This is going to be very technical post (which I have been trying unsuccessfully to publish since this morning...) which will cause the eyes of any non-glass person to glaze over and fall out so skip it unless you are interested in making your own molds with rims for slumping glass.
Coffee in the New York Skyline mug, the iPod song WAS Natural Beauty by Neil Young from Harvest Moon, but I decided I would probably slit my own throat if I listened to it any longer and switched (almost purely randomly) to Riding the Storm Out by REO Speedwagon. My husband is fond of saying the golden age of rock 'n' roll is 16 and today I am just proving him right...
Yesterday I made my first pass at slumping pieces for seder sets. I have been commissioned to do two of these sets and am finding interesting challenges. The thickness of glass I am using is 1/4" to 3/8" (more than two sheets of Bullseye and less than three) as I am using frit and every piece comes out slightly different in thickness. The parameters of the commission are to make a base plate with about a 1-1/2" rim and six smaller plates--each with a rim--which should fit inside the big plate. The little plates should be about 4" X 4" square.
I initially figured that the base plate should be 16" X 12" and I planned to use mullite (clay) shelf segments as the mold material. Yesterday I was rummaging through my stuff and I found some duraboard (ceramic fiber) pieces from old kiln lids and shelves and decided to try them first. Duraboard is a relatively soft material that can be cut with a utility knife, smoothed with sandpaper and has a fine grain for smooth detail. Mullite is very hard--I cut it with a tile saw and would need to use one of my diamond grinding bits for glass to round the top edges down.
For mold assembly, I tried two different approaches. For the big plate--because I saw a possibility for a need to change dimensions and because of the size of the scraps I had on hand--I used four strips 2" X 18" X 1", sanded down one top edge, and laid them out so that each piece would be one side of the plate in the appropriate length. I filled the seams with kiln wash paste.
For the small plate, I cut a one-piece mold out of a single piece of 1/2" thick Duraboard (or it could have been HD board--I have some of both lying around and have no need to tell which is which) and smoothed down the top inside edges with sandpaper.
For the glass I used scrap from a large piece that I did last summer and cooled too quickly causing it to break. (It is a sad thing when you have pieces of glass lying around which qualify as "scrap" and which are over 16" X 12".) However I made a serendipitous error in cutting the scrap: I ignored the measure twice, cut once principle becasue I was in a hurry and ended up cutting the big plate 15" X 12". You will see the serendipity of this error in the slumping result.
Into Big Bertha they went with two other large pieces and I extended the slump at the top by 10 minutes (1225 degrees for 15 minutes). The results were less than successful. Only one piece fully slumped even with the extended time. As one of the other large pieces (the control group if you will) did not fully slump either it is clearly not just the new molds (more fun with Big Bertha's firing schedules)--though they presented their own issues. It has been almost two years since I made molds like this for a dinnerware commission and for an Ikebana dish and I arrogantly forgot the lessons I learned that time.
Most molds for slumping glass have a slope to them--they do not just drop off abruptly as glass does not like to just drop off abruptly. So when you make a mold like I did you have to figure in the width of the slope (key when you are counting on a particular surface area inside the plate to put food or other little plates on). The actual flat surface size in the bottom of the big plate turned out to be just under 7" X 11"--Probably not big enough to hold the six 4" X 4" plates.
There is also a relationship between the width of the rim and the height of the drop that you need to take into account to avoid the edges of the glass pulling in and lifting up as it slumps. Here is where the serendipity came in: The sides of the big plate which I cut cirrectly had 1-1/2" rims and they pulled in a bit (1/4" in the corners and 1/2" in the middle--still too noticeable) but did not lift. But the edges that I cut too short (resulting in only a 1" rim) pulled in and lifted up.
I take away from this experiment a comfortable guess at the ratio between width and height as 2 to 1 for glass of a thickness 1/4" to 3/8", and a conservative estimate of 2.5 to 1.
The smaller plates were more difficult yet. I only planned on a 1/2" rim for them with a 1/2" drop and even though they did not slump nearly all the way they were already pulled in and lifted up. I am currently fully slumping the control plate and the smaller plate in the medium kiln and I am eager (really I am) to see how bad the little plates look at a full slump.
Now I have to redo my size calculations completely. The 1" drop on the large plate did not add to the balance of the piece and adds unnecessary complications so I am going to decrease it to 1/2". I will leave the rim at 1-1/2" for that piece. For the little plates I need to increase the rim size to 1" and I will need to see what this does to the interior flat base of the plate--it might not be balanced anymore at a 4" total size. If I have to increase the size of the small plates for balance I will also need to increase the size of the big one.
Passover is coming. I better get a move on.