Making Your Own Molds Part 2 - Choosing the Best Mold Materials
In my last article I talked about using castable materials to make your own high temperature molds for shaping glass in a kiln. But castibles may not always be your best choice. It really depends upon what you want to make. There are several groups of refractory mold material that can be used. Gil's definition of a Refractory for Kiln Workers is ... any mold material that will not burn, break, deform or explode when heated to 1500 degrees F.
Each of these refractory materials has different strengths and weaknesses. Some are better for bending other are better for casting. Some can be used for multiple firing and others can only be used one time. Understanding their specific attributes will make it easy to choose the right mold for the job.
Before we pick a mold material we need to decide what we want our glass to do. Generally speaking, there are four different glass shaping activities that can be accomplished with molds.
The first is BENDING glass (also called slumping, sagging, or draping). Bending molds tend to have "soft" shapes, that is they are usually gentle curves without a lot of detail. Because bending is usually done right at a temperature where the glass is becoming pliable and soft enough for gravity to change its shape, it is really not very fluid and it will not pick up a lot of detail from a mold. It is kind of like bending a piece of paper in that the shape can change, but the thickness of the paper (or glass) doesn't change. Unlike paper, glass at this viscosity does not like sharp bends, so the majority of the bending molds are bowls, partial cylinders or slightly undulated forms. Depending upon the type of glass you are using, you can do most of you bending in the 1100 degrees F - 1250 degrees F temperature range. As with most kiln processes, lower temperatures and longer hold times will usually give you the best results.
If you stay at the lower end of this temperature range, the glass isn't very "sticky" so a super thick layer of kiln wash is not needed. If fact a light coat or two will usually do the trick. I have read horror stories of beginners spraying kiln wash (good idea) on a heated stainless steel mold. But, they applied lots and lots of layers and it became so thick that it flaked off and left an irregular surface and bare spots on the mold (bad idea).
Usually metal and ceramics are the most popular materials for bending molds because they are durable and you can get many firings out of a single mold. If it is limited run or a one-of-a-kind project then castable materials may be the best choice.
The second activity to consider is STRETCHING the glass in the kiln. Some folks confuse this with bending, but it is really a slightly different activity. Here the glass is a bit hotter, say 50 - 75 degrees hotter that the bending temperature and the glass actually stretches. Think of taffy being pulled
Stretching mold are also often Suspension molds where part of the glass is supported and the rest of the glass is allowed to fall freely. Drop-out rings and elongated Floral Formers are typical suspension molds but detailed molds like the corrugated weave molds that require the glass to be softer than just bending consistence fall into this category. The thickness of the softer glass will decrease in the areas where it stretches and it will also pick up more detailed shapes than it can at bending temperature.
The next highest temperature activity is PATE-de-VERRE. Traditionally frit is placed in a mold and fired to the point that all of the particles of glass bond into a single unit without ever becoming a liquid. This is similar to but a bit hotter than tack fusing and is often carried out in the 1300 F to 1350 F range. Plaster based castable molds are usually the first choice for pate-de-verre, but some exciting pieces are being make with kiln washed ceramic fiber molds.
My fourth category is CASTING. This is usually done at fusing temperatures or when the glass is soft enough to behave like a liquid. It will puddle and flow into cavities and take on highly detailed shapes. It will also stick to other hot object like ceramic or metal molds that do not have enough kiln wash on them.
Generally plaster based mold material are used for casting molds because they do not requited a coating of kiln wash. Here durability is not the priority, that aspect is trumped by having a mold that will hold fine detail and will not crack and fail at high temps.
There is no reason that a person couldn't take the glass to any temperature above or below those that I have talked about. I mean it is your glass after all, you can do anything you want, right? I just mentioned these because they are the most common activities and knowing what you want the glass to do, bend, stretch, bond or flow, will help determine the best mold material for the task at hand.
I find that the best strategy or sequence of events is to first determine how the finished glass will be displayed. Knowing that in advance can help determine the best mold to use. Once the mold choice has been finalized, the glass can be cut and fused to fit the mold. Working through these questions in this order can save you a lot of time and frustration. Working in the reverse order can give you a lot of "kiln experience" because you may end up making the piece three or four times before you get the results you are after.
My reasoning is that the limitations of the display system can greatly affect the size and shape of the mold that you use. If you are making a bowl that sits on a table, your main concern may only be that the bowl sits flat without rolling around. But, if you are making a lighting fixture, a sink or any object where holes and hardware are involved, there might be varying degrees of accuracy involved that will need to be factored into the final shape of the mold.
Once you have a mold that fits your hardware, you can make a master template by wrapping the mold with tin foil. When flattened out, the foil can be used as a pattern for you glass.
OK, that's' all for now. In coming issues we will take a closer look at the different refractory materials.
Until Next
Keep a Warm Kiln.
By: Gil Reynolds
