Melt Test For Ceramic Materials
Set of 9 melt tests. On the left, was before being fired, at right after being fired at Cone 6 in oxidation. Read below what materials were tested, and some analysis of this melt test.
A simple melt test is the first step when investigating if a material has any potential in ceramics. Materials go through profound changes when heated to well over 2,000 degrees! They can melt into a puddle, vaporize and vanish, change color, change molecular ways. The first thing is to make a small dish or "drip tray" from a clay that is know to be safe at the temperature you are going to fire. It needs to be deep enough that if the material totally melts, it's not going to overflow onto the kiln shelf. What is usually meant by a melt test is to put a chunk of rock or powder in a drip tray or dish. Since we don't use chunks of rock or dry powder in a glaze-we grind up the rocks into a powder and suspend them in water-the melt test where you just use a chunk of rock or powder has its limitations. What I did in the melt tests pictured above, is grind the ingredients up enough that they would go through a #60 mesh screen, and mix them into a paste with water. This gives me something closer to real-world usefulness.
I keep a physical notebook, and often name a found local rock or clay according to where it first was recorded in my notebook. So for example, on page 3 of my notebook, I recorded 6 materials and labeled them A-F; the melt test labeled #1 above was D from page 3 of my notebook, therefore "3D." You can see in the photos above I scratched numbers into the actual drip trays above; each of these numbers is somewhat explained below.
1- Rock 3D (local rock picked up near stream)
The color change in this rock was dramatic. This rock could be ground up finely and used as-is for a yellow paint; however, for ceramics this becomes a red. It is SO MUCH easier to get red from clay than to go through the effort of grinding up a rock, that I decided not to do any further testing on this rock.
2- Rock 3E (local rock picked up near stream)
Before firing, this rock was significantly darker than 3D above, and yet the color when fired was not significantly darker when fired. Much like 3D, my reaction was it's not worth continuing to work with this rock since I could get this color in a glaze with far less effort from a clay
3- 5A (Volcanic River Rock)
This was in a huge (multi ton) pile of rocks that was taken off the roof of a building on campus, and dumped at the East 40. The result is kind of like the Holy Grail-one ingredient, one rock, beautiful glaze. This almost never happens. Here's the catch; I only have maybe 3 pebbles of this rock. It was picked out of a landscaping mix that was dumped at the East 40, and when I went back to the pile I just couldn't find enough of it. If I see this kind of rock in any quantity (like bags of just this rock at a gardening center) then I would want to pursue testing further.
4- 5D (River Rock)
This was also picked out of the huge pile of rocks mentioned above; when I was making paints from the rock, what attracted me was that it was on the green side. I thought it might make something close to a terre verte paint. Much like melt test #1, the color is much better raw (which can be used for paint) than in its fired state.
5- 5C (River Rock)
Another rock from the big pile deposited at the East 40. There are enough quite similar rocks that I would like to go further in testing this one. It has an interesting pink color; it is quite refractory, and did not melt at cone 6. Like nearly every rock, it's going to need other ingredients to make a glaze. This will need a flux added to help it melt, and possibly it will also need a glass former ingredient added. Being ground more finely assists in melting, and this test was cone 6; the hotter temperature of a cone 10 firing would also of course assist in melting it. There is promise in this one-but a lot more testing would be needed
6- East 40 Wild Clay
This is, pure and simple, clay that was dug from the NCC East 40. Note the severe cracks; this is why you don't usually make a glaze out of 100% clay. Clay holds a lot of water-first, "added" water that makes the clay be something you can form or pour, as opposed to clay in a dried powder form. But also, there is "chemical water" in the structure of the clay molecule that gets released when the clay is fired; that's why ceramics (fired clay) doesn't melt into a mud puddle when you add water like raw unfired clay would. This is a lovely red color at cone 6, and since this clay is plentiful on the NCC East 40 to say the least, this is a winner in terms of potential for a glaze ingredient.
7- 7B Pocono Purple
I call this Pocono Purple because it's a purple colored rock that I picked up from the side of the road in the Poconos, where it is extremely common and plentiful. It retains its purplish hue here at cone 6 oxidation. The rock on its own is dry to the touch, it's not melted into a glaze like #3, so it will other materials added to formulate a glaze. I've already done some testing on this one, and plan to do more in the future. This one shows even from the melt test that it has promise.
8- Front Yard
Yup, just like the name says, this is the clay I dug out of my front yard. Like test #6, it is a clay and so cracked; however, this clay obviously has less shrinkage than #6 since it cracked so little. I identified this as good for further testing, and have actually already developed and used a glaze from this clay. This clay has potential as a clay body, but because I don't want to dig up my tiny little yard, I'm sticking to glaze formulations from it. Although I did make clay beads and a lovely necklace from the beads!
