Rokosz Optical

Since the article on fused blanks first came out in S&T '84 numerous points have come up that need revision.

It is now Dec 19 2013.It has been some 30 years since the 14 incher was fused.It is still in one piece and in use and as far as I can tell has no figure change.Bit surprising considering the anneal was done without a computer controller,just three hand dial switches.What saved the mirror as well was that it was a "small mass" item in a much bigger hot chamber.The kiln had a chamber size of 30 inches square by 12 inches high.Do not try to "over mass" the interior space of your kiln.

The main consideration is that commercial potter type kilns are not well suited for fusing mirror blanks.A kiln better suited is what is termed an annealer kiln.The difference is in the layout of the heating coils within the chamber. Annealers have the coils in the top and sometimes sides.Potter type kilns only in the walls.The ideal  is  having coils in all surfaces - top,bottom,and sides.Potter types can be modified.It is very important to have an even as possible thermal heat gradient inside the chamber.If heat only comes from the sides the central areas of the glass will not have the same temperature  and this will affect the fused quality in that area versus the outer edge areas plus the overall glass anneal will be uneven.

As stated in the S&T article each kiln is different and you need to run some small scale experiments to get a feel for how your kiln "works". It needs to be stressed that once the fusion temperature has been held long enough to fuse the glass  one needs to open the lid several times in rapid sequence and dump the temp down to just above the anneal range as quickly as possible.(Wear protective infrared eye guards because of the infrared radiation from the kiln.)Deciding how long to hold the glass at the fusion point is part of the "art"  and only comes from repeated small test examples in your kiln.

Because of the need for this massive dump of heat I don't use any muffle type blocking of the lower side wall heating coils.The extra mass of the muffles only makes the job harder to rid the chamber of the now unwonted heat and to stop any more slumping of the glass components One could use a muffle but the fuse time would have to be included in the time lag..My home made kiln has a top lid (or top loader)layout.

As for the internal glass supports made from firebrick they don't need to be made from/as one piece.You can make a production type setup of similar parts.Firebrick is not the only material that can be used.

Annealing is a very important topic.One needs to search the web and get a handle on the approach and technical aspect  critical to a good anneal.The one critical point on fused ribbed mirror is the concept of"angles of reentry" meaning the various sharp differences in glass thickness and shape at the edge of the mirror.This aspect of ribbed mirrors means they need a much longer anneal than is seen in most anneal charts ( as at Bulleyes glass chart for thick glass pieces.)The short answer is go long ..very long.Also consider the type of material added into the kiln besides the glass - molds,supports etc.Stay away from any very hard dense kiln shelf material.Heating coils in the bottom of the kiln is very critical,as in monitoring the internal temp with more than one thermocouple.You wont to measure the temp at two or more different points inside the chamber one of which is as close to the glass as possible, even touching it .

Bulleye's web site has some very interesting technical articles on annealing,one of which - Tech Note 7 is critical for a successful anneal. The variation in temperature differences within the kiln chamber combined with the ramp down temperature affects the resultant stress level in the glass and the degree to which the mirror will hold a final optical figure.Too much stress within the glass will limit the optical surface wave rating the mirror will hold before springing back to it's old shape over time.

Annealing is very much an "art" as each kiln (shape,material coil placement,thermo couple placement) has it's own character and needs unique consideration.

To control the overall anneal one needs a digital controller. One that allows zone control with multiple thermocouples to vary the temp ramp up - soak - and ramp down temp in a controlled way can also be of help...A critical feature one needs to look for in controllers is the number of "segments" allowed in the overall heating/cooling cycle.The greater number of segments the better.Hobby type controllers have from 6 to 8 segments whereas a more industrial controller can vary between 30 to 60.This is important because the greater number of segments allows for a much slower ramp down of the temp from the anneal range.The more severe "angle of reentry"  (odd angles or severe variations in thickness) within the overall glass shape/form then the slower ones needs to cool it.