Smooth Surfaces on Drop Vessels

It is widely recognised that the usual results of kiln forming are one textured side and a smooth upper side. The common methods of having upper and lower surfaces both smooth is to blow the glass, avoid allowing the glass to touch the mould, and cold working the textured side to smooth.

The question arises about the possibility of getting smooth surfaces on the inside and outside of a drop vessel.  As the glass in a drop only touches the mould at the collar and edge, shouldn’t the glass be smooth on both sides?  The answer to that is in the temperatures and time used.

The temperatures used in a drop are not high enough to be certain of smoothing the outer surface.  But the soak times at drop temperatures are enough to create a fire polish on the upper/inside surface.  This indicates the blank in a drop should be placed with the texture up, facing the heating elements.  The smoother side facing the floor will be stretched and will remain smooth. 

The smoothing effect of firing with rough side up does depend a little on the depth of the drop.  Shallow drops will not have the same heat exposure that deeper drops do, assuming that a moderate heat is being used over three to four hours.

This implies that the design to show on the inside of the drop should be in contact with the separator when fusing the blank.

Firing Records

Bullseye Glass Company

To develop your fused glass practice, you need to record lots of information about your firings.  This tells you what has gone well and not so well.  It hones your expectations about how you should be preparing, scheduling, and analysing your experiences.  It becomes your detailed memory bank of results and gives directions for the future.  This should be done whether fired in your own kiln or someone else’s.

Categories of information for the record
There is quite a bit of information that needs to be included in such a record.  This is my view of what needs to be included  in your logbook for future reference.

Date
Record the date of the firing as that will give you historical information on similar projects.  It can show you what you have changed over time and the variations you have introduced.

Glass used
This is not only the type of glass (Bullseye, Float, Oceanside, Wissmach, Youghiogheny, etc), but the colours used.  This should include the manufacturer’s code numbers to enable you to replicate the glass used.

Lay up
This can be a description, a drawing or pictures of the set-up of the piece prior to firing.  This is vital to later understanding what you did in this firing.  Record any glues or stabilising elements you use. Any frits or powders used should be recorded. The placement in the kiln is important - centred, one corner or another, level/ height in kiln, etc., can affect the results.  You can make a sketch or take a photo to attach to the record rather than writing separate descriptions. How it comes out is recorded later.

Dimensions
The dimensions (h x w x d) including any variations in height are needed to compare with other projects.  This might be included in the lay-up diagrams or pictures, but it is most useful to have the dimensions and their variations recorded as numbers too.  You might think in terms of layers, but remember to record the thickness of each layer/piece (e.g., 2mm, 3mm, 4mm, 6mm, etc)

Kiln used
This is especially important if the kiln is not yours. Every kiln has variations and it is important to compensate for that in scheduling and placing of the piece in the kiln.

Process
This is essential in gaining an understanding for planning any modifications.  The process can be described by standard terms - e.g.,  sinter, slump, tack, contour, full fuse, casting, melt – or by your own terminology (if it is consistent).

Description
A statement of your project and aims is very useful for the future.  It is a reference point to use in comparing what you wanted with the results of the firing.

Support system
This includes essential information affecting the firing – shelf type (e.g., fibre, mullite, ceramic tile), mould type (e.g., ceramic, fibre, steel), and a description or sketch including any reference codes.

Kiln furniture. The kind and quantity of kiln furniture (dams, stilts, posts, etc) can affect the firing results, so need to be recorded.

Separators
This includes kiln wash (type, whether new or the number of uses), fibre paper type and amount, mould coatings, and anything else you may use to keep the glass from sticking.

Schedule
This is the thing most everyone remembers to record.  You need to record it each time you use it – even if you have used it many times before.  You need to record each step of the program.

So many times, people report that “it [the schedule] has always worked before”, only to discover that some element had been intentionally or accidentally altered from past firings.  I normally write the schedule in a logbook and then enter it into the programmer. I use the written record to check against what I have entered into the controller.  Then I know I have programmed what I intended.  I can also check on earlier, similar firings to see the variations I have used in the past.

Results
Drawings or pictures of the finished item are essential.  A description of the results is also needed as a picture does not tell the whole story.

