Logged in as Level 2 access: Logout
Blisters are evident on the fired glaze surface as a 'moonscape' of craters, some with sharp edges and others rounded. These craters are the remnants of bubbles that have burst during final approach to temperature or early stages of cooling. In some cases there will be some unburst bubbles with a fragile 'dome' than can be broken. Blisters can vary in size and tend to be larger where the glaze is thicker.
Is the glaze fluid enough?
Often glazes appear like the melt should have plenty of mobility to heal but this can be deceptive, a melt flow testing regimen is the only way to know for sure (melt flow testers have a reservoir at the top of a steep incline and the glaze runs down a calibrated runway). Generally a more fluid glaze will heal blisters much better (see section below on blisters occurring even after refire).
Are excessive gases generated during glaze fire?
Significant amounts of gases can be generated within the glaze itself due to the decomposition of some materials after melting has started (i.e. dolomite, whiting, manganese dioxide, clays, carbonate colorants, etc). Substitute these materials for others that melt cleanly. For example, use frits, supply CaO from wollastonite instead of whiting or dolomite, use cleaner clay materials, or use stains instead of metallic carbonates. If you are using organic additives be aware that some of these can generate considerable gases during decomposition; do tests without them, use an inorganic substitute or find way to disperse them better into the slurry.
You might be under estimating the amount of gases that are coming out. Are you holding the top temperature long enough? Perhaps a much longer than expected soak might be necessary (on very thick tile or sculptural pieces, for example, 24 hours might be needed). Could you do a test on a small piece to confirm this? It might also work to adjust the firing schedule to soak, decrease the temperature a little (so the glaze is still pretty fluid), hold it and then cool quickly for the next few hundred degrees to solidify the glaze.
Is the glaze recipe or chemistry the problem?The approaches to dealing with glaze chemistry issues differ in fast fire (e.g. tiles) and slow fire (studio pottery). In slow fire we want glazes that are mobile and can heal imperfections over a long soaking period. In fast fire we want glazes that remain unmelted until after 950C (gases from decomposition can occur up until this temperature) and then melt quickly after this.
Is the system is intolerant of gases?Gas release from decomposing materials in the body can continue until 950C. Many glazes begin melting long before this.
Is the glaze firing part of the problem?
Is it being firing in a gas kiln?
Is the body the problem?
Is the problem in the glaze mixing?
Is the problem glaze application?
Are you bisque firing? Is it done right?
All clays release gases from burning of carbon material and decomposition of other compounds. Some clays release sulphur compounds also. If the glaze is melting during release of these gases, they must bubble up through it. If the melt is stiff, the kiln is ramped up too quickly, cooled too rapidly, or the glaze melts too early, it will not have opportunity to heal properly.
Do blisters get worse even if you fire ware again?
This often happens and it is not easy to understand since one would think that there can be no source of gases if the piece has already been glost fired. Regardless of the reason if a glaze is not healing its blisters on multiple firings then it is not fluid enough. One does not fully appreciate how stiff the average glaze melt is until you work with crystalline glazes that are so fluid a bowl must be placed under the ware to catch the runoff. However the fired surfaces of these glazes are incredibly glossy and perfect. If your glaze melted more it would run more, however you can counter this by putting it on thinner. The melt fluidity of a glaze is primarily affected by the amount of flux, so you need to increase it. However if the flux you choose has a higher thermal expansion be prepared for the glaze to craze. This is actually a job for INSIGHT.
The perfect storm of high surface tension and high LOI: Blisters.
An example of how calcium carbonate can cause blistering as it decomposes during firing. This is a cone 6 Ferro Frit 3249 based transparent (G2867) with 15% CaO added (there is no blistering without the CaO). Calcium carbonate has a very high loss on ignition (LOI) and for this glaze, the gases of its decomposition are coming out at the wrong time. While there likely exists a firing schedule that takes this into account and could mature it to a perfect surface, the glaze is high in MgO, it has a high surface tension. That is likely enabling bubbles to form and hold better.
Carbonate gassing can cause glaze blisters
An example of how a carbonate can cause blistering. Carbonates produce gases during decomposition. This glaze (G2415B) contains 10% lithium carbonate, which likely pushes the initial melting temperature down toward the most active decomposition temperatures.
Let me count the reasons this glossy white cone 6 glaze pinholing
First, the glaze is very thick. Second, the body was only bisque fired to cone 06 and it is a raw brown burning stoneware with lots of coarser particles that generate gases as they are heated. Third, the glaze contains zircopax, it stiffens the melt and makes it less able to heal disruptions in the surface. Fourth, the glaze is high in B2O3, so it starts melting early (around 1450F) and seals the surface so the gasses must bubble up through the glaze. Fifth, the firing was soaked at the end rather than dropping the temperature a little first (e.g. 100F) and soaking there instead.
