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jo4550

Member Since 08 Sep 2010
Offline Last Active Apr 22 2014 05:40 PM
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In Topic: is gilding / gold leafing food safe ?

19 April 2014 - 06:56 PM

Hello Kennedy

It is normal for some glazes to change on a refire especially when you are approaching the softening point (I do not mean melting point) of the glaze.  This is the point where the fluxes start to be active.   This has become very obvious when running the laser decal classes and searching for the softening point for the decal to adhere.   There is a fine line where the glaze doesn't react and this is at the lower end of the temp scale.   I would hazard a guess that your glaze has a flux that has a very wide firing range and it is perhaps Gerstly Borate.  You seem to have reached a point at cone 06 in your glaze where the glaze is already outgassing and the cooling at 06 halts the process and and traps the gases changing both the colour and the integrity of the glaze.   

 

To resurrect this glaze AND burn off the lustre find a cool spot in your glaze firing and take back up to top temp.   This should restore the glaze.  I say cool spot as refiring is almost like going a cone higher than the original firing.

 

I would have attempted to save the gold and platinum surfaces by adding a couple more layers (with firings in between) and firing to your gold firing temperature before I took the drastic step of burning the lustre off.  To remove small areas of lustre it is common to use a gold eraser but it has to be used carefully as you can "matte" the surface of the glaze that you are rubbing.  Lustre can also be removed by using hydrofluoric acid but I personally wouldn't go down that path for H&S reasons.  It also mattes the surface of the glaze where the lustre has been removed.

 

Cheers

Johanna


In Topic: Your Best Guess On Overglaze/Lustre Technique

05 June 2013 - 08:19 PM

There are a couple of issues here but I would like to start off by pasting an excerpt from the Heraeus fact sheet on Lustres as I have run out of space to add any more attachments. This can be found in its entirety at http://tinyurl.com/lvt6m7c
Heraeus is one of the largest overglaze and lustre producers/suppliers in the world. They are based in Germany.



1 General Information
Lustres are based on metallic compounds dissolved in organic solvents. After firing they form a very thin layer (less than 0.1 μm).
Typical characteristics of lustres are their brilliance as well as their metallic iridescent brightness which occurs after firing on smooth substrates.
The lustre loses its iridescent effect on matt surface and appears matt.
Lustres are suitable for the decoration of glass, porcelain, bone china, earthenware

2 Firing Range
480-630° C / 896-1166° F for glass and lead-crystal.
650-900° C / 1202-1652° F for porcelain, bone china, earthenware and tiles.

3 Precious Metal Content
Lustres contain less than 6 % precious metal or are precious metal free.

4 Properties
4.1. Mechanical Resistance
The mechanical resistance of lustres does not achieve the same standard as most
ceramic colours and precious metal preparations because the formed lustre film is
very thin. Therefore, we recommend that customers test the decorations under
their own conditions to achieve the required resistances.

From this excerpt you will see that lustres applied to tiles used as a splash back may be a short term measure as constant cleaning with an abrasive will soon wear off the lustre.


However I am assuming that you have taken this into consideration.

I am finding it hard to get a good image of the tile to show that it has a creamy base so I cannot comment on how many firings this would have had but I suspect only one overglaze firing.

As you can see from the excerpt lustre can be fired anywhere up to 650-900° C / 1202-1652° F depending on the surface it is on. The controlling factor is the softness of the glaze that it applied to at the temperature you are firing to. The softer the glaze becomes the more the lustre will sink in. This is especially so for low earthenware glazes.

As I have said in previous posts Duncan products are rarely, if ever, used by the porcelain painters (the main users of Overglaze techniques). They buy products that are are manufactured by the suppliers and not just repackaged and rebranded as in the case of Duncan overglaze products. I have also never heard of lustres being described as Lustre Overglaze.

Regarding Duncan's instructions that you supplied here are my thoughts.

1. Any glaze or surface is overglaze compatible. (I don't understand the need for this statement)
2. The firing temperature is suspect. Maybe it is a typo and it is meant to be cone 016. I don't know whether they are offering a normal lustre because lustres starts to burn off after 900.C and they state to fire to 1000.C I would normally recommend firing a cone 6 glaze to 760-780.C however I have refired them successfully to 810.C In your situation where you will need extra mechanical strength I would suggest you run a test and see how the cone 6 glaze interacts with the MOP at 900.C. It is a chore but you won't know until you try as each glaze behaves differently.

The MOP that I use is a Fay Good product and can be purchased in the USA from China Painting today. Here is a direct link http://tinyurl.com/mg3ml75.

#Ceramics Monthly January 2013 contains a techno file on Lustres
#Pottery Making Illustrated May-June 2013 has an article on how to use lustres.

Johanna DeMaine

In Topic: Videos of people making relief decorations in monochrome...

21 May 2013 - 09:01 PM

Here is a youtube link of an old Wedgwood 1930's film that shows you the sprigging process.


Here is also a link to 2010 CAD archived feature on sprigging
http://ceramicartsda...-making-sprigs/

Johanna

In Topic: Newbie questions about firing and mildew

05 May 2013 - 01:51 AM

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.
Attached File  Black spot.pdf   492.53KB   23 downloads

Regards

Johanna

In Topic: is gilding / gold leafing food safe ?

04 May 2013 - 09:57 PM

thanks for the link Johanna! Posted Image

also, would you recommend a respirator ? I searched for paint respirator on the 3M website ( http://bit.ly/10yLzPU ) but I'm not sure which one to go with...

EDIT: I think I will go for the 3M 6100 half piece mask with the P100 Particulate filter when handling plaster and 6001 Organic Vapor filter when working with luster/china paint

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.westernsa...newnorth1.html"



Regards
Johanna