Comments on results
You should also give a commentary on the results of the firing.  This should include successes as well as disappointments.  Thoughts for future similar firings should be written down.  They will be forgotten soon, if you don’t.

How to keep all this information
As you can see there are many elements that need to be recorded as they each can affect a firing. I see these as a minimum, and you will add elements important to you for this list.

It does not matter much in what form you keep the information.  It can be a ledger, spreadsheet, database or your phone or tablet that you carry with you always.  There are several apps for recording the kiln firings that can be used.  What is important is that you can record the information immediately, or as you prepare the work for the kiln, into the chosen form of recording.  I use a logbook and convert that in my leisure moments to a spreadsheet (usually at new years day).  This allows me to compare information over time and especially the kinds of firings that I rarely do.  It also allows me to search by various processes.

It is important that you back up any electronically held information to the cloud or other device to protect against loss or corruption. 

Forms
It is useful to have a form for compiling this record.  A number of elements of the records can be reduced to tick boxes to ease the recording.  It helps to remind you of the information you need to log for each firing.  Bullseye have an excellent form that you can use or adapt to your needs. There are a few apps that can be used on phones or tablets which are useful for those who record everything on their phone.  Remember to back it all up to the cloud for preservation in case of loss or damage.

Darkening leads

There are several ways to darken the leads in leaded panels. Three are to:

use patina on the leads,

brush with on stove blackening with a soft brush, and

simply brush after cementing.

A certain number of people use black patina to darken the leads after cementing and cleaning the panel.  This certainly gives a black result, but it introduces an acid to the panel. I do not do this, nor do I recommend it.

Another method of darkening is to apply stove blackening or black oil paint to the panel to make the leads dark.  I recommend that you put very small amounts on a soft brush and then brush over the leads.  It might have to have a little more colour added for a large panel, but that is better to add more than to clean up large areas of smudged black over the glass, especially with painted glass.

credit: PicClick UK

But...

You can darken lead came without patina or black colour.  You finish the panel with the scrubbing brush to push whiting against the fillet of lead light cement against the leads as normal.  This has the effect of cleaning the glass as well as stiffening the cement at the edge of the cames. Remove the excess whiting as normal.

But, before picking out all the excess cement once the scrubbing brush process is finished, use a soft brush, such as a shoe brush, over the whole panel.  This can be mechanised by using a soft bristled mop in a drill motor on a slow to medium speed.  This will pick up colour from the cement and spread it evenly over the lead and solder joints. It will give a dark grey appearance to the whole of the leading and solder joints as well as polish up the glass. 

The degree of shine will be dependent on the amount of time you wish to spend, but can be a polished to a very dark grey to black colour.  This will last longer than simple black colour brushed onto the leads, as it is bound by the linseed oil in the cement to the surface of the leads. Also, it quickly dries so that not so much black is transferred to your hands as you handle it.

Making lead cames black during the finishing of a leaded panel is as simple as brushing over the cames before picking out all the excess lead light cement.

As Fast as Possible Firings

I have long advocated that it is best to avoid as fast as possible firings because the way controllers work.  They compare the temperatures several times a minute (the number depending on the manufacturer) to determine the rate of increase.  This allows big overshoots at the top temperature with fast rises.  This was reinforced this morning by observing a different factor.
 
I took a piece out at 68°C to put another in.  During the time the kiln was open, the air temperature dropped to 21°C.  I filled the kiln and closed the lid and idly watched the temperature climb before switching the kiln on for another firing.  It took a bit more than two minutes for the thermocouple to reach 54°C with the eventual stable temperature being 58°C.  I had not been aware how long it takes the thermocouple to react to the change in temperature.  Yes, it takes a little time for the air temperature in the kiln to equalise with the mass of the kiln, but not two minutes.
 
With a two-minute delay the recorded temperature can be significantly behind the actual air temperature.  For example, a rate of 500°C per hour is equal to 8.3°C (15°F) per minute or 16.6°C (30°F) overshoot of the programmed temperature. Even at 300°C it is a 10°C (18°F) overshoot.  This effect, added to the way the controller samples the temperatures, means the actual overshoot can be significant for the resulting glass appearance.
 

This is just another small element in why moderate ramp rates can be helpful in providing consistent results for the glass.

