Showing posts with label Cleaning. Show all posts
Showing posts with label Cleaning. Show all posts

Wednesday 31 January 2024

Care of Ceramic Kiln Shelves

Mullite kiln shelves
credit: IPS Ceramics


The most popular and easily available ceramic shelves are made from Mullite, Cordierite, and CoreLite. Other hard specialist kiln shelves are available. They are made of other materials. Shelves are also made from other materials such as refractory fibre board, vermiculite, and fire-resistant ceiling tiles. This concentrates on the care of ceramic shelves.

Composition and Characteristics

This table gives some information about the characteristics of the materials involved in these shelves.

 

Name

Thermal Shock Resistance

Brittle

Strength

Composition

CoreLite

Low

Yes

Moderate

Ceramic with a high silica content

Cordierite

High

Yes

Strong, but heavy

Magnesium, iron, aluminium oxide, silica

Mullite

High

Yes

Strong. but heavy

Silica, Aluminium oxide

 

CoreLite is a trade name for an extruded ceramic shelf. It is strong, but brittle. It is subject to thermal shock below 540ºC/1000ºF. This suggests the ceramic has a high silica content as the quartz inversion is at 573°C/1063°F, where the ceramic has a sudden expansion on heating and an equal contraction on cooling. The cooling rate at this temperature is normally slow enough to avoid breakage.

credit: Clay Planet


cordierite - composed of magnesium, iron, aluminium oxide, and silica. hard, brittle, and with low expansion characteristics.

credit: refractorykilnfurniture.com


Mullitecomposed largely of silica and aluminium oxide. It is strong, brittle, and has good thermal shock resistance.

Care

There is enough information from considering the composition of these shelves to indicate they are all brittle and have differing vulnerabilities. These have implications for storage, use and cleaning.

Storage

If storing vertically, take care to avoid setting down on hard surfaces. If they are in a rack, have a separate slot for each shelf. This avoids friction between shelves and possible surface scratches. The most useful material for these racks is wood, or harder materials covered with wood. These racks can be horizontal or vertical.

If it is not possible to have a separate rack for each shelf, do not lean them on each other. Shelves leaning against others or against hard surfaces can become scratched. Provide a cushion against scratches such as cardboard, or thin plywood.

When moving the shelves, avoid setting them down on their corners, or bumping the shelf anywhere against hard structures.

Use

Reduce firing speeds to less than 220ºC/430ºF per hour up to 540ºC/1005ºF, especially for CoreLite shelves. Cordierite and Mullite shelves are not as sensitive, but still can be broken by fast firing rates in this temperature region.

Cover a large portion of the shelf at each firing to avoid uneven heating of the shelf. It is best to evenly distribute moulds and other things that shade the heat from the shelf around the shelf to help avoid thermal shock breaks.

If you cannot or do not want to cover the whole shelf, elevate the mould(s). This helps to keep the whole shelf at the same temperature when only small parts of shelf are covered. It does not seem to matter so much when flat glass is in contact with the shelf. But continue to observe the moderate ramp rates below 540ºC/1005ºF.

It is even more important to elevate damp or heavy moulds from the shelf. These kinds of moulds shade the heat from the shelf immediately below them while the rest of the shelf heats rapidly. This difference in expansion over parts of the shelf becomes too great for the shelf to resist.

Another thing to avoid is cutting fibre or shelf paper on top of the shelf. It often creates long shallow scratches in the shelf. These can be the source of bubbles, but more often, flaws on the back of the fired pieces.

Cleaning

Care is needed to avoid mechanical damage during cleaning. Scraping can create scratches in the shelf. These are difficult to remove or fill smoothly. So, scraping needs to be done carefully.

Any sanding also needs to be done carefully. If you use power tools, it is very easy to create shallow depressions that will be the source of bubbles in future firings. It is slightly more time consuming to manually sand the kiln wash with a sanding screen with or without a holder. But it preserves the flatness of the surface.

If it is decided to wash the shelf primer off the shelf, consider how difficult it is to wash a very persistent baked on substance. It requires thorough scrubbing to remove all the hardened material. Power washers are not advised since the high water pressure can abrade the surface of the shelf.  But if you do decide on washing, you need to air dry for several days afterwards. Then kiln dry slowly to just below boiling point of water. Soak at that point for several hours, or until a mirror held above the open port does not fog up.

