Wednesday, 28 December 2022

Damming for Exact Shapes

 Many times, exact dimensions of the final piece are not critical.  When they are and the piece is 9mm and thicker, or has irregular amounts of glass near the edge, damming is required.

 If the dimensions are rectangular, you can use straight edged refractory materials, usually sawn up broken kiln shelves, vermiculite, or fibre board strips.  

 These need to be kiln washed and lined with fibre paper.  The dams should be lined with 3mm fibre paper that is 3mm narrower than the final height of the piece.  This allows a bullnose shape at the edge to form.





 If the shape is a circular or irregular shape the dams can be made from thick fibre board or vermiculite.  The lining of the dams is the same as for rectangular shapes.  

 The use of 3mm fibre paper means that you have to make rectangular shapes 6mm bigger in each direction to achieve the exact final dimensions.  For circular or irregular shapes, the edge will need to be only 3mm larger.  This is because the edge goes around the whole shape, rather than only one side.

 

Wednesday, 21 December 2022

Simultaneous Fusing and Slumping

“I sometimes slump at the same time as I do a tack fuse. Is slumping at this higher heat bad for the mould? “

Image credit: Creative Glass

Mould

 It is possibly not bad for the mould, but it does depend on your temperature and heat work.  Ceramic moulds are typically fired to 1200° or 1300°C so higher kilnforming temperatures are unlikely to affect the moulds.  The speed at which the target temperature is reached is of concern though.  Ceramics have what is called quartz inversions.

 Two of the constituents of ceramics – cristobalite and quartz – have significantly large expansions at 226°C and 570°C / 440°F and 1060°F.  Rapid rises through these two temperatures risks breaking the ceramic mould.  This is not the case with steel moulds, of course.

Glass

 There may also be effects on the glass.  Slumping typically ranges between 620°C to 677°C (1150°F to 1250°F).  Tack fusing typically is done in the 740°C to 790°C (1365°F to 1455°F)range.  This is a significant difference even at the higher end of the slumping range and the lower end of the tack fusing range. 

 Some of the effects are:

·        The marking of the slumped glass will be greater at tack fusing. 
·        The glass will slip down the mould more. 
·        Any pieces applied to the base are likely to slide during the slumping process.
·        There is a risk of creating an uprising or bubble at the bottom as the glass slips down the side of the mould. 
·        There is more risk of creating needle points at the edges.

 Performing two processes at the same time risks difficulties.  Inevitably, compromises will need to be made between slumping and tack fusing.  Eventually, it will come to a time when the two process won't work together.

  

A slump taken to tack fusing temperatures is at risk from uprisings at the bottom, needling at the edges, excessive marking on the back, slipping down the mould and thickening

Wednesday, 14 December 2022

Scientific Notes on Annealing

 The course from which this information is taken is based on float glass.  This is a soda lime glass just as fusing glass is.  The general observations – although not the temperatures – can be applied to fusing glasses.  This is a paraphrase of the course. It relates these observations to kilnforming.  The course is IMI-NFG Course on Processing in Glass, by Mathieu Hubert, PhD. 2015 

 

Viscosity vs. Temperature for a borosilicate glass
Graph credit: Schott

Viscosity Influence on Annealing

 Viscosity increases with reduction in temperature.  So high viscosity (low temps) cannot release stress; low viscosity (high temperature) cannot maintain shape – it will deform.  The range of viscosity is small.  The viscosity must not be so high that the stress cannot be relieved, nor must it be so low that the glass is unable to retain its shape. (p.6).  This indicates there is an inverse relationship between temperature and viscosity.  This is something we experience each time we fire. 

 The mathematical definition for strain point - high viscosity - is 1014.5 Poise.   And the annealing point as 1013.4 Poise, where if the glass is all the same temperature, the stress can be relieved in about 15 minutes.  (p.7-8)  

 As kilnformers we talk of the annealing range in terms of temperature, because that is what we can measure. The annealing occurs within a small range of viscosity. This has a relation to temperature that is not the same for all glass compositions. 

 The definition of the annealing as the range of viscosity at which annealing can occur is important.  