Contacting the Wholesaler

Credit: lunchshow.co.uk

Preparation

Whether preparing for a trade show or direct approach to a wholesaler, you need to get things to a good standard before making contact.

This can be a lot of work, but it will benefit your sales across all the venues you have work placed.  Wholesale gets your work out to multiple locations, develops your relations with shops and, by extension, to communities and to customers that you would not be able to reach by yourself.

Presentation materials

You need to have a clear identity to your work – something which binds the separate pieces together, making it clear that it is your sensibility that runs through the works.

You need to have a pricing strategy that runs through your range of products.  This will be connected to the target market that you have identified.  This is important to getting your work taken up by wholesalers and stores.

Excellent photographs of your work are needed in all the literature you produce for the buyers. Photography can make all the difference. Include photos that highlight your story and integrate with your products.  These should be consistent across all your materials.  They should have a unity of style whether shot on models, on background (normally plain white or black), and with props that support the story of your work.

Line sheets

credit: sewport.com

Line sheets are simple listings of each of your products with a code, title, sizes, and prices with each product line and variation on its own line on the paper. Create simple, readable line sheets with clear instructions on how to order, minimum order levels for discounts, and all your contact and banking details. If you have a required means to get in touch, make sure that is included on the line sheet.  The line sheet is essentially an order form which each of you will have a copy, and from which you will create the invoice.  The agreed payment terms, including supply dates should be written on the line sheet when agreed.

Information on product and maker

Write the material for promotion of your work and yourself clearly and concisely.  Start with the most important information about the business and products. Details and methods should come much later.

Be consistent in the way you describe your products.  Always consider the target market.  The wholesaler will be much more dispassionate about the products than the ultimate buyer will be. The descriptions show the trade what your target customer is like and so they can see what the fit between your work and the shop’s offering is.  Use the concepts and words that are familiar to the ultimate customer. 

Review your literature many times, proofread, and finally get a friend to look over the materials for style, spelling, consistency and accuracy.

Your complete contact information is required on every piece of printed material.  It also is needed on all online and email communications.

The meeting

Do the work to know who you are contacting by name and job title.  Use the person(s)’ names, refer to the business, store or shop, indicate you have followed any blog or social media postings, and if relevant, be knowledgeable about the local area.

Be selective in what of your work you present to the buyer. You don’t have to have all your lines of work in shops.  You probably could not cope if you did.  Consider what you can supply in quantity that will fit with the shop’s offering.  This will relate to the materials you can source in bulk (for discounts), and what you can produce quickly and easily.  To offer the best prices at a profit, you will need to determine processes that can be streamlined; designs that can be simplified; processes that can be done by less skilled people.

Remember the buyer’s interests during the meeting

They are looking for a range of work that has wide appeal – to both/all genders.  A price range for different works to appeal to a range of customers is needed. 

You need to demonstrate you know what is going on in the design and crafts field and can respond, keeping your offerings fresh and contemporary. Colours, themes, shapes, interests change, and you need to show you are aware of current trends.

Owners are looking for exclusivity.  Store owners want to sell items that no other venue in their area has available.  It is arguable that the more of your work a shop sells, the more exclusive the two of you should become.

Part of the appeal for the shop is signed and dated pieces.  It adds to the caché of the work.  This should be done discretely to avoid distracting from the whole of the work. 

Often owners expect not only well-made items, but displays too.  At the least, you should have presentation boxes that show off your work well.

credit: theproductmart.com

Trade Shows

The alternative to trekking around shops is to attend trade shows.

Trade shows are a place to make contacts – shop owners, fellow crafts people, and representatives.  Yes, you are there to get orders, but the people you meet may be your future customers. Your contacts might help you understand the market better, or move your work in different directions.  It is a place to gauge how your work is perceived, and what you might change or re-enforce.

Networking can help you in gaining new contacts, and even friends in the crafting community.  Visit other stands and arrange to have a coffee break during the show with those you find compatible. You can compare notes on the show or general business stories with those who are not in direct competition.

Have your product display in evidence at the show.  If that is not possible for some reason, have photographs of it in your literature that you can hand over. This enables potential buyers to see how your work might be displayed in their shop.