There is more information on removing kiln wash here and here.


Summary

Ceramic kiln shelves are hard, but subject to scratches, impact breaks, excess dampness, failure due to uneven temperatures, and to rapid rises in temperature below 540ºC/1005ºF.

Wednesday 31 May 2023

Causes of Large bubbles

 Let’s think about moisture and large bubbles from under the glass. It is not the water, but the gasses created by the decomposition of materials that can cause the bubbles. There are other causes of large bubbles too. The most common causes are discussed here.

The usual explanations are:

  • ·        Uneven shelf
  • ·        Heat resistant particles under the glass
  • ·        Uneven heating
  • ·        Glues
  • ·        Organic material
  • ·        Moisture
  • ·        Amount of gas

 

image credit: Warm Glass

Uneven shelf

Shallow depressions in shelves can cause large bubbles. Occasionally, the shelf can be damaged in various ways causing scratches or dings in the shelf. Air can be trapped in these depressions. And it does not take much volume of trapped to be a problem. The heat of kilnforming causes the air to expand. As the glass becomes less viscous with increased temperature, the pressure from the expanding air forces the glass upwards. The amount of air and the amount of heat work combine to create bubbles from simple uprisings to large thin walled or even burst bubbles.

There are some things that can be done to detect and avoid bubbles from forming. It is possible to screed powdered kiln wash over kiln washed shelf. This gives pathways for the air to escape. It does leave a more marked bottom surface than kiln wash.

Using 1mm or 2mm fibre paper allows air from under glass. You can maintain a relatively smooth surface with Papyros or Thinfire over the fibre. Even Thinfire or Papyros on its own will allow air from under the glass.

Checking for depressions can be done by spreading kiln wash powder over shelf and drawing a straight edge over the shelf. Depressions will be shown by the presence of the powder. It can also be done with powdered glass frit.

Particles under glass

Any particle resistant to kilnforming temperatures holds the glass up while it is forming so creating an air space. It is important to ensure the shelf is clean as well as flat. Small pieces of grit or dirt that are resistant to high temperatures will hold the glass up from the shelf enough to create a bubble – small or large depending on the temperature. Vacuuming the shelf before adding anything to the surface before each firing is important to bubble free results.

Uneven heating

This is sometimes cited as a cause of bubbles. If so, the heat would need to be very localised. This is possible if the glass is very near elements. In general, the temperature is equalised at a distance equal to the width of the elements.

Glues

A wide variety of glues are used in kilnforming. Those available to enthusiasts all burn away leaving gasses between layers. These gasses - if trapped - can thin the glass below as well as above the glue’s position. This will give the impression that the bubble has come from between the shelf and the glass. Most often the bubble forms between the glass layers, pushing a bubble only into or through the top layer. The solution is to avoid using glue or minimise it and place it only at the edges.

Organic material

Organic materials can be a problem. When you are using a large or thick fibre paper sheet under a piece of glass, occasionally the gasses from burning out of the binder can be great enough to create a bubble. Although normally, this only leaves a grey to black mark on the underside of the glass. Vermiculite boards need to be fired before use, as they contain significant amounts of binder.

Inclusion of organic materials such as leaves, twigs, or bones, leads to bubbles. Very long soaks below the softening point of the glass are required to allow the organic material to burn out of the objects.  The time required increases from an hour for leaves to 24 for bones.

Moisture

Moisture is very often cited as the source of bubbles. It is possible that the steam from water may be trapped in shelf depressions, or the areas held up from the shelf. And anytime there are no precautions to allow the air from under the glass, or between sheets bubble formation can be promoted. If adequate precautions are taken (flat shelf, clean shelf, bubble squeeze) the moisture will evaporate before the glass is hot enough to form a seal around the edges and trap any steam. It is another good reason for moderate ramp rates at the beginning of a firing.

Amount of gasses

Of course, if there is a lot of moisture there can be problems. Simply applying kiln wash in four coats does not leave enough water in the shelf to be a problem.

If you have washed the kiln wash off a mullite shelf, there will be a lot of water in it even after it feels dry. Then it does need to be kiln dried before use. To avoid breaking the shelf you need to fire slowly to 99°C/210°F and soak there for a couple of hours with the vents open or lid propped up a little to allow the moisture out of the kiln.

