Wednesday, 8 November 2017

Kiln Wash Removal


There are a variety of ways to remove kiln wash.  Many depend on whether the surface is flat, smooth curves, angles or textured.  Some are applicable to both.

Flat surfaces are the easiest to deal with.

Abrasive methods work well with a variety of tools. 

They can range from large paint scrapers to smaller ones with a Stanley blade inserted. 






Coarse open mesh plaster board (dry wall) sanding sheets are very useful. There are frames that you can fix them to, but sanding without the frame works well too.





Using power tools to sand the shelf is not advisable.  It is too easy to remove lots of material, including the surface of the shelf – even the hard, ceramic ones.  This leads to minor depressions in the shelf and consequent bubble difficulties when firing.

Do not be tempted to sandblast as that will, almost certainly, create small depressions in the surface of the shelf.  Sand blasting is only possible on steel moulds.

Wet

Wet methods are applicable if you are concerned about the dustiness of the process.  You can dampen the kiln wash on the shelf and sand or scrape as above.  You will create a paste or slurry in front of the scraper which can be bagged and put in the waste.

You can also use a lot of water and the green scrubby washing up pads.  Unless you use a lot of water, the kiln wash builds up in the scrubbing pads.





Some people use vinegar or chemicals such as lime away with the water. Both are acids – lime away being much the strongest.  I am sure these are used on the basis that kiln wash is based on lime.  However, the material that makes the kiln wash stick to the shelf is china clay which is barely affected by the chemicals.  In addition, the alumina hydrate is impervious to many chemicals available to kiln workers.

One drawback to using wet methods, is that the shelf is wetted and needs drying before use.  The amount of water used in applying kiln wash is minor in relation to washing or soaking the shelf to remove the kiln wash.

Do not be tempted to use pressure washers. Yes, they will remove the kiln wash, but also leave little divots in the shelf which will cause later problems.

Smooth curves
Kiln wash on moulds with smooth curves can be removed with flexible sand papers or the plaster board sanding screens.  Normally, the coating of kiln wash is thin and does not require a lot of pressure or effort.

It is possible to dampen the kiln wash and take it off with scrubbing pads.  Make sure you do not use excessive pressure.  If you have wetted your ceramic mould, you need to dry it very carefully, to avoid having the mould break in the next firing.  This is because trapped water can turn to steam and the pressure will break the ceramic. It is best to let the mould air dry for a week or so before putting it into the kiln to thoroughly dry at about 90°C for a couple of hours.

Do not be tempted to use a pressure washer or water pick, as both can erode the surface of a ceramic mould.

Curves with angles

Moulds with angled areas such as at the bottom or corners of a rectangular mould need a flexible abrasive to clean out the angles.  You can fold a piece of sand paper to use the folded edge to do the final cleaning out of the angles.

The same can be done wet, but all the precautions about wet removal of kiln wash need to be observed.

Textured
Textured moulds require much more care in cleaning the kiln wash away, to avoid damaging the images and textures.  The flat upper surfaces can be dealt with as though it was a flat kiln shelf.  The indentations need to be more carefully treated.  Folded pieces of sand paper can be used to clean the delicate areas.

To ease cleaning of textured moulds it seems best to use kiln washes without china clay as the binder.  These will brush out of the mould with a fibreglass bristled brush.  It is now popular to use boron nitride - often sold as Zyp - as a coating for these moulds.  This needs to be brushed out and renewed with each firing.


Removing kiln wash from glass

Kiln wash stuck to the glass can present greater problems, because you want to avoid marking the glass.  It is best to start with the least aggressive abrasive, such as a green scrubby, and progress toward more aggressive and abrasive methods.  When using the more aggressive methods, try the finest grit first to see if that will work, as it makes for less work cleaning up the grinding marks from the glass.

For flat glass, you can work with a succession of finer loose grits, or a succession of finer diamond hand pads.  




Flexible diamond impregnated sheets can be used for curved surfaces.  Again, this requires a succession of finer grits to get to the polished stage.





You can use small hand held rotary tools with diamond and felt pads to polish out stuck kiln wash.  This helps to remove some of the labour of polishing the glass.





Some people advocate the use of acids to remove the kiln wash.  However, you must remember that glass is an alkaline material and acids will tend to mark the glass.  Vinegar is a mild acid, but prolonged exposure will etch the glass.  Strong chemicals such as lime away or etching cream or hydrofluoric are all strong acids and will mark the glass after brief exposure to them.