Take material to the show that you can hand out to prospective purchasers or representatives.  This could be inexpensive samples, the essential business cards, literature, and of course, the line sheet that you complete with the orders at the show, or be taken away if you cannot get them to commit immediately.  Be sure you get material from them too and that you record what they were interested in, so you can contact them after the show.  A contacts book for you to keep information in is essential.

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Selection of a gallery or shop in which to place your work is a complex interaction of commission levels; the value you place on your time in preparing for and attending craft fairs or putting your work online; the perceived prestige of the shop/gallery; the potential relationship between you and the outlet; and the relationship of the consignment, wholesale and retail prices.

Sintering

This is a process used in glass to stick glass together without any change in appearance of the separate pieces.  It has various names - fuse to stick and lamination are two.

General description
“Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it…. Sintering happens naturally in mineral deposits [and] as a manufacturing process used with metals, ceramics, plastics, and other materials.

“The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum….
 
“An example of sintering can be observed when ice cubes in a glass of water adhere to each other, which is driven by the temperature difference between the water and the ice.”
https://en.wikipedia.org/wiki/Sintering
 
Applied to glass this means that you can make a solid piece out of multiple touching or overlapping pieces that do not change their shape.  This is done by using low temperatures and very long soaks. 
 
The usual process is to take the glass at a moderate rate up to the lower strain point.  The rate of advance is slowed to 50°C or less to a temperature between slumping and the bottom of the tack fuse range.  The operator must choose the temperature, largely by experimentation. 
 
The slow rate of advance allows a lot of heat work to be put into the glass.  This, combined with a long soak (hours), gives the molecules time to combine with their neighbours in other particles.
 
Sintering can be done in the range of 610°C to 700°C.  The lower limit is determined by the strain point of the glass being used and practicality.  

The upper limit is determined by the onset of devitrification. This  has been determined by the scientific studies of sintered glass as a structure for growing bone transplants.  Devitrification reduces the strength of the bonds of the particles at the molecular level.  These studies showed that the onset of devitrification is at 700°C and is visibly apparent at 750°C regardless of the glass used.  Therefore, the choice was to use 690°C as the top sintering temperature. 
 
For reasons of practicality the lowest temperature tested was 650°C.  Indications were that at least an additional two hours would need to be added to the sinter soak for each 10°C reduction below 650°C.  This would make for a 12-hour soak at 610°C.  For me this was not practical.
 
My recent testing has indicated some guidelines for the sintering process:
 
The ramp rate has significant effects on the strength of the resulting piece. 

• A moderate rate (150°C) all the way to the sintering temperature needs a two-hour soak at the top temperature. 
  
• A rapid rate (600°C) - as used in medicine – to the sintering temperature requires approximately six-hours soaking.
• A rapid rise to the strain point followed by the slow 50°C per hour rate to the sinter temperature requires a three-hour soak.

 
The temperature range of 610°C to 700°C can be used for sintering.  The effects of the temperature used have these effects:

• With the same rates and soak times, lower temperatures produce weaker glass.
• The lower the temperature, the longer the sinter soak needs to be for similar strengths.  Generally, the soak at 650°C needs to be twice that of sintering at 690°C.
• Lower temperatures produce more opaque glass.  In this picture all the glass is clear powder and fine frit in the ratio 1:2, powder:frit.

 

The annealing of sintered objects needs to be very cautious. The particles are largely independent of each other, only joined at the contact points.  The annealing soak needs to be longer and the cool slower than for simple tack fusing. 

• Testing showed that annealing as for 12mm is adequate. 
  
• There was no advantage of annealing as for 25mm as that did not increase the strength.

 
Porosity
Although the structure of the sintered glass appears granular, it is not porous except at or below 650°C.  At the lower temperatures, the glass becomes damp on the outside and weeps water.  At 670° and 690°C the outside became cool to touch but did not leak water.  This observation depends on evenly and firmly packed frits.
 

Grain structure at 650C

Grain structure at 690C

The keys to successful sintering of glass are the use of a heat work through slow ramp rates, and long soaks throughout the whole firing.