 

 


Wednesday 1 December 2021

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.

Wednesday 24 November 2021

Playing in the Sandbox


This process provides flowing, abstract images that can be used as autonomous pieces or formed into other objects, such as free drops, bowls, cut for jewellery or into pattern bars.  The appearance provided is unique to this combination of using frit and pressing.

In principle, this process is the same as creating sand pictures.  The process is in three stages: making the box, adding frit, and pressing.


The Sandbox
Determine the size of the box.  It should not be more than two-thirds the size of your kiln shelf depending on thickness.  Thicker glass pressed to 6mm will spread more than thinner.  As a guide, 12mm should have an allowance to spread to about 1.3 times the original size; 19mm should have an allowance to spread about 1.5 times the original dimensions.

Cut two sheets of the same size from clear fusing glass. One will be the front. The other will be the back.

Determine whether the image you are creating will be portrait, landscape, or square.  Orient the sheets in the appropriate way to have the top away from you.  Choose the top piece of the pair and cut two 6mm strips from the designated top.  This gives you a lip to be able to pour the frit into the box easily.
 
Box formed with bottom and sides glued to back and front.  The filling lip shows on the right.

From another piece of clear glass cut two 6mm strips for the sides.  If you cut them the same length as the side of the glass, they will stick above the back about 3mm. You can cut this off, but it really is not a worry for the construction of the box.  These strips form the spacers to allow the frit to be poured into the box.  Their thickness will determine the amount of frit needed to fill the box.

Get out the back sheet and clean and prepare it for attaching the strips. My preferred method is to glue the bottom 6mm strip on its edge with super glue.  It is advisable to wear plastic gloves when gluing the strips, to avoid sticking your fingers to the glass.  Super glue cures quickly and does not delay the construction of the box.  It burns out cleanly without any health and safety concerns.  Place a thin film of super glue on one edge of the strip.  Attach it to the bottom by placing it carefully at the edge of the sheet.  Do the same for the sides.
  
When the strips are stuck down to the back, place  a thin line of super glue on the top edge of the strips in preparation for attaching the top sheet.  Using a strip of wood placed at the bottom of the backing glass will help in placing the sheet accurately. Lower the sheet from contact with the bottom to the strips forming the sides of the box.

When the glue is cured, inspect the sides of the box for gaps. If there are gaps, use clear Sellotape to seal the gaps in the sides. It will burn off cleanly in the kiln.


Adding the Frit.
Place the box on an easel or other support so it is slightly tipped backwards.  This helps ensure the box does not fall toward you while working on it.  It also allows the frit to slide toward the bottom rather than bouncing off the other frit.
 
The early stages of filling with the box on a stand

The size of frit you choose to use will affect the final appearance.
·        Generally, powder will appear greyer and more opaque than frit. This is due to the multiplicity of tiny bubbles between the grains of powder.
·        Fine and medium frit provide more clarity than powder.
·        Coarse frit provides the most clarity, but with fizzy bubbles between pieces of frit.

When preparing to place the frit in the box, it is a good idea to take small amounts out of jars and place it into small cups to avoid contamination of the main source of the frit.

Pouring the frit into the sandbox

You can use a jeweller’s scoop or a teaspoon to move the frit from the cup to the box.  Tip the frit into the box above where you want the colour to be placed.  


Moving the frit with a skewer

If the frit does not land just where you want it, you can move it with stiff wire that is long enough to reach the bottom of the box.  Gently sweep the frit with the end of the wire toward the place you want the coloured frit to be.

Using a jewellers scoop to add the frit.

Continue adding colours to create the profile and shapes you wish.

You can make additional alterations to the way the frit is placed.  You can poke the frit from one layer into lower layers with a stiff wire by pushing the wire directly downward.  You cannot do this more than 2 or 3 centimetres deep, as the frits and powders become compacted.

A thick copper wire being used to poke down from an upper layer to the lower ones.

When filled to the top or to your desired level, use the fourth strip to close the box.  If full, glue the strip to the top.  If not full, cut strip to the length needed to drop into the opening of the box.  Place a couple of drops of super glue on the top of the already placed strip to keep it in place while moving to the kiln.




