the things that will attack the aluminium oxide coating of the elements are carbon, wax, halogens (such as chlorine or fluorine), molten metals (such as zinc, aluminium), lead glazes, alkaline metals, borax compounds.

All these elements attack the element coating.  And each time you fire the slight difference in expansion between the core of the wire and the coating creates cracks in the coating.  The exposed core forms new coating to fill the gaps.  This over time reduces the thickness of the element wire.  As the wire thins, the resistances increases, causing more fissures in the coating to occur, accelerating the aging process.

The next in this series is about how firing practices can affect the life of elements.
Firing Practices

Other relevant posts:
Nature of elements
Maintenance

Simple Investment Mould Materials

There are a lot of differing recipe options for making plaster moulds. A simple general purpose investment mould making material and method follows:

Equal parts of powdered silica (sometimes called silica flour or flint), plaster of Paris and water by weight.  For example:

1 kilo silica
1 kilo Plaster Paris
1 kilo water

(Do not measure by volume)

Mix silica and plaster of Paris dry in separate bucket by hand.  If you can use a closed container that is best.  Otherwise use breathing protection and do the mixing outside.  Silica is very bad for your health.

Measure the water into a separate bucket with enough volume for three times the amount of water. Slowly sprinkle the entire contents of the dry mix into the bucket of water.  Do not dump it in!

Let the mixture sit for 2 minutes (slaking).  Then mix by hand slowly to prevent bubbles. Using your hands allows you to feel any lumps that are present and break them down gently. Depending on temperature and amount of water, you have 15-20 minutes before the mix begins to become solid.

When mixed thoroughly, pour carefully and slowly into a corner of the mould box or container to reduce the occurrence of bubbles within the investment material or against the master.

When the pour is finished, tap the mould container to encourage any bubbles to the surface.

You can take the investment and master from the container once it is cold to the touch. Remove the master from the investment material carefully to avoid damaging the surface of the investment.

For pate de verre, you can use the mould almost immediately.  For casting, it is important to have a dry mould.

Let the whole air dry. Depending on the temp, humidity and density this can last from several days to several weeks. A way to tell how dry the investment is, is by weighing the mould when it has just hardened. When it has lost on third of its weight (the water component), it is ready for kiln drying. This removes the chemically bound water from the investment material. 

This is only an outline of what to do.  Investment moulds are extremely complicated in their chemistry, physics, and use.

Glass Dust

This is from Greg Rawls' website.  He is a glass worker and a certified industrial hygienist. A huge amount of practical information on safety in glass working is available on his web site:

http://www.gregorieglass.com/Health_Saf ... mical.html

Ground Glass

OSHA classifies glass dust as a “Nuisance Dust”. Ground glass does not cause silicosis. You can wear a respirator if you are concerned about exposure.

Glass is made from sand, which contains silica - a naturally occurring mineral silicon dioxide (SiO2). Crystalline forms of silica, also known as “free” silica, can contribute to the development of silicosis under prolonged exposure conditions.

It is important to understand the difference between glass and crystalline silica because exposure outcomes are extremely different! Glass is a silicate containing various other ingredients which have been melted and upon cooling form an amorphous, or non-crystalline structure. While silica (SiO2) is a primary ingredient in the manufacturing of glass, when glass is formed under heat, the crystalline structure is changed to an amorphous structure and is no longer considered crystalline.

Ground glass is rarely respirable because the particle is too big. Always use wet methods when grinding glass! Water captures the dust. Sometime other chemicals are used to add colour to glass such as arsenic, lead, cadmium. These are usually present in low concentrations and are bound to the glass and not readily available but could present an exposure issue under some circumstances.