: jo4550 - Viewing Profile

Hello Sue
What you are describing is quite prevalent in refiring glazed ware at at China paint temperatures. It is often called mildew but it is actually black spotting that occurs on the UNGLAZED surfaces when the clay body has NOT been fired to full maturity. Hence you will normally find it under the work (where it touches the shelf)and footrings. It is not trapped carbon from poor bisqeing. It is mostly found in older pieces that have been around a while. It is carbon being trapped in the open pores of the body and this can be generated from a variety of sources.

The carbon can become reabsorbed and trapped from the media being used so it is more prevalent to occur from oil based mediums than water based mediums. This coupled with poor circulation and ventilation in the kiln during firing allows the pores of the unglazed clay surfaces to reabsorb this new carbon. This carbon can be burnt out with repeated firings especially if the temperature is raised slightly higher. The easy answer to all this is that if you are making things from scratch make sure you fire to the full maturity of the clay or otherwise get a clay that vitrifies at the temperature you want to fire to. Then this problem shouldn't occur. Always make sure you have the lid or door of the kiln slightly ajar in the overglaze firing until at least 650.C This allows the gases generated from the mediums to be dispersed. Always allow plenty of space between the shelves in the kiln during an overglaze firing. Always try to fire to the highest temperature in your overglaze firing depending on your glaze type. eg if using porcelain fire to 800-820.C rather than the lower 760-780.C option. This allows more carbon to burn out. If at all possible fire your pieces lifted OFF the shelf by little wads or pins.

I have attached a small .pdf made from page 125 of "Ceramic Faults and their remedies" by Harry Fraser. This deals with this topic in greater detail and it saves me from having to type it all out.
 Black spot.pdf (492.53K)
Number of downloads: 15

Regards
Johanna

dontstareatthesun, on 05 May 2013 - 10:14 AM, said:

Hi Kennedy
You seem to be on the right track with your choice. Here is an extract from an article I wrote called "Health and Safety and Overglaze (in paticular lustres)" which is reprinted here on my website http://overglaze.dem...rg/?page_id=460


"My method of protection from fumes is as follows. I use a fume booth constructed by my husband together with a respirator while I have developed a method of working that limits my exposure to the hazards involved. I use a resist method whereby I estimate that 90% of my time is spent using lustre resist which is quite inert. The other 10% is the actual application of the lustre, as all the fine lines apart from some gold pen work is achieved by resist. I am confident that I am limiting my exposure to lustre.

As well as this I wear a respirator. It is a Norton brand 7700 series silicone half face mask model with 2 N7500-1 organic vapour cartridges. It is extremely comfortable to wear despite the fact that I wear glasses. These cartridges are not suitable for clay dust etc. For that you need a particulate filter. However it is not enough to just wear the respirator. It must be maintained. When you have finished using it the inside must be wiped and then the whole lot stored in a sealed (clip lock) bag). This extends the life of the cartridges and keeps dust out. The cartridges need to be replaced when fumes can be smelled through the respirator. Norton has recently been taken over by North Safety Products. A web link to view is http://www.westernsafety.com/newnorthrespirators/newnorth1.html"



Regards
Johanna

http://chinapaintingtoday.com/store Look under Precious metals. I can recommend the Fay Good 12% liquid bright gold. It is a German product. It is the one I use. Another link to gold is
http://www.thegoodstuff.com/ghlb1.html. This is also a German product. Germany has a history of producing the best gold.

Golds and lustres are best bought from China/Porcelain painting suppliers. I don't know of too many painters who use Duncan products. They are aimed more at the hobby market.

Regards
Johanna

Thrown_In_Stone, on 15 March 2013 - 02:09 AM, said:

Several questions spring to mind here. How are you glazing? Are you glazing the inside and outside separately and in what order. If so what is the drying time between the inside and the outside? Are you brushing on the glaze? Are you spraying the glaze?

I ask these questions as it seems to me that you may have a problem with crawling (glaze pulling away from the surface during the firing). I have experienced this when dipping glazes on thinly constructed work. The bisque becomes saturated when more glaze has been added to a surface. Bubbles or blisters appear on the unfired glaze during drying. These can be smoothed back by rubbing. However in the majority of cases the glaze crawls during the firing because of the lack of adhesion of the glaze to the surface.

With brush on glazes this can also occur when subsequent layers can lift off the first layer be it ever so slightly.

With sprayed glazes this can also happen when too thick a layer is applied in one hit rather than several thinner layers.

One way or another the bisque can becomes saturated. Saturated areas do not have good adhesion with the glaze.
With reference to your statement "about 70% of the problem pots have the problem on the side facing the elements" it may be that the side facing the elements initially heats up at a more rapid rate than the rest of the pot and trapped moisture causes further problems with poor glaze adhesion.

I note also that your bisque temperature is higher than your glaze temperature. Could this be contributing to the poor adhesion?

Regards
Johanna

downtoearthpotter, on 07 February 2013 - 03:22 PM, said:

Of the 3 main Ball Clays available in Australia (Ball Clay FX, Ball Clay R and Clay Ceram) Ball Clay FX has a very similar Chemical analysis. The TiO2 content is fractionally more whilst the Fe2O3 is slightly less. I have successfully substituted Ball Clay FX for OM#4 in glazes tests. However which ever you choose should be tested first as analysis change depending on what part of the pit is being mined.

I have found the Glaze Program Matrix by New Zealand ceramist Lawrence Ewing to be spot on. This program has analysis of all the main materials used in Australia. It's database of materials used world wide is extensive. It is really easy to substitute Australian materials for materials used elsewhere and make the necessary adjustments. It is the best money I have spent on glaze calculation programs for my needs in Australia.

Johanna