The Pressing
Prepare the shelves
You will need two shelves for each pressing. One is the base to hold the glass and the spacers.  The other is to provide the weight to press the glass thinner.

Clean off old kiln wash from the shelves. Experience shows that adding new kiln wash over old for this process promotes the sticking of the kiln wash to the glass.  Add new kiln wash that performs well at extended times at upper temperatures.  I find Bullseye shelf primer works very well.


Once partially dried, with the pink beginning to pale, you can smooth the surface brush marks.  Some use balled up material such as tights to rub over the surface.  I find very good results from rubbing lightly over the kiln washed surface with a sheet of paper between the palm of my hand and the shelf.  The advantage of doing this smoothing while slightly damp is that no dust is created that needs to be cleaned away.  The disadvantage is that too much pressure will pull bits of kiln wash from the shelf.

Do not use fibre papers as the separator.  The glass will be moving within the space between the shelves.  It will pick up and incorporate parts of the fibre paper, if used.

If you have shelves of different thicknesses, reserve the thickest shelf for the upper, pressing one.  If all your shelves are the same size, put a second on top for adequate weight, or add heavy bricks or a steel weight to the top shelf.  (Note: if you use bricks for weights, they need to be dried first.  A two-hour to three-hour soak at 95C should be sufficient.)

Placing
Place the sandbox centrally on the shelf.  If you are doing more than one, ensure there is plenty of space between the pieces and from the edge, so they don’t contact each other, or drip over the edge of the shelf.  The allowances given for the size of the sandbox are a guide.

Two sandboxes placed on separate shelves

Place spacers of the desired thickness around the four corners of the shelf to restrict the extent of thinning.  This also regulates the evenness of the glass across the whole surface.  Usually, 6mm is a desirable height for the pressing.  Other thicknesses can be chosen for different purposes.  The spacers can be steel washers, although they will spall in the cooling stages of the firing.  If you have pieces of ceramic of the desired height, they can be used.  Fibre paper stacked up to the appropriate height are surprisingly robust spacers.  They also provide a cleaner set of spacers than steel.

A corner of the shelf with the 6mm fibre spacer

Place the upper shelf gently down onto the glass piece. The glass at this stage is taking the whole of the weight of the pressing shelf.  The shelf must be placed both gently and evenly down onto the glass to avoid breakage.

Check that everything is in place. This may require additional, directional light such as from your mobile phone or a torch.  It is now ready to fire.



The Firing
This assembly of materials has a lot of mass.  It is 2 to 3 times the normal mass for a standard firing.  

Pressing shelf placed on top of the glass sandbox

This promotes variations in practice:
  • ·        Even with this additional mass, you can fire quickly.  This is because the glass is in small pieces and that the mass of the shelves gains heat slowly. 
  • ·        The greater mass does require longer soaks than a normal fuse firing. 
  • ·        The upper temperature for a full fuse is required to get the glass to a sufficiently low viscosity to allow the glass to move.
  • ·        The long soak at the top temperature does not promote devitrification as in normal fusing.  My speculation is that the glass is not exposed to the air, so the devitrification cannot form. 
  • ·        A further difference in a pressing firing is that the annealing can be at the rate for the final thickness of the glass.  The mass of the shelf and weights above the glass means the glass is cooling evenly from both sides, unlike normal fusing.  The glass may be cooling more slowly than programmed, but the programmed rates limit any possibility of too rapid a cooling.


A schedule for a 12mm thick Bullseye piece with a 19mm upper shelf might look like this:
300°C/hr to    670C       for   180 minutes
300°C/hr to    816C       for   180 minutes
AFAP      to    482C       for  120 minutes
55°C/hr   to    427C       for   0 minutes
99°C/hr   to    370C       for   0 minutes
200°C/hr to    50C         for   0 minutes
Off

A piece of 19mm should be slower:
150°C/hr to    670  for   240 minutes
150°C/hr to    816  for   240 minutes
AFAP      to    482  for   120 minutes
45°C/hr   to    427  for   0 minutes
90°C/hr   to    370  for   0 minutes
180°C/hr to    50    for   0 minutes
Off  

Both these schedules assume the final thickness of the glass will be 6mm.  The schedule for glasses other than Bullseye only needs to have the top and annealing temperatures altered to the ones appropriate to the glass.