9- 25B Easton Cliff Rock
Along route 611 in Easton, this is literally falling off the cliffs onto the shoulder and side of the road. This was a VERY interesting melt test. Almost always, impurities in rocks and clays such as iron and manganese get much darker when fired to ceramic temperatures; this one, astoundingly, fires white! What is hard to see in the photograph is just how refractory this rock is; it ended up a powder after being fired. All the other rocks tested in this batch melted enough to fuse into a hard mass. One even formed a true melted/vitrified glossy surface. A white rock is like a white canvass; it can be used in a white glaze, but it has the potential to be a part of a base glaze to which varied colorants could be added to make any number of colors. I have done more work with this rock, and to the point where I have a glaze I made from it that I really like and works great on a clay body I use a lot.
I keep a physical notebook, and often name a found local rock or clay according to where it first was recorded in my notebook. So for example, on page 3 of my notebook, I recorded 6 materials and labeled them A-F; the melt test labeled #1 above was D from page 3 of my notebook, therefore "3D." You can see in the photos above I scratched numbers into the actual drip trays above; each of these numbers is somewhat explained below.
1- Rock 3D (local rock picked up near stream)
The color change in this rock was dramatic. This rock could be ground up finely and used as-is for a yellow paint; however, for ceramics this becomes a red. It is SO MUCH easier to get red from clay than to go through the effort of grinding up a rock, that I decided not to do any further testing on this rock.
2- Rock 3E (local rock picked up near stream)
Before firing, this rock was significantly darker than 3D above, and yet the color when fired was not significantly darker when fired. Much like 3D, my reaction was it's not worth continuing to work with this rock since I could get this color in a glaze with far less effort from a clay
3- 5A (Volcanic River Rock)
This was in a huge (multi ton) pile of rocks that was taken off the roof of a building on campus, and dumped at the East 40. The result is kind of like the Holy Grail-one ingredient, one rock, beautiful glaze. This almost never happens. Here's the catch; I only have maybe 3 pebbles of this rock. It was picked out of a landscaping mix that was dumped at the East 40, and when I went back to the pile I just couldn't find enough of it. If I see this kind of rock in any quantity (like bags of just this rock at a gardening center) then I would want to pursue testing further.
4- 5D (River Rock)
This was also picked out of the huge pile of rocks mentioned above; when I was making paints from the rock, what attracted me was that it was on the green side. I thought it might make something close to a terre verte paint. Much like melt test #1, the color is much better raw (which can be used for paint) than in its fired state.
5- 5C (River Rock)
Another rock from the big pile deposited at the East 40. There are enough quite similar rocks that I would like to go further in testing this one. It has an interesting pink color; it is quite refractory, and did not melt at cone 6. Like nearly every rock, it's going to need other ingredients to make a glaze. This will need a flux added to help it melt, and possibly it will also need a glass former ingredient added. Being ground more finely assists in melting, and this test was cone 6; the hotter temperature of a cone 10 firing would also of course assist in melting it. There is promise in this one-but a lot more testing would be needed
6- East 40 Wild Clay
This is, pure and simple, clay that was dug from the NCC East 40. Note the severe cracks; this is why you don't usually make a glaze out of 100% clay. Clay holds a lot of water-first, "added" water that makes the clay be something you can form or pour, as opposed to clay in a dried powder form. But also, there is "chemical water" in the structure of the clay molecule that gets released when the clay is fired; that's why ceramics (fired clay) doesn't melt into a mud puddle when you add water like raw unfired clay would. This is a lovely red color at cone 6, and since this clay is plentiful on the NCC East 40 to say the least, this is a winner in terms of potential for a glaze ingredient.
7- 7B Pocono Purple
I call this Pocono Purple because it's a purple colored rock that I picked up from the side of the road in the Poconos, where it is extremely common and plentiful. It retains its purplish hue here at cone 6 oxidation. The rock on its own is dry to the touch, it's not melted into a glaze like #3, so it will other materials added to formulate a glaze. I've already done some testing on this one, and plan to do more in the future. This one shows even from the melt test that it has promise.
8- Front Yard
Yup, just like the name says, this is the clay I dug out of my front yard. Like test #6, it is a clay and so cracked; however, this clay obviously has less shrinkage than #6 since it cracked so little. I identified this as good for further testing, and have actually already developed and used a glaze from this clay. This clay has potential as a clay body, but because I don't want to dig up my tiny little yard, I'm sticking to glaze formulations from it. Although I did make clay beads and a lovely necklace from the beads!
9- 25B Easton Cliff Rock
Along route 611 in Easton, this is literally falling off the cliffs onto the shoulder and side of the road. This was a VERY interesting melt test. Almost always, impurities in rocks and clays such as iron and manganese get much darker when fired to ceramic temperatures; this one, astoundingly, fires white! What is hard to see in the photograph is just how refractory this rock is; it ended up a powder after being fired. All the other rocks tested in this batch melted enough to fuse into a hard mass. One even formed a true melted/vitrified glossy surface. A white rock is like a white canvass; it can be used in a white glaze, but it has the potential to be a part of a base glaze to which varied colorants could be added to make any number of colors. I have done more work with this rock, and to the point where I have a glaze I made from it that I really like and works great on a clay body I use a lot.