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

Craft and Wholesale Pricing

Craft fair pricing 

Image credit: craftprofessional.com

An important element to be considered in setting prices for pieces at a craft fair that are also consigned to a shop, is that the price the shop is selling your item at, is also the price you should be asking at craft fairs.  Without doing this you risk losing the consignment arrangement with shops and stores.

Also remember that when attending craft fairs or pop up shops, you are doing the setup, travel, marketing, sales, etc., that the shop is doing in a commission arrangement. That cost needs to be reflected in your price for the item at a craft fair, even if different work is being offered.  Especially if the craft fair is in the region of any of the shops or galleries you have placed similar items, you need to be fair to the shops in your pricing to be able to continue supplying them.  The same principle should apply to your online sales.

Some people solve this problem by having a craft fair range and a consignment range.  The craft fair items can be offered at a slightly lower price, if you must. But remember to factor in the costs of craft fair and pop-up shops even when pricing craft fair lines.

Wholesale pricing

Image credit: erplain.com 

Also note that your price on consignment needs to be high enough that you can take wholesale orders without losing money.  Wholesale orders are where the buyer pays you for the work in advance or upon delivery without taking a commission. They normally expect to buy at a lower than commission price and multiples of the work you are offering.

Maybe you feel you do not want to do wholesale work.  Stop a moment to consider that instinctive reaction. Wholesale means you have made the sale already without having to wait for the ultimate sale to occur, as you would on consignment. You do not have the administration of keeping track of stock in various galleries. You have certain, and almost immediate income. All these things make wholesale attractive.

It is the expectation that the wholesale price will be half or less of the retail price.  To make the piece affordable to the wholesaler and for you to still make a profit, you need to add something to the consignment price to enable the items to be sold by the wholesaler and the consignee at similar prices. This is most often less than the full difference between wholesale and consignment prices because you receive the money up front.  Cash can mean quite a lot.  You also need to have enough cushion to be able to give a discount on orders for multiples of the piece.  And that means you need to set the minimum order number to get that discount.

The bottom line (as they say in accountancy circles) is that you need to set the wholesale price first and then work back to the consignment price and retail price.

The wholesale price should be about 10% above the absolute minimum price where you can make the desired profit.  This enables you to offer that amount of discount for orders of multiples of your work.  This then implies the retail price is twice the wholesale plus taxes.  The consignment price will be about 70% to 60% of the retail price (based on 30%-40% commission levels).  More realistically, you can visualise the consignment price to be 10% to 20% higher than the wholesale price, leaving the shop to add their percentage on top of your price.

Image credit: seobook.com

Selection of a gallery or shop in which to place your work is a complex interaction of commission levels; the value you place on your time in preparing for and attending craft fairs or putting your work online; the perceived prestige of the shop/gallery; the potential relationship between you and the outlet; and the relationship of the consignment, wholesale and retail prices.

Firing multiple layers

Glass Stela
Credit: Stephen Richard

Fusing multiple layers is prone to the creation of multiple large bubbles.  It also needs a strategy to schedule for thick layers.

Avoid bubbles

A widely recommended strategy for stacks of glass is to fire in pairs of layers. Then combine the fused two-layer pieces in a final firing. 

It is easier to fire two layers of glass than 6, 8 or 10 layers. The heat up is easier and less time consuming for multiples of 6mm than multiples of 3mm. The bubble squeeze schedule is simpler.  It also allows inclusions between the initial two-layer sheets and then between the layers of 6mm sheets.

This multiple firing strategy reduces the risk of large bubbles in a stack of multiple pieces. It seems the weight of the 6mm layers forces the air out from between the thicker glass more effectively than thinner layers. 

It is also a simpler set of firings.  If you were to want to make up a 12mm thick piece from 3mm sheets, your heat up will be very long compared to firing two layers in three firings.

E.g. Stone* recommends a heat up for 2 layers of 3mm glass:

240C/hr to 250C, no soak

400C/hr to 500C, no soak (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak)

500/hr to top temperature.

This is about 2.3 hours to top temperature without the bubble squeeze and 6.7 hours to cool.  This means that you could fire twice in one day, if organised well.  If you are planning a final tack fused layer that should be done in the last firing of the combined layers.

