|Monthly Tech-Tip |
Refiring pottery and porcelain comes with many issues that can ruin ware, knowing about these will help you do it successfully.
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Refiring ceramic ware can be a necessity because of the process (e.g. decal, lustre or successive glaze firings) or to repair defects (e.g. crawling).
However, refiring ware can lead to problems. The most common is daunting, avoiding that involves careful control of the firing schedule. The second firing is actually the first time the fired ceramic matrix, as it existed coming out of the first glaze firing, is heated upward to maturity. That ceramic is much more brittle than it was in the bisque or dry state and it will propagate quartz inversion cracks very well. It is also dense and has little ability to withstand the variation in thermal expansion that comes when the temperature across a piece of ware is not kept even. A second firing to the same temperature also gives the clay body double the heat treatment. Dark burning clay bodies can respond by going darker or changing to red to brown. Vitreous bodies, especially, when fired beyond their point of maximum density, can blister and bloat on a second firing. Warping and plucking are also a good possibility. The incomplete decomposition of any body materials will continue in a second firing, possibly generating gases that could cause boating or glaze blisters.
Glazes with high melt fluidity often blister, or blister more, on a second firing. The glaze materials, as a mix, were melted on the first firing and combined to form a glass. On the second firing that glass will melt completely, and seal the body surface, at a much lower temperature than on the first firing. Frits, by contrast, have already been melted, thus glazes containing higher percentages of frits, will yield fewer surprises because it is actually their third firing. Reactive glazes can mellow on second firing (the phase differences created the first time around can bleed together). Stains that depend of fast firing with change color.
The color of stain/glaze pairings that must be fired quickly to survive incompatibilities will certainly change (e.g. zinc glazes with zinc-hostile colors). Glazes that depend on crystallization for the visual effect will certainly fire different on the second time, often crystallizing more.
Can overglaze decoration be done on already glaze-fired ware? Yes. The glaze will melt even better than the first firing and colors will embed, dissolve, bleed, and crystallize according to the stain medium into which the pigment or oxide is mixed. Of course, plenty of CMC gum in the overglaze colors will be needed to make brush strokes adhere and stay put.
Can crazing be fixed by refiring. No. Crazing happens because of a mismatch in thermal expansion between body and glaze, firing can only hide that temporarily, it will come back.
Can we predict how refiring will affect thermal expansion (CTE)? If you can get through this paragraph you will realize the answer is no. CTE could increase or decrease in refiring due to many factors. If a body is high in feldspar then the second firing is going to give the feldspar more time to dissolve the particles of quartz, that will reduce the thermal expansion. If the body has less feldspar and plenty of kaolin the conversion from kaolinite to meta kaolin will advance further but whether that reduces or increases thermal expansion will depend on the amount of quartz present and the degree to which the feldspar glass can absorb it. If pyrophyllite is present the degree to which its natural mineral form is changed in firing 2 will increase CTE. The same if wollastonite is present. If the quartz is of a larger particle size the CTE will change less on refire (if there is significant feldspar present). The particle size of the kaolin will also determine the extent to which its conversion and dissolution will proceed first and second firing. If frit is present in the body the picture is more complex depending on whether it is a low or high-expansion product. The density of the body and the degree to which it was first fired past zero-porosity point will affect first firing completion of vitrification. Bodies are often overfired past zero porosity point as standard practice, these are less predictable in fired character because decomposition of mineral species is occurring. If bodies are underfired as practice then the second firing will certainly mature them more is sintering is not the primary bonding mechanism, CTE could increase or decrease according to many other complex factors. The thickness of ware, area of contact with shelves, thickness of shelves will determine the evenness of maturity development on ware cross-section. Different placement of ware in the kiln such that different parts are element-facing will affect the development of CTE distribution across a piece and the evenness is its distribution. The amount of very fine quartz in the ball clay used in the recipe will affect whether the feldspar-rich glass is able to absorb more quartz or meta kaolin. If glazes are applied thickly they may impose compressive forces that may affect or overcome the body’s imposition of its own CTE. If the body contains talc as a flux it is likely to mature more on second firing, whether that increase reduces or increases CTE is dependent on many other factors. Interactions of all of the above can affect microstructure development or decay that that in turn affects CTE in complex ways.
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 slower the firing the more likely this is to happen (refires must be done more slowly to avoid dunting). This type of bloating is not easy to explain, only a few mitigating measures are obvious (putting ware in a cooler part of the klin, avoiding glazes of very high boron content, using a stoneware or whiteware instead of a porcelain, using higher quality body materials).
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.
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.
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.
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 the way it started firing. The mug on the bottom looked like the plate on the right, but look what happened after refiring at 1950F in oxidation! The tin is likely a catalyst for the crystallization that occurred in the original result. Could be a fragile mechanism. This underscores the need for a period of oxidation at the end of a normal reduction firing.
Rutile blue glazes are difficult, blistering and pinholing are very common. You must get it right on the first firing because pinholes and blisters will likely invade on the second. On the second firing the melt fluidity increases, the glaze runs and creates thicker sections in which the bubbles 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. Drop and hold firing schedules can help a lot (coupled with a slow cool if needed).
This Cone 10 matte mug has been refired to attach decals. The fired matrix of the body is now brittle and dense and contains millions of quartz grains of many sizes. During the refire up through quartz and cristobalite inversions each of them experiences sudden volume increases. This does not happen in the glaze because its quartz particles were dissolved in the melt and converted to silicates during the previous glaze firing. The suddenness of the expansion depends on the rate of temperature increase and its extent depends on the size of the quartz particles. The body's passage through these two zones stretched the glaze and cracked it. Had the glaze fit been better (under some compression) it would likely have been able to survive.
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.
|By Tony Hansen|
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