 First, the viscosity value remains the same over many types and styles of glass.  The temperature required to achieve that viscosity varies, leading to different annealing temperatures for different glass. 

 Second, there is a range of viscosity - and therefore temperature - during which annealing can occur.  The annealing point is 1013.4 Poise, at which viscosity the stresses in glass can most quickly be relieved (generally within 15 minutes for 3mm glass).  However, the stress can be relieved at greater viscosities up to almost the strain point - 1014.5 Poise. (p.8).  At higher temperatures, the glass becomes more flexible and cannot relieve stress.  At lower temperatures (beyond a certain point) it becomes so stiff that stress cannot be relieved.  Again, those temperatures are determined by the viscosity of the glass.

 

Annealing Soaks

 Annealing can take place at different points within the range.  Bullseye chose some years ago to recommend annealing at a higher viscosity, i.e., a lower temperature.  This has also been applied by Wissmach in their documentation although initially the published annealing point was almost 30°C higher. 

 The closer to the strain point that annealing is conducted, the longer it will take to relieve the stress.  Annealing at the strain point is possible, but it is impractical.  Apparently, it would take at least 15 hours for a 6mm thick piece (p.8). 

 However, the trade off in annealing a few degrees above the strain point – requiring longer annealing soaks – is reducing the amount of time required by the annealing cool, especially for thicker or more difficult items.

 A further advantage to annealing at lower temperatures and slower rates is that it results in a denser glass – one with lower volume (p.3). Arguably, a denser glass is a stronger one.

 


Annealing Cool

 After annealing, the glass should be cooled slowly and uniformly to avoid formation of internal stresses due to temperature differentials within the glass.  Stresses that are unrelieved above the strain point are permanent.  Stresses induced during cooling below the strain point are temporary, unless they are too great.  To avoid permanent stress, the cooling should be slow between anneal soak and strain point (p.9).  Although glass can be cooled more quickly below the strain point, care must be taken that the temperature differentials within the glass are not so great as to cause breaks due to uneven contraction.

 Annealing cool factors for flat pieces are about three times that for cylinders and five times that for spheres (p.26). Or the other way around – spheres can be annealed in one fifth the time, and cylinders in one third of the time as flat glass of the same volume.   This indicates how much more difficult it is to anneal in kilnforming than in glass blowing.

 The industrial cooling rate for float glass of 4mm is 6 times the rate for 10mm although only 2.5 times the difference in thickness (p.27). This indicates that the thicker the glass, the slower the rate of cooling should be.  But also, that there is not a linear correlation between cooling rate and thickness.

 Glass with no stress has a uniform refractive index.  Stresses produce differences in the refractive index which are shown up by the use of polarised light filters.


Source: IMI-NFG Course on Processing in Glass, by Mathieu Hubert, PhD. 2015 (available online www.lehigh.edu/imi).

https://www.lehigh.edu/imi/teched/GlassProcess/Lectures/Lecture09_Hubert_Annealing%20and%20Tempering.pdf

Monday, 12 December 2022

Firing Small Pieces

 Do you run small pieces of glass through the whole cycle or just bring it up to your degree posted and cool down?

 

Picture credit: Eva Glass Design

It would appear easy to ignore the need to anneal small pieces.  They can anneal with short heat soaks.  In industry the anneal of sheet glass is 15 minutes for 4mm/0.019” glass.   In kilnforming the 30ºC - 40ºC/54ºF – 72ºF below the annealing point is where annealing is effective.  If you are certain that the natural cooling rate of your kiln is more than 15 minutes for that temperature range, you can simply turn off after top temperature.

However, it is not a good practice unless you intend to confine you kilnforming to small pieces.  All glass needs to be annealed to be sound.  Small pieces may need only 15 minutes and often that can be achieved with the natural cooling rate of your kiln.  But pieces of 6mm/0.25” thick and over 100mm/4” in any direction need to be annealed with longer soaks and slower cools.  This is done with a hold of the amount of time appropriate to your glass and layup.  There is an excellent table from Bullseye that gives the hold times and rates for cooling glass of different calculated thickness. 

