(This article was written for Amateur Telescope Making Journal in 1998. As of 2000, the glass kiln is being used for other projects. This article is posted in the hopes that it will show the feasibility of building glass kilns for amateur telescope projects. However, it is a very complex & difficult project, and the safety issues must be addressed. --Peter Abrahams) Steve Stadleman of Portland, Oregon has begun casting glass blanks for use by mirror makers. Steve converted his garage into a glassworks, using commercial wiring of 400 amps to supply 22 kilowatts of resistive heat to a kiln. The home-made kiln is built of structural steel and refractory brick, with internal dimensions of 44 by 44 by 18 inches, permitting a 40 inch blank. There is a computerized electronic controller wired to thermocouple pyrometers in the kiln. These thermometers are a simple bimetallic junction, resistance is measured a variable number of times per second to indicate temperature. They are the K type of thermocouple, of iron & aluminum, with a ceiling of 2400 degrees F, at which point they melt. Steve is a machinist, and built the kiln, with the electronics package built by a friend. Corning 7740 Pyrex, in the form of glass rods, is the source material, and is placed in a circular mold of refractory material. Heating the furnace to melting temperature is done slowly, so the heating elements do not suffer thermal shock. One day is used to get up to the melting point of pyrex, 2150 degrees F. At this point, a 20 inch blank measures 1/16 inch larger in diameter than it will when cooled. This called for some creative engineering from the kiln builder. The molten glass remains at this temperature for a five hour 'soak'. A shorter soak will lead to an increased number of bubbles in the glass. A longer soak wastes time, and in addition, molten glass is reactive and can dissolve refractory elements. Steve uses a very slow annealing process. The heat is very gradually diminished, for with no electricity the blank would cool in about 3 days, which gives a blank that is adequate for non-optical uses. A 25 inch blank, 2 inches thick, is annealed for 9 days. There are three major problems that can adversely affect a glass mirror blank. First, the glass must be homogenous, if it is melted from dissimilar types of glass and not thoroughly mixed, the different expansion rates of the two glasses can cause stress over the thermal cycle of a night. Because these blanks are made from commercial supplies of Corning Pyrex, they are melted from glass rods of identical composition. Second is strain, generally caused by inadequate annealing. There are tables & formulas for the required duration of this process, and Steve is proceeding much slower than required. The blank must be kept at a uniform temperature during annealing, with the top and bottom of the disk at the same temperature. There are thermocouples in the kiln, on the bottom, within the firebrick, 1/16 of an inch from the lower surface of the glass. Thermocouples are placed in contact with the top of the disk when annealing starts. These are monitored & heat is applied to keep the temperature even. Thick blanks are annealed for 10 days to over a month. The glass is cold when placed in the kiln, and cold when it is removed. It is tested for stress by placing it between sheets of polarized material, that are oriented at right angles to each other. Stressed materials show characteristic patterns of dark lines under crossed polars, and the first blanks show no signs of stress. The third problem to afflict mirror blanks is bubbles. The blanks produced to date have submillimeter bubbles in varying quantities. Several experienced mirror makers have inspected the blanks and believe they are suitable for use, and others want to grind one before deciding. Most of the bubbles are near the top of the blank, and the bottom is the side used to make a reflecting surface. Efforts are underway to minimize bubbles in future blanks. There are no bubbles in the source Pyrex, they arise from the air between the stacked rods. Steve is now using the largest available rods, 1.5 inches in diameter. Another possible source material is Schott Borofloat, a glass somewhat similar to Pyrex in sheet form, up to 20 mm thick. A blank melted from this would be bubble free at least 20 mm from the surface. This material will be used in some of the next blanks. By the time this sees print, the blanks will be surface ground, edged, and possibly curve generated. Currently, they are rough castings only. Steve is willing to build another furnace for larger mirrors. There have already been inquiries for 50 to 60 inch blanks. He could also make blanks that are not circular. Tapered blanks (thick in the middle and thick at the edge), and blanks with cellular backs, are being planned. This is currently a craftsman's project. Several blanks have been made available to mirror makers, but commercial supply is not feasible at this date. At this date, blanks are priced by the pound. A 25 inch blank, 2 inches thick, is $980. A 24 inch blank, 4 inch thick, weights 140 lbs. and is $1750. A 36 inch, 3 inches thick, weighing 244 lbs., is $3000. A 40 inch, 2 inches thick, is $2500. (As of 1999, prices are obsolete & production is uncertain). "I knew before I started that I couldn't make a perfect blank, but I wanted to be able to make a very good one, that might be sold at a reasonable cost, and wanted to make them larger than are currently available." Steve Stadleman 503-359-4862 e-mail: sandl@hevanet.com http://www.hevanet.com/sandl/ Notes: Illustration: 18 inch blank under polars one of the first shipped to (Vaughn Parsons) at Intermountain Optics, call: Vaughn Parsons, Intermountain Optics. saw 24 inch 4inch thick. 801-262-0411 home offc. 801-266-8738 There is no definite melting point, just going from 4-5000 poises at melt from millions of poises at solid, viscosity will be about that of cold honey, every time you melt it, some material volatizes ====================== Date: Thu, 07 May 1998 16:35:23 -0700 From: Mel Bartels The couple of Stadleman blanks I saw had minimal and non-objectionable bubbles and grit - very similar to what United Lens put out about 20 years ago. I didn't see any bubbles that would be exposed on a typical f/4 or f/5 curve. Even if they were exposed, they are handlable. I had two bubbles that exposed during fine grinding on my first 24". I just ran a toothbrush over them when changing grit sizes. I suppose this is a matter of taste and judgment. But from my vantage point, his blanks looked pretty good - I would certainly have no hesitation in using them. Mel Bartels ===============