My Home-Made Crucible Furnace

Click on any of the pictures to get a larger view.

This site is all about the crucible furnace and controller I built in February 2000 using Mark Lauckner's "The Color Pot - Electric Glass Melting Furnace" videos.    

If you'd like information about the videos used to make this crucible furnace go to Mark's website at http://www.mayneislandglass.com/instructionalvideos.html and and look at "The Color Pot" videos.  The videos are very well organized and very detailed.  There are actually two videos in the set.  The first one shows Mark constructing this furnace step by step from boxes of bricks and piles of angle iron, all the way through making all of the wiring connections.  The second video shows the construction and wiring of the controller unit as well as footage of Mark actually using the furnace.  He then skips forward a few months and shows how to do maintenance on the furnace after he's used it in production.  He shows how easy it is to replace the element and some of the fire bricks that have gotten damaged.  He also shows how to do a complete rebuild on another furnace that he's been using for several years.  

The fact that Mark put this video together by himself using a home video camera and a remote control doesn't matter a bit.  The information you need to build and use this furnace are there.  All in all it's a very well done, very detailed presentation and I highly recommend it to anyone that is reasonably handy and wants a superior crucible furnace! 

SOME CHANGES:

Out of necessity, I did several things differently than the videos instructed.  In Mark's video the construction is all shown using an arc welder.  Since I didn't own a welder when I made this furnace you will see in the pictures that I had to adapt the instructions to allow construction using angle braces, nuts, bolts and lock washers.  I used multiple steel rivets to attach the two sets of hinges rather than having them welded.  (I have since bought a MIG welder and actually welded my second furnace which cooks 50 lbs. of batch at a time.  However, the one shown here is still my primary furnace and has never been welded.  - Updated 10-2006)  I designed my own lid switch mounting bracket, and I designed my own handle to lift the lid using a piece of all-thread and a prefab "U" handle, again because of the lack of a welder.  

1.	2.	3.	4.

1.  With the gathering lid raised the lid switch is visible mounted on the back left hand side of the unit.   

2.  With the main lid raised.  The unit is plugged into the controller at the top of the picture.

3.  The main heating chamber.  The crucible shown holds 8 lbs. of glass.  The furnace will also hold a 15 lb. crucible.  In the right hand corner of the heating chamber the R-type thermocouple is visible.  (I use LaClede Christy round-bottomed crucibles, not the one shown here.  They have much thicker walls and last much longer).

4.  Looking down the crucible with the gathering lid open.  (The metal shown was aluminum sheet.  Don't use aluminum - use sheet steel.  If a glob of hot glass lands on the aluminum sheet it burns right through. - Updated 10-2006)

5.	6.	7.	8.

5.  Close up of the handle I designed to lift the gathering lid.  I'm fairly ambidextrous, but I designed this to be lifted with my left hand while I gather with my right hand.  (After obtaining a welder, the all-thread handle was replaced with a sturdier 1/2" steel rod handle similar to the one in Mark's video.)   

6.  I was uncomfortable with the R-type thermocouple sticking out the back of the furnace without being protected.  It comes covered by an alumina sheath with the bulky threaded apparatus shown on the right hand end in this picture.  I have kids and dogs and could just see this getting hit by something and breaking the alumina sheath, so I added a three piece cover consisting of the female threaded coupler in the center, a male threaded coupler and a flange that attaches to the back of the unit to give additional support and protection.  This whole contraption allows the thermocouple to extend into the heating chamber a little over 2 inches which is well within the recommended distance.

7.  A very poor "top view" of the lid switch and its mounting brackets.  The mounting brackets were my own design because I couldn't source the parts Mark shows on the video.  The function of the lid switch is to shut power off to the unit whenever the gathering lid is raised.  This avoids any accidental shock while gathering.  In this image, the silver "tongue" of the Microswitch is pointing up near the upper left corner of the picture.  The yellow tape you see is simply my wire indicator.  One of the unused connectors on the Microswitch is visible near the bottom of the picture.  The Microswitch itself is mounted to the aluminum bracket on the right side of the picture by screws.  There are spacers, made of several lock washers, between the Microswitch and the bracket.  The concern I had in designing this mounting was to minimize the heat transfer from the frame of the unit to the Microswitch since the housing of the Microswitch is plastic.  In operation, the brackets never get over 105 degrees Farenheit, even with the unit at 2300° F.

