On April 4 I started to blow glass, but the furnace was acting up. Ramping from 1900 to 2100, it went to 2000 and then really didn’t get any hotter. The temp readings were acting up and not settling down, so I set it back to 1900. April 5 I tried again. This time I was watching and saw the temperature reading go past 2000 no problem, but at about 2050 it started to “go unstable”. Eventually it read UUUU which means “no reading, upper limit”.
The only reasonable causes were; broken thermocouple, faulty wiring or connections, or controller failure. The sane response was to shut the furnace off and do a complete check of all components.
After turning the furnace off, I noted the crucible was welded to the maintenance lid by spilled glass. It would not budge, even hot. In an effort to free the crucible from the lid, I blocked the maintenance lid up a bit (about 1/2inch) and left it.
Sure enough, when I returned 12 hours later the crucible was free.
Once the furnace was completely cold I removed the crucible, lids and alumina board to have a look. The crucible is in excellent shape, though full of glass. The kanthal heater wires also look in perfect shape, which is amazing for the age of the furnace. The lids were all good, but there was glass on the top lip of the crucible and on the bottom of the rammable gathering ring. This was what welded the crucible to the ring and thus the lid.
I’ll have to remove the glass carefully so the crucible doesn’t weld next time I run hot.
The next steps are to inspect all the wiring and connections, and then to make sure all connections are tight. The most likely cause of the temperature readout issue is a loose thermocouple connection as these are usually pretty robust if not touched. Only if the connections are all tight will I start further tear down.
My action plan for the late spring (May-June) is to first check the wiring connections. Second is to remove the grog in the base of the hot box and sieve it so it’s clean, then reinstall the two lids and take the furnace up to 2100F to check the wiring and controller. With the crucible out of the furnace, this can be done much quicker as the crucible is the limiting factor on temperature rise.
While this is going on, I’ll also clean up the gathering port ring and crucible lip. If the heating test is good, I’ll install the ring and crucible to the again cold furnace and start it up.
With the furnace cold, I removed the broken lid and had a look.
The metal is actually thick sheet metal, and in good shape, so no change needed. The brick lid was broken into four pieces, so was discarded.
The new lid was about 1/2 in too long as was the last lid, so I cut the new lid down to size, and now it fits perfectly without distorting the sheet metal. I cemented the cut face with two coats and cured it in the annealer at 100F. I then installed it in the metal frame. As it turns out, painting the fame clogged the fastening clamps, making it very difficult to properly secure the brick in the frame. In the end a set of pliers and a ratchet got the job done.
The furnace is clean, crucible vacuumed out and all is ready to go hot.
Now that the furnace is cold, I’ve had a good look at the lid. In addition to trimming the bricks back to 13.5 x 13.5 inches, I need to replace the metal strip holding the bricks in place.
The original plans from Mark required 4in galvanized sheet metal flashing, which I could not find. I took aluminum flashing and cut it into 4in strips, and that has worked fairly well for years. I’ve replaced parts of it at least once when the tightening mechanism tore the thin aluminum, but it seems to need replacement again as it’s been deformed by the too-large lid.
This time I plan to obtain some 4in sheet metal, somewhere between 24ga and 30ga, with me leaning towards 24-26 ga at the moment. As I cannot source a roll of 4in sheet metal flashing anywhere, I will try Metal Supermarket and see if they will cut me a few 4in slices off some sheet metal.
Today I made four larger pumpkins, which filled the annealer and pretty much emptied the pot. Afterward, I turned off the furnace, which will now cool from 2100F to around 50F over the next week or so. The furnace is very well insulated, so it takes a long time to cool.
Once shut off, I need to attend the lid that cracked and trim it to fit the steel frame more securely. I plan to modify the current lid (assuming the crack is not too bad) as well as the spare lid I made last year as both were made to the same size, which is definitily too long. Afterward, I plan to restart the furnace in the last half of November, unless the weather is really horrid.
Yesterday I blew glass again, making some larger pumpkins as well as some new color Christmas balls. I recently received my glass order from Sylvie at Colourfusion (in Ontario), and it included cobalt blue (opaque) and a light blue (opaque). The light blue with a cobalt stripe is really nice, as is the straight cobalt c-ball.
I also ordered and received a new 4in optic mold from Steinert Industries. This one is a beautiful ribbed mold that is closed on the bottom. My other molds are open bottom. The nice thing about a closed bottom is that it can be used for small or medium gathers equally well. This enabled me to make both small and medium pumpkins.
On Saturday’s session, the gathering port lid fell out of the steel frame as I was starting my second c-ball. It landed with a thump on the furnace. Although it cracked in half, it still covered the crucible. Thinking quickly (things are at 2000F) I was able to push the pieces together with my large tweezers and move the lid to where the steel support would fit and close. I was then able to adjust the attachment clamps to firmly hold the lid so I could finish the session. There was a bit of brick dust on the top of the molten glass, but I gathered it out easily on the next piece.
When I shut off the furnace later this week for maintenance, I will have to modify the lit to better fit the support.
As to the glass, the new bags of Spruce Pine are melting well. When I first received the order over a year ago August, the bags were rock hard and the batch had many hard lumps that would not break. For some reason, just sitting around in my shed for a year has resulted in batch that is still hard, but breaks up into nice batch with a single bag-drop onto concrete as is considered normal. The glass is also really for the entire pot. I do love melting Spruce Pine.
I’ll take some photos of the pumpkins and c-balls later in the week and post them here.
It’s time to make some fall glass. Today at noon I started up the glass furnace. It’s a slow process: it takes 8 hours to heat from 100F to 900F at 100F/hour, then 16 hours to get to 1300F at 25F/hour, and then a further 6 hours to reach holding temp of 1900F.
Once it’s at 1900F, I’ll start charging with raw glass batch. The batch I use is Spruce Pine, or SP87 which is the classic batch for glassblowers. I should have a fresh pot of clear glass early next week. Then it’s time to make some nice things.
I shut down my glassblowing studio in May 2013 in order to attend a glass conference in Calgary. I usually shut down when I’m going to be away for a few days. In addition to this shutdown, I needed to rebuild the gathering port lid for the furnace.
I planned to rebuild the lid over the summer, but discovered the garage wall needed rebuilding due to a poorly installed hot tub by a previous owner. So instead of rebuilding the lid I rebuilt the side of the garage!
Later I started to rebuild the lid, but again there were setbacks. The original lid was built from 3in bricks, but I only had 2.5 in bricks. I tried many times to get 3in bricks, but they simply aren’t available.
In the end I redesigned the lid to use my 2.5 in bricks. I also had to modify the hinges for the different lid height, but that wasn’t too difficult. I also had to relocate the door shutoff switch, and I took the time to paint the furnace black.
Now the furnace was done, but my studio was full of leftover wood. Next came fall, and cold weather. I didn’t feel like blowing. Then came 2014 and spring, then summer then fall, then winter. To wet, too hot, too wet, too cold. I didn’t blow at all from May 2013, 2014, and most of 2015 – until this past month.
Finally in November I decided to fire up the studio again. Of course much cleaning was needed, but in the end the studio is again hot and running every Friday. It’s rather nice to be blowing again.