The proportion of plaster/silica to water is critical in order to fabricate a strong mold. If you don’t add enough plaster/silica to the water, the ratio will be such that the mold will be soft and readily crack when subjected to heat. USG, a major manufacturer of plaster, recommends consistencies (proportions of plaster to water) and recommends that you weigh your materials. Note: The USG recommendations are for plaster to water and not for plaster/silica to water, but for our usage, we will use their calculations to estimate the ratio of plaster/silica to water. Weighing the plaster/silica to water defines the exact density of your mold.
Once you’ve calculated the volume of investment you need, it is necessary to convert this information in weights of plaster/silica to water. It is assumed that you have subtracted the volume of the model to be cast from the volume of the mold box into which you will be pouring your plaster/silica into. Remember, we’re working with cubic feet.
The next step is to select the consistency you will be using; low numbers equal higher density molds. The higher the number the faster the materials will set. It is really not necessary to go above 60 consistency. Most casters use 50 to 60 from the chart below. I use 55 and find this ratio quite adequate. Another important aspect of doing accurate measurement is that if you find it necessary to add more plaster/silica, the expansion and contraction of the different layers of your mold will be similar and you will not get any separation between those layers. For very large molds that require you to mix many batches of mold mix, this weighing method is very important.
Follow this Example
Multiply the cubic feet needed by the amount of both plaster/silica and water to give you the amount of each that you need. Example: The measurement of your mold box is 14” x 18” x 4”. Using our calculation of length x width x height we find it to be 1008 cu. inches. This number is then divided by 1728, the cube root. Our answer is .5833 cu. ft. Now our model to be cast is 12 x 16 x 2; this calculation is 384 cu. inches or .2222 cu. ft. Subtract the model from the mold box and you get .3611. If we want a mold with a consistency of 55 we take .3611 and multiply it by 73 which gives us 26.3 lbs. of plaster/silica. Next we multiply .3611 by 40 which gives us 14.4 lbs. of water. If you have to divide up your final weights because it all won’t fit into one bucket do it evenly. Divide the number of buckets into the weights of each. This may seem like a lot of effort, but it really takes all the guesswork out of making larger molds.
It is helpful when using this system to premix your plaster and silica in equal amounts. Both ship in 50 lb. bags.
Note: The download pdf below is the actual page from Glass Notes and has a bit more information.
Click for Plaster/Silica ratio chart
The cane test never fails to establish the annealing temperature of any unknown glass.
This test was demonstrated to my classes back in the 70's (a long time ago) by Fritz Dreisbach. I and many others have used this test and it has never failed to be accurate. Accuracy is dependent on the accuracy of your pyrometer and thermocouple.
1. Make a few canes of glass from the glass to be tested. The canes should be about 18" long and the thickness of a #2 pencil. Estimate what you think the annealing temperature or annealing point is. As an example, let us assume the A.T. to be 1050° F (565° C).
2. Bring the oven up to the assumed 1050° (565° C) A.T. and stabilize it at this temperature for a couple of hours. Place the cane in the oven as shown in the photos below, either method will work, and check it after one hour. Even though the oven is hot the cane will not shatter.
3. If the cane is bent, your oven is too hot and you need to lower the temperature about 50° F (28° C) and stabilize for a couple of hours. Repeat Step 2. Always use a new cane when repeating the test. Keep lowering the temperature and repeating Step 2 if the cane continues to bend after one hour.
4. If the cane is not bent after one hour but is slightly bent after 4 hours, you have hit the annealing temperature on the nose. A word of caution: if you are annealing thin ware such as long–stemmed goblets or any ware that is top heavy, it is advisable to lower the annealing temperature slightly and hold your soak longer. Keep a cane propped in the corner of your annealer, out of the way, to check your annealing.
5. If the cane is still not bent after 1 hour plus an additional 4 hours, the oven is too cold. Raise the temperature 20° to 30° F (11° to 17° C) and stabilize for a couple of hours. Repeat until you achieve 4 above. Remember to record the A.T. for the oven being tested. Doing this test requires patience. Because pyrometric temperature readings may vary for each oven, it is important to repeat the cane test for each oven.
6. Each oven should be tested to determine the optimum annealing temperature. An oven that is cool by as little as 50° F (28° C) can increase the soak time necessary to remove stress by as much as ten times.
7. Soaking at the A.T. is the most effective way to remove stresses induced in forming hot glass.
It is assumed that your thermocouple and pyrometer are in working order and reading your oven temperature accurately.
Shown above are two methods of how to do a cane test
There is more info on this test in my book
A chart for annealing thick castings
This annealing chart is derived from Corning's method as shown in Mclellan and Shand. The chart was originally based on annealing a flat glass slab of uniform thickness. Everything your glass is not. The original chart is set up to calculate annealing and cooling rates when the heat is radiating equally from the top and bottom. Since your glass is enclosed in a thick mold and radiating at differing rates the chart needed to be adapted to meet the annealing and cooling characteristic of thick art studio castings. Dan Schwoerer of Bullseye Glass adapted the original chart to meet real world studio cast glass characteristics and is laid out to accommodate Bullseye glass. To make the chart work for your glass just substitute the requisite annealing temperature. The chart below is that chart. The artist should keep in mind though that this chart like any chart is not a "one chart fits all" and should be used only as a starting point to anneal and cool your particular idiosyncratic casting. Keep good records and chart your time temperature curve to meet your needs. Make sure your oven does not have thermal gradations (hot/cold spots) and make sure your pyrometer is reading accurately. If your oven and thermocouple is funky no chart will help you achieve stress free work. Xanax and Zoloft can cure your stress but will have no effect on your glass. Take enough though and you won't care.