8.  Side view of the lid switch.  The tongue is sticking out toward the viewer, just to the left of the yellow tape.  This tongue is depressed by the frame of the gathering lid when the gathering lid is closed.  This closes the circuit and allows power to the elements.  This view shows the configuration of the aluminum brackets (heat dissipators) and the spacer configuration under and between the brackets designed to minimize heat conduction from the frame where the switch is mounted.

THE CONTROLLER:

9.	10.

9.  Not fancy but pretty cool.  The neat thing about this controller is that it is portable!  The controller is plugged into a 240 volt outlet via the cord at the lower left side of the box.  The cord you see plugged into the right side of the controller is from the crucible furnace.  The other two wires you see coming into the box a the top left are the thermocouple leads and the lid switch leads.  When I build a second one of these furnaces I'll be able to switch them out for maintenance simply by slipping the thermocouple out of the first one and inserting it into the second; sliding the two leads off of the lid switch on the first unit and sliding them onto the second unit's lid switch; and finally unplugging the first unit from the controller and plugging the second unit in.  Viola!  Instant change out from one unit to the next, saving the cost of all of the controller components and the thermocouple (total of about $430).  Click HERE for a PDF file of the wiring diagram I use for all of my controllers (Updated  1/10/06).

10.  Inside view of the controller box.  Hanging down in front is the Fuji PXV3 computer controller.  The big black thing mounted in the back upper left is the mercury displacement relay (MDR).  I have to thank Roger Peterson for helping me out with the wiring for this controller.  Although Mark's videos cover construction of the controller, there are apparently some differences in electrical code between Canada (where Mark lives) and the US (where I live).  Therefore, some of the information Mark provides in the video on wiring this controller needed to be changed for US consumers.  Mark has been given the wiring diagram Roger designed and I used to build this controller and I'm sure he'll be giving copies of it with any future US sales of the videos.    One thing I would note about the controller - use an 8" x 8" box like Mark recommends in the video.  The 8" x 6" box I used was too small for my fat fingers to comfortably work inside.

COOKING WITH GLASS:

11.	1385° F.	1495° F.	1600° F.

1800° F.	2000° F.	2200° F.	2300° F.

11.  The first charge in the furnace.  I filled the cullet all the way to the top so that it would glaze the entire inside surface of the crucible, including the top lip.  This is intended to keep small bits of the new crucible from spalling off into the glass, potentially contaminating the whole batch with little white flakes.  The purple color around the inner surface of the gathering port is a coating of kiln wash to help keep sloppy gathers and/or stringers from eroding the brick.  (Added 5/8/00 -->) I stopped putting kiln wash on after the first time because of the potential for the kiln wash to flake off into the crucible.  I decided I'd rather risk having to replace the bricks more often than contaminate the crucible full of glass and have to go through the lost production time of an unscheduled shut down.

The cooking schedule for the Effetre compatible cullet I use is as follows (Updated 5/8/00):

1. Load the crucible at room temperature.
2. Ramp up to 500° F. over a period of 1 hour and soak for 1 hour.
3. Ramp up to 850° F. at 200 degrees/hour (1 hour 45 minutes) and soak for 1 hour.
4. Ramp up to 1800° F. at 200 degrees/hour (4 hours 45 minutes) and soak for 30 minutes.
5. During the soak, charge the crucible (add more cullet) several times until the melted glass is within a half inch of the top rim of the crucible, leaving enough room for displacement when dipping a large marble.
6. Cook Step:  Ramp up to 2250° F. at 300 degrees/hour (1 hour 30 minutes) and soak over night (8 hours).
7. In the morning take a gather.  Pull an 8mm diameter rod and check for bubbles when it's cool.  If there are any bubbles larger than 1mm then keep cooking and recheck every hour.  Otherwise, go to the squeeze step.  
8. Squeeze Step:  Ramp down to 1800° F. over a period of 1 hour and soak for 2 hours.
9. Take a gather.  Pull an 8mm diameter rod and check for bubbles when it's cool.  There shouldn't be any!  My bubbles actually disappear shortly after reaching 1800° F.
10. Ramp up to working temperature in 30 minutes.  I use 2150° F., but it will vary based on the glass and the techniques being used.
11. At the end of the day, if it doesn't need to be charged, ramp down to 2000° F. and hold over night.  This will conserve some electricity because the furnace is so well insulated.  Otherwise, if charging is necessary, repeat from step 5 above.  (NOTE:  I used to ramp down to 1500 ºF overnight to save even more electricity.  However, I found that this large swing in temperature contributed to cording in my glass and I no longer do it. - Updated 10-2006)

This furnace is extremely well insulated.  When I went to turn it off the first time I programmed the controller to ramp down from 2075° F. (what I initially thought my working temperature should be) to 75° F. over a period of ten hours.  When I went back after ten hours to check on it the main chamber temperature was still 567° F.  After 24 hours the main chamber temperature was still 155° F.

