Making convex clock glasses at home

Peter Planapo

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Hi all,

I need a convex clock glass of about 150mm diameter, but the correct size isn't available.

I had a possibly crackpot idea that I could do it like this:

1. cut a disc or circle from flat glass (I have a glass circle cutter), slightly bigger than the required final diameter to account for the convex curve
2. make a concave mould from plaster (using another bigger clock glass as former)
3. heat my glass disc over the mould with a very powerful blowtorch (which I also have), slowly reducing the temperature, over a few minutes, after the glass has sunk into the mould for annealing.

I've read that this can be done in an oven (it probably is, commercially), but the temperature would need to go to at least 1100C or 2000F so the domestic oven's out, and most commercial ceramic kilns fall short as well. Also, of course, I'm not about to lay out huge sums of money on a professional kiln for one or two clock glasses.

I wonder if anyone's tried this. Perhaps they could give some pointers, like e.g. what kind of plaster could be used, plaster of Paris or possibly potter's clay? And anything else a newbie ought to know. The only glassworking I've done was making capillary pipettes in chemistry class, but that was a while ago.

Many thanks,
Peter
 

bruce linde

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I think Glass starts to soften and be pliable at around 700°… But you might want to send an email to my friends at AdamsChittenden.com. They make all kinds of custom glass pieces for everything from wineries to scientific labs to large tubes to hold mercury in antique clocks (insider info!)

try asking tom at info@adamscittenden.com ... tell him bruce sent you
 

bruce linde

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Jim Hartog

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Hello Peter,

You might consider buying a convex glass that is slightly too big and grind the edge down to get the diameter that you need. May take a while if you need to remove more than a millimetre.

Jim
 

THTanner

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Clock Glass slight convex(curved)diameter(5.9" or 150mm)

This site sells similar items to clock glass for gauges that fit the VDO products. I am not sure where they get the glass and often the convex curve is not uniform on the gauges since it comes out a bit then goes flat across the center. But perhaps they have others that might work for you:???:

(sorry - tried to post the vendor, but the item came up)
 

Ralph

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I see at least one 150mm glass on an auction site you probably know about.

Ralph
 

Shipsbell

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Hi all,

I need a convex clock glass of about 150mm diameter, but the correct size isn't available.

I had a possibly crackpot idea that I could do it like this:

1. cut a disc or circle from flat glass (I have a glass circle cutter), slightly bigger than the required final diameter to account for the convex curve
2. make a concave mould from plaster (using another bigger clock glass as former)
3. heat my glass disc over the mould with a very powerful blowtorch (which I also have), slowly reducing the temperature, over a few minutes, after the glass has sunk into the mould for annealing.

I've read that this can be done in an oven (it probably is, commercially), but the temperature would need to go to at least 1100C or 2000F so the domestic oven's out, and most commercial ceramic kilns fall short as well. Also, of course, I'm not about to lay out huge sums of money on a professional kiln for one or two clock glasses.

I wonder if anyone's tried this. Perhaps they could give some pointers, like e.g. what kind of plaster could be used, plaster of Paris or possibly potter's clay? And anything else a newbie ought to know. The only glassworking I've done was making capillary pipettes in chemistry class, but that was a while ago.

Many thanks,
Peter
Sounds like an interesting experiment, I don't know if plaster of paris would work but what do you have to loose but time.
If you do this report back. Patrick
 

Peter Planapo

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try asking tom at info@adamscittenden.com ... tell him bruce sent you
Thank you Bruce, I've had a look at the impressive website and sent an email.

You might consider buying a convex glass that is slightly too big and grind the edge down to get the diameter that you need. May take a while if you need to remove more than a millimetre.

Jim
Good idea Jim, I have in fact tried that in the past and managed to crack the glass which wasn't cheap. Probably I was clumsy and the wheel too coarse. If I do it again I'll be gentler.

This site sells similar items to clock glass for gauges that fit the VDO products
Thank you, good tip. The shipping cost would have been $15 USA-UK but the seller kindly pointed me at stuff that's locally available, specifically laboratory watch glasses that come in 150mm at £5.85 post free.

I see at least one 150mm glass on an auction site you probably know about.