To convert from degrees per hour to hours per degree
If your controller cannot be programmed for degrees per hour it will be necessary to do the following conversion to convert from degrees per hour to hours per degree. Find the row you will be using (Thickness) and subtract the degrees in the Initial Cooling Range. Example: Your casting is solid and it is 3 inches thick: Subtract 800° from 960° (960° - 800° = 160°) Divide the answer (160°) by the degrees per hour (3°) 160 ÷ 3 = 53.3 hrs. (round it up to 54 hrs). This answer is the number of hours required to lower the temperature to 800 degrees at the given rate of 3 degrees per hr. Repeat for the Second Rate and for the Final Cooling Rate.
If the glass is not set up in such a fashion that it can cool equally from top and bottom or is anything besides a flat slab of uniform thickness, select an annealing cycle for a piece that is twice the thickness of the thickest area of the piece.
Even a very conservative annealing cycle may not work if the annealing oven is not capable of cooling evenly. If your glass checks, check your oven before you blame the chart.
This chart re-printed through the courtesy of Dan Schwoerer, The Bullseye Glass Co.
Click for a copy of the chart
The Free Standing Pot Furnace
This freestanding pot furnace is a general design that has many variations and configurations. This particular furnace is round and not that difficult to build. Construction is not unlike an invested pot furnace as shown on page 158 of my book Glass Notes. One of the advantages of a round pot furnace is that the flame fires tangentially. Firing in this manner gives the flame an opportunity to fully develop as it circulates around the perimeter of the furnace. This is important as you want to get every last BTU available from your burner flame. Tangential firing is also much easier on your refractories as you do not have a flame impinging directly on the refractory.
The size of your furnace is determined by the size of your crucible. I would recommend building your furnace so that it is capable of accepting two different size crucibles a 200 pound capacity crucible is not that much smaller than the 300 pound capacity crucible. When building this furnace make sure you leave about 2-4 inches of clearance from the edge of the pot to the side wall.
The drawings below should give you an idea of the furnace layout. The flue depicted is a simple brick one. I would recommend using a recuperator in place of the traditional flue. Designs for recuperators can be found in Glass Notes as well as where they can be purchased. If you do not have a recuperator on your furnace one can be retro-fitted. Recuperators save a lot of energy.
Designs for this and other style furnaces can be found in Glass Notes.
Crucible Formula for Pot Furnace (for slip casting) pg. 296 Glass Notes
Back in the 70's, some of us made our own crucibles. I did not remember who developed the formula since it was written on a scrap of paper with no names but I suspected that it came from Dudley Giberson originally as he was the one that first published that type of information for the studios and universities in his "Joppa Catalog of Fact and Knowledge." I sent an e-mail to Dudley asking him about the formula and here is his reply.
"The formula you have below is a variant of the one I published in my ''Joppa Catalog of Fact and Knowledge," June '77. It came from Mark Peiser and goes like this:"
Mark Peiser Formula (in pounds)
18 –––––Tenn. Ball Clay
21 –––––Mullite (325 mesh)
21 –––––Mullite (200 mesh)
8 quarts water
2 oz. of defloculant, Darvan or sodium silicate*
Halem variant of Giberson formula which is a variant of the Peiser formula
7 –––––Tennessee Ball clay #9
9 –––––Mullite (325 mesh)
9 –––––Mullite (200 mesh)
About 1 oz. of defloculant, Darvan or sodium silicate
*Defloculate with Darvan #7 (Darvan is a defloculant and used to disperse ceramic suspensions to minimize their water content.) This product is manufactured by the R.T. Vanderbilt Co. http://www.rtvanderbilt.com/spec_ag_2.htm. You many be able to obtain it from a ceramic supply company. If you can't get this product you can substitute sodium silicate. There is a limit to the amount you can use when defloculating. If you go beyond the limit it will cause your casting slip to lump. Add a few drops at a time. You should see an immediate thinning of your slip.
Building a Glass Studio in the 60’s and 70’s
I found this document while going through papers that resided in a box long forgotten in the back of a closet. I have no idea who compiled the information and who it was distributed to. I would love to know who researched it. Prior to being able to purchase a furnace and all that goes into a studio we were required to build our own. I’ve built many furnaces in my day as well as annealers and all the other stuff needed to have a working glass studio. This document gives you some idea of what our costs were but if memory serves me I think they were somewhat higher than outlined in this document. The costs are cheap by today’s standards and I have to question how accurate these prices really are. I wonder how many of today’s glass teachers are capable of building a studio from scratch?
Click on the cover and it will download all 5 pages.