Using an annealing soak and a cooling cycle for every firing is a good practice.  This gets you into a habit, so that you do not skimp on the anneal and cool for larger, thicker, or tack fused pieces.  If your kiln cools more slowly than you have scheduled, that's ok.  The kiln does not use any electricity to heat the elements.  No additional electricity cost or wear on the kiln occurs.

Wednesday, 7 December 2022

Fire Polishing of Frit Castings


Image credit: Obsession Glass Studio

 Fire polishing castings is relatively difficult.  Even though people may suggest temperatures for this kind of fire polish for castings from frit:

  • ·        They are relevant to particular kilns. 
  • ·        They are also dependent on the ramp rate. 
  • ·        The presence or absence of a bubble squeeze is important. 
  • ·        The size of the casting is relevant.

 The objective is to get a fire polish without distorting the shape of the piece.  The general procedure is to fire slowly to the softening point. This is to ensure the casting is of similar temperature throughout. The softening point for fusing glass is around 540°C/1000°F. You should soak at that point for a time to ensure the glass is all at that boundary between brittle and plastic.

 You may prefer to use a bubble squeeze soak to achieve the same thing.  This has a slightly higher risk of distorting the piece.  If you do use the bubble squeeze, it should be done at the lower end of the bubble squeeze after a slow rise.  The casting will not be subject to much change at 600°C to 620°C/1110°F to 1150°F, if the soak is short.

 The rates to be used are dependent on the size and thickness of the piece.  Larger and thicker pieces need slower rates than thin ones.  Fire at an initial ramp rate for twice the thickness to be sure of heating thoroughly.

 When the softening point is reached, or the slump soak is complete, proceed at a rapid rate to the tack fusing temperature. To get the result you want you will need to observe.  Peek at frequent intervals. Be prepared to advance to the next segment when the gloss appears on the surface.  Your controller manual will tell you how this is done.

 

Sunday, 4 December 2022

 The wonders of thick and thin glass films.


Applying Functional Films to Glass Substrates

Posted  on 

Source: https://mo-sci.com/applying-functional-films-to-glass-substrates/

Glass is a hugely versatile material. Tempered glass, for example, can be produced simply by changing the heating and cooling process during manufacturing. Changes to the shape of glass lenses alter their optical characteristics, while the introduction of pores into bulk glass enables a range of high-tech applications like bio-scaffolds and catalyst supports. Modifications to the chemical composition of glass – for example, through the use of glass modifiers – can change almost all of its properties, enabling the production of corrosion-resistant labware and high-resistance electrical components.

Depositing films on glass provides a way of augmenting the properties of glass without changing the glass itself. From thin-film solar cells to heating elements integrated directly onto glass surfaces, these films enable products and components which combine the properties of glass with those of other materials and technologies.

Film deposition can largely be divided into two categories: thin film deposition and thick film deposition. As we mention in our article on glass films, there is some overlap between the actual thickness of films in these categories – however, “thin films” and “thick films” remain distinct primarily due to differences in the technology used to produce them. Thin films typically range from less than a nanometer to several microns in thickness and are typically produced by sophisticated processes such as vapor deposition. Thick films, on the other hand, generally range from several microns up to a millimeter in thickness; and are usually deposited in the form of inks or pastes via processes like screen printing or tape-casting.

Depositing Thick Films on Glass

Electronics

Thick films are widely used in electronics: alternating layers of conductive and resistive materials can be deposited and patterned onto a substrate to build up electric circuits. While ceramic substrates are common, it is not unusual for these films to be deposited onto glass instead.

In these applications, thick films are typically deposited on glass via screen printing, forming layers between 5 and 20 μm in thickness. Insulating thick film pastes will often contain glass in the form of frit to provide high resistivities.7 After deposition, these thick films are typically fused at high temperatures before the next layer is deposited, providing a reliable and low-cost route to the fabrication of microelectronic devices.

One alternative application of thick is the production of printed heater elements on glass substrates.8 Directly depositing a heating element onto glass enables the construction of self-defrosting windows or glass appliances (such as kettles and cookers) which provide uniform heating with the modern appearance of glass.

Thick films offer the advantage of versatile and low-cost fabrication, making them ideal for the production of electronic components throughout a wide range of industries. For precision applications, however, thin film technologies provide much greater control over film thickness and surface characteristics.