Ralph
I don't remember the exact size of my broken one, but it wasn't at the time available. There are one or two sellers who sometimes have a good selection but some sellers are very expensive, and as often as not the exact size is greyed out (out of stock). One can usually get away with +/- 1mm but not more in my experience, it either won't go in the rim or it falls through.

If you do this report back. Patrick
I shall certainly do that.
 
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leeinv66

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I also would be interested to hear how you get on with this. Glass to me always seems like such a mystical material that only the experts in the field know the secrets to.
 

woodlawndon

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I also would be interested to hear how you get on with this. Glass to me always seems like such a mystical material that only the experts in the field know the secrets to.
Me too. The place I worked at needed very specialized glassware for chemistry and needed it often so we had a glassblowing lab. These guys were very specialized at their craft and hard to find. The best craftsman was a Polish guy, a magician that could create anything with glass that an engineer dared to dream up. I used to spend time in the lab just to watch these guys work. I'm retired but have often thought about seeing what they thought about creating stuff for my clocks.
Don
 
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Cheezhead

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Before you struggle with glass, you might try making a clock "glass" from clear acrylic plastic. Use your plaster mold as described and use a hair dryer to heat the plastic to conform to the mold or else use the oven in your stove. Harbor Freight has a cheap heat gun if you prefer, much hotter than a hair dryer, might even soften glass. If acrylic does not work as you want, it will cost nearly nothing.

When a window in my house breaks, I replace it with clear acrylic. The sun has not affected the acrylic windows over the years and I can not see any difference. Another benefit in addition to resistance to breaking is that the plastic is a better heat insulator than glass.
 

Peter Planapo

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Before you struggle with glass, you might try making a clock "glass" from clear acrylic plastic.
Thanks for the idea, and I'm sure it would work. I'd do it on a modern clock but I think I'd rather not have plastic parts on a 150 year old clock, especially visible parts. And I'd have to buy the plastic. The first thing I'll try, I think, will be just to see if a piece of my 2mm glass will soften and sag, or crack and be rendered useless, under a blowtorch. There will be technique to learn; I have plenty of glass.
 
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gmorse

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Hi Peter,

I think you'll need to find a way of bringing the whole piece up to roughly the same temperature at once, to avoid cracking.

Regards,

Graham
 
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shutterbug

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Sounds like an interesting experiment, I don't know if plaster of paris would work but what do you have to loose but time.
If you do this report back. Patrick
With that kind of temperature, I think you'd want to try it outside. I think it would burn the mold.
The idea of a larger convex glass cut to fit is the most practical suggestion. A diamond cutter with lots of coolant would be needed. Find the center of the lens, mount it on something like a lathe to turn it, then introduce the cutter and grind it to size.
The cutter could be mounted on something like a Dremel tool, and the tool mounted on something that could control the feed rate.
An inventive mind could construct everything needed except the Dremel tool and cutter :)
 

Evernia

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Wouldn't your glass circle cutter work on this level of concavity rather than grinding all the excess away?
 

shutterbug

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A glass cutter is an iffy alternative. The cutter should only make one scribe mark, and then it has to break evenly. In a circle, that becomes pretty hard.
 

Jim Hartog

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Hello Peter,

When I grind glass (which is seldom), I grind parallel to the edge, not perpendicular. Diamond hand files are not that aggressive but take forever.

Jim
 

Jim DuBois

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Making your own convex glass in a home shop by either remelting it over a form or by edge cutting or grinding an oversized glass...neither is likely to work for those of us who don't do these sorts of things often. Glass melts at something like 1300-1500 degrees F. Even if we could get the glass to that temperature and it reforms to the convex shape desired, it would have to be cooled slowly and in an even way across the glass. Uneven cooling or too fast cooling will result in cracked or shattered glass. Trimming the edges of an oversized blank is also a difficult process to cut well/properly and not break the glass. Very slow to grind it and also likely to crack if not done slowly and carefully.
 
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shutterbug

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You really need a steady stream of coolant on it as you grind. After grinding, the rough edges should be smoothed out with a very fine diamond wheel or a ceramic wheel. That amounts to a small bevel on the sharp edges.
 
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