Results
The pressed glass will have the texture of the shelves on both sides.  Normally, no kiln wash will be stuck to the glass.  If there is kiln wash to be removed, you can do this by abrasive means – sandblasting, diamond pads, wet and dry sandpapers or Dremel style tools.  It is important to keep the glass damp during this process.


Untreated result of pressing



If the surface of the glass is without sticking kiln wash or other marks, you can use it with the matte surface without further kiln work.  You can also fire polish the piece to get a glassy surface, once you have thoroughly cleaned it.


Alternatives
Tape box together
After super gluing the bottom and side strips, you can bind the box together with clear Sellotape.  Pull off at least three strips of tape and set them where you can reach them easily.  Place the upper sheet on the prepared base. Move the box to the edge of the work surface so a little of the box hangs over.  The first stage is to place a strip of tape at right angles to the side to bind the top to the bottom.  Do this for each of the three sides.  When the top is securely attached to the base and sides tape along the length of each of the three sides. 

This shows on the lower left a loosened piece of sellotape on the edge of the sandbox.


This process avoids any difficulty in attaching the top.   Attempting to use only Sellotape to bind the box together is very difficult and requires at least three hands.

Spacers for the frit
Spacers do not always need to be strips on edge.  The spacers can be one or two wider strips placed on their sides to provide the needed height.  They can be coloured, forming a border; but remember the border will become curved. The strips will need to be glued to the back.  The top can be attached with super glue, or taped to the sides and back.

Pressing without a box
It is possible to use the pressing technique without a box or frit.  You can arrange clear and coloured cullet on the shelf.  The arrangement needs to be such that there are no gaps between the pieces.  This means that the glass will probably be 3 to 4 layers thick.  Be careful to avoid creating thick layers of dark colour by interfiling with clear. Place the spacers at the corners of the shelf for the thickness desired and fire.  The slower rate of firing (as for 19mm) should be used.

This sandbox process is a combination of arranging frits and pressing.

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

Wednesday 28 July 2021

Vitrigraph Pot Liners



Stainless steel vitrigraph pots are durable replacements for ceramic pots that do not last many -  if more than one - firings. But cleaning is not straight forward. Most recommendations seem to concentrate on cleaning by banging the metal to break the glass away from the sides and bottom.  This seems more brutal and noisier than necessary. It will eventually dent the metal and possibly become unusable.

An unlined ceramic pot


An alternative is to line the sides with 1mm or 2mm fibre paper.  Paper of this thickness has enough fibres that the paper will stick together and not contaminate the pull.  It will still protect the metal from glass sticking At the conclusion of the firing and after the cool down, you can remove the fibre paper and have clean sides.

Instead of placing the glass in a bare pot, you can  line the pot with fibre paper 
 

It is possible to put a piece of fibre paper on bottom with a hole in it to match the pot’s hole.  There is a slight risk of drawing fibres into the pull, although I have not experienced it yet.

This method also works with ceramic pots.

Wednesday 30 June 2021

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.

Wednesday 27 May 2020

Oxidisation of Foil




Often, life intervenes between foiling and soldering.  This frequently results in the foil not accepting the solder very well, because of the mild tarnish that has occurred in the meantime.

It is a good idea to clean the foil of any possible corrosion before soldering when there has been an interval between the two processes. It is enough to clean foil with a mild abrasive such as a foam-backed scrubber from the dish washing, or fine steel wool.  I prefer the scrubber as it does not introduce another metal.  

Some people prefer a vinegar and salt solution to apply to the tarnished foil.  I am concerned about the introduction of an acid into the process causing further problems later.  I don't recommend this method of cleaning.

I then coat the exposed foil with a film of paste flux to protect from further tarnish. This acts better than any loose covering of cloth or plastic to protect from oxidation.  The purpose of flux is to both provide a "wetting" agent for the foil to accept the solder,  and to prevent oxidisation. Liquid flux cannot provide protection, once dry,  from the copper tarnishing.  I prefer the use of paste flux to reduce boiling of flux and to keep the copper free from corrosion.  The paste flux will not indefinitely prevent oxidisation, but will do so for a week or two.