So, continuing my search for a lead glaze that I could fire at higher temperature I reduced the lead content but also ran a test on different temperatures from 1220°c to 1260°c. But the results on my test tiles were not really reliable nor conclusive except that too much red iron oxide produced dull tenmoku like results. Thus I mixed a batch with 4% red iron oxide and tested this batch on four different clays: two different red stoneware bodies, a buff stoneware clay and a mix of different reclaimed stoneware clays. I fired these to about 1260°c in oxidation:
The results were just too unreliable and too far removed from the lovely quality of traditional English lead glazed ceramics. Enough and time to go back to basics with a traditional low fired Earthenware lead glaze on red terra cotta clay with slip decorations.
Thus, a new start to some more tests. In order to keep my interest up but also to see how these new glazes sit on three-dimensional surfaces I've done some little bowls which will be used as glaze test pieces along with some test tiles
Set of test tiles and pots are very promising.
So, I adjust the oxide quantities and fired them on some other pots, including some pinch pots made from other red clays.
Finally I'm getting the results I'm looking for with soft and very shiny glaze results. So, I do another glaze test with the same glazes but increasing the final kiln temperature by about 15°c to encourage a bit more movement and fusing of the glazes.
Based on these results I feel safe to move on to larger pieces of ceramics and I first test them out on two bigger platters made from earthenware terracotta clay.
The slip decorating process for the these platters is very difficult as the slip needs to be applied to leather hard clay and I don't want it to be dry when working into it. That means that I have plan carefully the different steps while decorating both sides of a pot without accidentally smearing the decorations. Honestly, this is a bit of a nightmare!
Once the decorated pieces were slip decorated and bisque fired I dipped them into two different glazes: a brown and a green lead glaze. They were then fired in oxidation. I noticed that I get a really nice effect when applying the brown glaze over the green glaze, like here. I don't get the same if I reply them in reverse order.
Unfortunately both my platters had glaze shelling off the edges. I assume this could have be done to various factors: the slip and clay don't fit very well and shrink at different rates, the slip and also the glaze was applied too thickly where it was shelling off, because I burnish and compress the rims to ensure those rounded edges they are compacted and offer little grip for the slip to adhere itself....
I also had a crack on one of the platters. This is the first time this has happened to any any of my platters. Cannot think why that is and am hoping it won't happen again.
So, I decided to partially reglaze these platters and refire them at a slightly higher temperature in an attempt to disguise and cover up the shelled off glaze and fill in the crack. Even though there was an improvement I still get shelling problems around the rims.
So, what can I do in future to prevent this from happening again. It has been suggested that I vary the firing temperatures by increasing the initial bisque firing temperature. I currently don't have the time to do these tests. I think the way I apply the slip and and glaze may help. For my next set of plates I have adjusted my approach to include:
Sponging the surface of the plate to give the surface a texture and key for the slip to adhere to.
Wet the edges with a spray so that they aren't as dry and this may also help the slip to stick to the clay body.
Instead of dipping the large areas I brush on the slip in thinner layers thus also preventing a excess of slip gathering on the rims where the drips would otherwise collect.
Brush on the glazes to also prevent too much glaze pooling around the edges and pulling the slip off the body of clay.
I also decided to build up layers of the two glazes across the plates and blend the brushstrokes with my fingers in order to blend the colours together.
This is the result:
I have no problems with shelling off nor any cracks. Phew! The glaze application is a bit patchy and thus there are variations in depth of colour. I quite like this but may reglaze this platter to see whether more glaze would improve it. Because of the layering and blending of glazes they are much softer and blend don't compete with the slip pattern.
I apply the same approach to a large vase and a large figure and here are their results:
I did not expect to have such purple patched on the vase where the glaze is so thin that the body of clay comes through.
The large figure has come out really well. I am not sure that I like the black slip decorations as they are a bit harsh. I would change those in future. I prefer the softer black slip decorations on the vase which were created by dabbing the slip over stencils and not by using a slip trailer.
When Dave, our course leader, first suggested that I may want to try out slid decorations with honey lead glazes I was not taken by the idea. I didn't particularly like the examples of traditional historical slipware. However, as is often the case, when you learn and get familiar with something it grows on you. Now, having gone through this process this far I'm really liking the results I've been achieving and plan to continue with them.