Rutile blue glazes: Love the look, hate the trouble to make it
A closeup of a cone 10R rutile blue (it is highlighted in the video: A Broken Glaze Meets Insight-Live and a Magic Material). Beautiful glazes like this, especially rutile blues, often have serious issues (like blistering, crazing), but they can be fixed.
This is when you should program a firing yourself
This is Polar Ice cone 6 porcelain that has been over fired. The electric kiln was set to do its standard cone 6 fast fire schedule, but a cone in the kiln demonstrates that it fired much higher (perhaps to cone 7 judging by the bend on the cone). This is a translucent frit-fluxed porcelain that demands accurate firing, the over fire has produced tiny bubbles and surface dimples in the glaze. The mug rim has also warped to oval shape. The lesson: If you are firing ware that is sensitive to schedule or temperature, use large cones and adjust if needed. If it fires too hot like this, then program to fire to cone 5 with a longer soak, or cone 5.5 (if possible). Or, program all the steps yourself; that is definitely our preference.
LOI it not important? Think again!
This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters. The LOI (loss on ignition) of these materials can affect your glazes (e.g. bubbles, blisters, pinholes, crawling).
What can you do using glaze chemistry?
There is a direct relationship between the way ceramic glazes fire and their chemistry. Wrapping your mind around that and overcome your aversion to chemistry is a key to getting control of your glazes. You can fix problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. Substitute frits or incorporate better, cheaper materials, replace no-longer-available ones (all while maintaining the same chemistry). Adjust melting temperature, gloss, surface character, color. Identify weaknesses in glazes to avoid problems. Create and optimize base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. Create glazes from scratch and use your own native materials in the highest possible percentage.
Serious blistering at low fire
An extreme example of blistering in a piece second-fired at cone 03. The glaze is Ferro Frits 3195 and 3110 with 15% ball clay. The twin of this mug was first fired at the same time, it had almost no blisters. The difference was that it was applied thinner. However, has this piece been slow cooled in the kiln these blisters would likely have healed.
Can this 5 lb thick walled bowl be fired evenly in an electric kiln. No.
When electric kilns, especially large ones are tightly packed with heavy ware, the shady or undersides of the pots simply will never reach the temperature of the element side, no matter how long you soak. In this example, the inside of this clear glazed cone 6 bowl has a flawless surface. The base is pinholed and crawling a little and the surface of one side (the shady side), the remnants of healing disruptions in the melt (from escaping gases) have not smoothed over. The element side is largely flawless like the inside, however it is not as smooth on the area immediately outside the foot (because this is less element-facing). Industrial gas kilns have draft and subject ware to heat-work by convection, so all sides are much more evenly matured.
Can you bisque fire at cone 02? Yes. But why? How?
The buff stoneware mug on the right was bisque fired at cone 02, the one on the left at cone 06. The cone 02 mug was immersed in the clear glaze for 1 second and allowed to dry. The other was glazed on the inside first, allowed to dry, then glazed on the outside with a 1 second dip. Of course, the cone 02 one took longer to dry. In spite of this, the glaze is thicker and more even on the one bisque fired to cone 02. How is the possible? The secret is the thixotropy of the glaze. When that is right, a one second dip will give the same thickness and evenness whether dry or bisque, 06 or 02. Why bisque fire to cone 02? To get a glazed surface free of pinholes on some stoneware clays.
Orange-peel or pebbly glaze surface. Why?
This is a cone 10 glossy glaze. It should be crystal clear and smooth. But it contains strontium carbonate, talc and calcium carbonate. They produce gases as they decompose, if that gas needs to come out at the wrong time it turns the glaze into a Swiss cheeze of micro bubbles. One solution is to use non-gassing sources of MgO, SrO and CaO. Or, better, do a study to isolate which of these three materials is the problem and it might be possible to adjust the firing to accommodate it. Or, an adjustment could be make to the chemistry of the glaze such that the melting happened later and more vigorously (rather than earlier and more slowly). The latter is actually the likely cause, this glaze contains a small amount of boron frit. Boron melts very early so the glaze is likely already fluid while gases that normally escape before other cone 10 glazes even get started melting are being trapped by this one.
Why is the terra cotta glaze on the right crystal-clear while the other bisters and clouds?
The answer is: Firing schedule. These are the same glaze, same thickness, Ulexite-based G2931B, fired to cone 03 on a terra cotta body. The one on the left is fired to cone 03, soaked half and hour and the kiln is turned off. The one on the right is fired to 100F below cone 03, soaked half an hour, then at 108F/hr to cone 03 and soaked 45 minutes, then control-cooled at 108F/hr to 1500F. The blisters likely heal on the slow cool. The micro-bubble-clouds likely dissipate on the first soak and gradual rise to temperature.
Out Bound Links
In Bound Links
Copyright 2003, 2008 http://digitalfire.com, All Rights Reserved
INSIGHT is ceramic chemistry