However, it is a much longer schedule recommended by Stone for 6 layers of 3mm glass:

• 25C/hr to 125 for 20’
• 30C/hr to 250 for 20’
• 40C/hr to 375 for 20’
• 50C/hr to 520 for 15 (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak before continuing at the same rate to the top temperature).
• 150/hr to target temperature

This is about 18 hours to top temperature without the bubble squeeze and another 18 hours to cool.  This strategy requires 1.5 days, assuming all the layers are even.  The same amount of time is required for both strategies, but the chance of large bubbles is dramatically reduced.

He recommends for 3 layers of 6mm glass:

• 200C/hr to 250, no soak
• 340C/hr to 500, no soak
• 400C/hr to 600, no soak (a bubble squeeze could be introduced here by changing the target temperature to 650 with a 30-minute soak)
• 500C/hr to top temperature.

This is about 2.5 hours to top temperature and 18 hours to cool without the bubble squeeze.

This means that it only takes 2/3 of the time to fire 3 layers of 6mm glass than it does to fire 6 layers of 3mm glass.  Yes, you lose some time in firing the pairs of 3mm glass, but you gain in reducing the risk of creating large bubbles that will ruin your final piece.

Inclusions

If you are putting elements between the initial two-layer pieces for fusing, you need to introduce a bubble squeeze.  Putting elements between the fused pairs will also require a bubble squeeze on the final firing.

Tack fusing the final layer

Note the times indicated above are for even layers.  If you have uneven layers or are tack fusing, the times will be extended much further than the ones noted there.

For a tack fused set of top layers, you will need to add those in the last firing, or do a sharp tack firing before the last firing.  In the case of a tack fused pair for the top layers you will need to reduce the rates of advance for the last firing by about 1/3. This would mean:

• an initial rate of 135C,
• a second ramp of 230C,
• a third of 270C and
• the fourth of 335C instead of the rates for even layers. 

You will also need to reduce the top temperature.  Observation will be required to determine when the correct profile has been achieved.

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

When firing multiple layers of glass, the risk of creating large bubbles can be reduced by firing pairs of 3mm sheets, and then combining the results into one stack.

*Graham Stone. Firing Schedules for Glass, the Kiln Companion, 2000, Melbourne Australia.  ISBN 0-646-39733-8

Maintaining Consignment Relationships

Stay in communication.  There are several ways to do this.

photo credit: Careers in Sport 

Get on their newsletter list and put them on yours.

Check with them occasionally as a reminder you are around.  Some pretexts for making contact are: you may need to be paid or want to know what is selling or not; to check on stock levels; you may also offer promotional opportunities and want to know what they would like to be put forward.  But do this remotely (email, telephone, etc). An in-person visit should be done by appointment.

Make sure you can restock when the shop needs it. And inquire regularly if more stock is needed.  Find out their delivery hours and stick to them. Restocking requires an inventory list.  This is also the time to bring any new lines you have developed.  Ask if it is OK to bring it before visiting.

Remove old stock. Placing new or seasonal items in the shop can help with sales.  You can get advice from the shop.

Promote the shop.  Use your social media, include them in your list of outlets.  This mutual promotion gives rewards of increased visibility and with determination, sales.

Be inventive. Create ways of communicating your continuing interest in the mutual business benefit both parties receive.  Use anniversaries, local events, things unique to their business, etc., as occasions to be in touch without any commercial objective.  There are lots of creative ways to be in touch.

Selection of a gallery or shop in which to place your work is a complex interaction of commission levels; the value you place on your time in preparing for and attending craft fairs or putting your work online; the perceived prestige of the shop/gallery; the potential relationship between you and the outlet; and the relationship of the consignment, wholesale and retail prices.

An important element of maintaining relationships with your galleries is keeping in contact, being interested in how their business is, and responding to their communications.

Other posts on consignment:

Consignment

Consignment Agreements

Consignment Rates

Consignment Venues

Recovering from Devitrification

An explanation of what devitrification is, can be found in the link.

Mild devitrification is generally a smeary appearance on the surface.  Most often this can be corrected by either removing the surface, adding a flux or putting another surface over the piece.

mild devitrification
photo credit: Bullseye Glass Co.