POWER CONSUMPTION DATA (New 5/8/00):

I ran the crucible furnace with a utility usage meter attached between the controller and the wall socket for about four weeks. I started out monitoring the usage as I was doing different things, like ramping up to cook, or just running at working temperature, or ramping to shut down, etc. Eventually, I went to just recording the meter in the morning when I raised the temperature for the day and at night when I lowered the temperature.   My cycle was to charge the crucible once per week and cook it for 8 hours at 2250º F.  At nights and over weekends I would lower the temperature and let it hold at 1500º F.  During the day I would raise the temperature up to my working temperature of 1950º F.  (New temperatures have been noted in the cooking schedule above - Updated 10-2006)

After compiling my statistics it turns out that it costs around 75 cents per day on normal 8-hour workdays that I don't do a charge cycle. On days that I do a charge cycle it costs a little more, but not much.

When the furnace is idle overnight at 1500º F. it runs at about 350 to 400 watts per hour.   When it's at working temperature of 1950º F. it runs at about 950 to 1000 watts per hour.  When it's cooking at 2250º F. it does something interesting. It gets up to between 1800 and 2000 watts per hour for the first couple hours. Then, it drops back to between 1500 and 1600 watts per hour for the next few hours and then it drops down even further to around 1000 watts per hour for the last couple hours. It obviously has something to do with the insulating quality of the furnace and the amount of time the residual heat takes to build up.

My power costs about 5 cents per kwh. So, based on average usage rate of about 15 kwh per day,  I calculated that it costs me less than a dollar a day to run this furnace 24 hours per day, 7 days per week.  

PROGRAMMING THE CONTROLLER (Updated 5/8/00):

For programming information about the Fuji PXV3 controller, Roger Peterson compiled a document that helps you set up and program the Fuji PXV3 or the PXW series.  He used to have it on his web site, but I now have it HERE (added 8/19/03).  The Fuji owner's manual is very hard to interpret without Roger's information, or a LOT of practical experience.  Also, working together with Roger, running experiments with the Fuji, we've uncovered several "features" that aren't explained very well in the Fuji document.    (I actually use PXR3 controllers now as well as the original PXV3.  The programming is similar with the added benefit of being able to run two separate 4-segment programs, or one 8-segment program, just by switching one parameter.  This makes it convenient to make the ramp up program for an annealer the first 4-segment program and the ramp down portion for the annealer the second 4-segment program.  When it ramps up it will stay at the final temperature in the ramp program until you change the parameter to run all 8 segments.  Yes, this will repeat the ramp up program before continuing on to the ramp down portion.  However, since the annealer is already at annealing temperature the ramp up portion of the program has no effect.  It simply adds a soak time of however long you initially programmed it to ramp up.  By subtracting this soak time from the ramp down process the program works wonderfully.  Updated 10-2006)

HINGES and ROD REST (Updated 12/9/00):

Several people have asked how I dealt with the issue of the hinges since I didn't weld any of the parts.  The answer is that I used steel rivets to attach the hinges.  I first bent the hinges for the upper lid to their final shape by heating them and hammering them to shape while holding them in a vise.  I then pre-drilled holes in the hinges.  I then mounted the lower hinges by drilling holes in the frame and attaching rivets as I went.  By doing it this way I didn't have to worry about hole alignment when I went to put the rivets in.

The other question people have asked is what I use for a rod rest while I'm doing my gathers.  These pictures show a simple rod rest that I made from a piece of 3" angle iron.  I tapped threaded holes near the top edge and screwed two 10-32 screws into them.  I seated them with nuts so they wouldn't unscrew and then bent them to the "V" shape you see in the picture.  It works great!

12.	13.	14.	15.	16.

12.  All four hinges riveted in place.

13.  A close-up of two of the hinges.

14.  View of how the rod rest sits on the crucible furnace.

15.  A close-up of the back side of the rod rest.

16.  A close-up of the front side of the rod rest.