Depositing Thin Films on Glass

Optics

One of the primary application areas of thin films on glass is in optics: in fact, the most famous application of thin films is probably the household mirror, which is produced by depositing a thin metal layer on the back of a sheet of glass to increase its reflectivity.

Depositing thin films on glass can produce optical interference effects, which result in certain regions of wavelengths being transmitted, reflected, or absorbed.2 Such films modify light by virtue of their nanoscale structure rather than the color of the bulk material itself, enabling optical parameters such as reflectivity and the color of transmitted light to be precisely tuned by changing layer thickness. The wings of some species of butterfly use the same fundamental “optical thin film” principles to produce their striking iridescent coloring.3

Low emissivity (or “Low-e”) glass is a major application of thin film deposition on glass. Produced through successive deposition of thin metal oxide films on glass, it allows the transmission of visible light while reflecting radiated heat (i.e., the infrared portion of the spectrum). Such optical films enable Low-e windows to reflect the sun’s light in hot environments or to prevent heat loss through windows in cold environments.

Similarly, these films enable anti-reflective coatings, which reduce glare in architectural applications as well as in consumer electronics.

High-precision deposition of thin optical films enables the construction of specialist optical filters such as dichroic filters. These filters rely on extremely precise film deposition to transmit or reject specific wavelength bands for precision applications in research and industry.

Electronics

Glass substrates for thin films have a number of special roles in electronics — particularly in the fabrication of transparent conducting films (TCFs). TCFs are a special type of film made of materials that are optically transparent and electrically conductive. They are fabricated by depositing or growing thin films of materials such as metal oxides — or even graphene — on glass substrates (with the glass offering the additional benefit of blocking infrared wavelengths of light). TCFs are applied in a range of devices, including LCD and OLED displays, touchscreens, and photovoltaic panels.4

Depositing conductive traces directly onto glass substrates enables circuitry and functional electronic components to be integrated into glass, with widespread application in aviation, automobiles, and consumer electronic devices such as smartphones.5 Other applications of thin films on glass substrates in electronics include the manufacture of thin film resistors and transparent electrodes produced by sputtering metal films onto glass.6

At Mo-Sci, we are experts in creating custom glass solutions for unique and demanding applications: whether that is glass substrates for a specific thin film application or ultra-pure glass frit for the production of resistive thick film pastes. To find out more about our services and capabilities, get in touch with us today.

References and Further Reading

  1. Bach, H. & Krause, D. Thin Films on Glass. (Springer Science & Business Media, 2003).
  2. Anderson, A.-L., Chen, S., Romero, L., Top, I. & Binions, R. Thin Films for Advanced Glazing Applications. Buildings 6, 37 (2016).
  3. Butterflies Hack Light Waves to Produce Brilliant Color — Biological Strategy — AskNature. https://asknature.org/strategy/wing-scales-cause-light-to-diffract-and-interfere/.
  4. Rosli, N. N., Ibrahim, M. A., Ahmad Ludin, N., Mat Teridi, M. A. & Sopian, K. A review of graphene based transparent conducting films for use in solar photovoltaic applications. Renewable and Sustainable Energy Reviews 99, 83–99 (2019).
  5. Kim, H.-G. & Park, M.-S. Fast Fabrication of Conductive Copper Structure on Glass Material Using Laser-Induced Chemical Liquid Phase Deposition. Applied Sciences 11, 8695 (2021).
  6. Thin Film Applications | Bourns. https://www.bourns.com/pdfs/thinfilm.pdf.
  7. Zargar, R. A. & Arora, M. Screen Printed Thick Films on Glass Substrate for Optoelectronic Applications. in Photoenergy and Thin Film Materials (ed. Yang, X.) 253–282 (Wiley, 2019). doi:10.1002/9781119580546.ch6.
  8. Radosavljevic, G. & Smetana, W. 15 – Printed heater elements. in Printed Films (eds. Prudenziati, M. & Hormadaly, J.) 429–468 (Woodhead Publishing, 2012). doi:10.1533/9780857096210.2.429.