|Monthly Tech-Tip |
Refiring can be an issue with the body and the glaze. Or both. Vitreous bodies, especially when fired beyond their point of maximum density, can blister and bloat on a second firing. Glazes with high melt fluidity often blister on second firing. Clay bodies can turn color, going darker. Glazes can crawl. The more raw and coarser particled materials there are in body and glaze the greater chance there is that these did not complete gasing or decomposing on the first fire (and thus will continue on the second). The second firing is actually the first time the fired ceramic matrix, as it existed coming out of the first kiln load, is heated upward to maturity. The glaze materials, as a mix, were melted on the first firing and combined to form a glass. That glass will melt completely, and seal the body surface, at a much lower temperature than on the first firing. The new dynamics can easily yield unexpected results. Frits, by contrast, have already been melted. Thus glazes containing higher percentages of frits, will yield fewer surprises on second fire. Likewise with finer particled bodies made from materials having lower carbon content and lower LOI.
More information coming soon.
The body is pure Lincoln fireclay. It matures fully by about cone 8. The first firing (left), was to cone 6. The second firing, right, was to cone 7. The higher temperature, theoretically should have given the glaze additional opportunity to smooth out. It is not always logical why this happens, especially with a melt-fluid glaze like this. It appears the re-melting glaze formed blisters on the heat-up and the molten glass had sufficient surface tension to maintain these throughout the cycle. They only broke when increasing viscosity of the cooling melt overcame the surface tension.
A nice thing about this is that the percentage of metallic oxide is comparatively low compared to other metallic glazes. And, it is iron oxide, which is not toxic at all.
Bloating. These teapots have been refired to cone 6. This is just a standard porcelain body (feldspar, kaolin, silica, ball clay) that vitrifies fully at about cone 7. The second firing is obviously vitrifying to an equivalent of more than that.
Rutile blue glazes are difficult, blistering and pinholing are very common. You must get it right on the first firing or pinholes and blisters will often invade on the second. The melt fluidity increases, it runs and creates thicker sections in which the bubbles just percolate and just do not heal well during cooling (even if it is slow). When finishing leather hard or dried ware do not disturb thrown surfaces any more than necessary. Make sure that ware is dry before the glaze firing. Do not put the glaze on too thick. Limit the melt fluidity (so it does not pool too thickly in any section). Do not fire too high.
Refired to 1950F. The recipe is very flux heavy (high feldspar) dolomite:spodumene matte, zero silica, 4% tin oxide and less than 1% iron oxide. Sounds like crystallization territory. The plate on the left is the way it normally fired. On the right that way it started firing. The mug on the bottom has been refired at 1950F in oxidation and the color is back. The tin is likely a catalyst for the crystallization that occurred in the original result. Could be a fragile mechanism. Anyway, a period of oxidation at the end of the reduction firing should solve this.
Plus the glaze ran even more. The main problem was that the original firing was taken too high, about cone 02 (seven hour schedule). This body nears zero porosity there and is beginning to decompose. That generates gases. The second firing was taken to cone 03 in four hours. But the glaze just percolated more. However freshly glazed bisque ware in that same firing came out perfect. Lessons were learned. Fire faster. Keep it cone 03 or lower. Do not put the glaze on too thick. Use self-supporting cones to verify the electronic controller, they are much more accurate than regular cones.
This piece was made in Puebla, Mexico by a potter using traditional techniques. I have sawed it in half and refired one half at 1850F (orton cone 04). That half (the right one) has not shrunk, that means it has not exceeded the temperature of the original firing (the piece was likely glazed in the dry state, not bisque fired). Notice how the refire has darkened the body color. This is probably happening because the lead bisilicate glaze is maturing the surface of the clay (without the glaze it would take a much higher temperature to darken it to this extent, and, that would also entail shrinkage to a dramatically smaller size).
Yes. I paid $70 US to get a good one. It runs at a very high speed. It is battery powered and has many ends. It uses the 18650 lithium ion battery (also used to make power packs for laptops, power tools, even bikes and cars). I got this one on Ebay, it was listed as "Electric Mini Chargeable Engraving Pen Machine For Carving Grinding Metalworking". It is good quality and came with many unexpected accessories. It easily ground through the glaze and into the porcelain far enough to remove the speck. Then I just needed to fill up the hole with glaze and refire. This could also be used to remove burrs and nicks that might make a piece unsaleable. Can you engrave (e.g. text) into a glaze or porcelain? No. Even though there are heads intended to do this, the spinning motion gives the tool a mind-of-its-own about where it wants to go. And it hops. A lettering template might help but it would still be very difficult to get a deep enough scratch into the surface to be visible.
These were fired at cone 04. Both were glazed at the same time (with three brush-on coats of Spectrum 742 Dark Red). After the first firing they looked the same. But the one on the right was refired (to repair glaze issues) and it has lightened in color. This glaze is not opacified, its deep color relies on depth of application.