Removing the devitrified surface

Sandblasting and grinding are two common methods of removing the surface. If you have access to a sandblaster, this is the easiest method of removing the surface.  You can remove the surface with manual methods too.  You can use wet and dry sandpapers, starting with coarse ones and proceed through grades to at least 400grit (0.037mm).  The flexibility of the sandpapers is that they can conform to uneven surfaces that tack fusing provides, to remove devitrification in depressions as well as the high bits. Diamond hand pads and sheets do the job more quickly, but are more expensive.

Acid etching is another surface removal method. There are various etching creams on the market which will remove the surface. You need to apply and leave for a long time to allow the acid to work on the glass surface.  It is best to keep the acid paste damp to enable the acid to work over a long period.  A piece of cling film will work well.

Making a new surface

You can provide a new surface by using devitrification sprays.  There are both commercial products and do it yourself ones that work.  The do it yourself product is a borax solution.  The method for making the solution is given here.

Borax powder

You also can give the devitrified surface a new one by covering it with clear powders.  Powders sifted evenly over the surface until there is a thin covering over all the piece will give a new surface concealing or covering the devitrification.  Fine frit does not work so well, as more needs to be sifted over the surface.  This will not be applicable to tack fused pieces, as the whole piece needs to be taken to a contour or full fuse to make sure the powder or frit is completely smooth.  This will make the tack fused areas flat.

Left to right - devitrified surface, powder covering, fired piece
Photo credit: Bullseye Glass Co.

When dealing with devitrification, the whole of the surface should be treated, not just isolated areas.  Treating isolated areas will most probably leave a difference in appearance between the treated and untreated areas.  It is not worth the risk of having to fire yet again.

Dealing with devitrification usually involves removing the devitrified surface or making a new one.

Bubble Squeeze Temperature

“My bubble squeeze temperature is higher than my slumping temperature.“ The writer goes on to say that their bubble squeeze is at 1250°F/676°C vs. a slumping range of 1150°F/620°C - 1175°F/634°C.

I applaud the writer for doing the slumping at the lower range of slumping temperatures. This allows the glass to relax into the mould with fewer marks being picked up.  The temperatures might require significantly long soaks depending on the span, depth, shape, weight of glass, etc.   But it is a good practice to get work done at as low a temperature as practical.

 But...

There does seem to be a misunderstanding on how a bubble squeeze works. Like most things with glass, any process works over a range of temperatures.  Bullseye glass begins to soften about 540°C. This continues to about 680°C where the transformation range begins – that is, the glass is behaving more like a viscous liquid than a softening solid.  A bubble squeeze or a slump can begin anywhere in this 540°C to 680° range.  At the lower end of the range, any slump will take “forever”.  At the top end, some slumps may occur too quickly and have mould and stretch marks on the bottom. 

Credit: Fusedglass.org

However…

This note is about the relation of bubble squeeze to slumping temperatures.  If you can slump an item at 620°C, you can also perform a bubble squeeze at that temperature.  Both processes rely on the glass becoming “soft” enough to relax into the shape below it.  It may be that you will need a very long soak to press out air in a bubble squeeze at 620°C, but it can be done if you are willing to wait a long time.  

Many people begin their bubble squeeze at 620°C for fusing glass with a soak.  I am not sure that a soak is required at this point, as slowing the rate of advance over the next 50°C will have the effect of increasing the heat work the glass receives without the need of a soak at the beginning of the bubble squeeze ramp – unless you have a rapid rate of advance toward the bubble squeeze.  

They then progress slowly (maybe 50°C or less, depending on thickness) for the next 55°C to 60°C and soak at that higher temperature for half an hour, or more for difficult shapes.  This additional heat work allows the glass to gradually become more plastic and deform more slowly than at a higher temperature bubble squeeze.  This is often called a cautious bubble squeeze, since it starts at a lower temperature and moves gradually to the top of the bubble squeeze range.  It removes the single shot bubble squeeze at a higher temperature, when air might already be trapped. 

In general terms, the slump can be carried out at or below the softening point of the glass.  This softening point is also the maximum temperature for a bubble squeeze. For example, float glass has a softening point of about 720°C, so a bubble squeeze and slumping can be in the 660°C to 720°C range.  Some glasses have even higher softening points, and others have much lower softening points than Bullseye or Oceanside.

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