Citric Acid Cleanser

Christopher Jeffree has kindly outlined the reasons for the effectiveness of citric acid as a cleaner for removing refractory mould residue and acting on kiln wash stuck to glass.  This is his work (with a few personal notes removed).

"Citric acid works well for removing the plaster scale that builds up in vessels used to mix plaster, and it helps to remove traces of investment plaster and kiln wash from glass.  Its metal-chelating properties probably help with dissolution of calcium deposits, but I am less clear why it is so good at removing kiln wash.  The constituents of kiln wash are kaolin and alumina hydrate, neither of which I would expect to be soluble in dilute acids.  Equally, the refractory materials in investment formulae I would expect to be insoluble.  However, kaolin forms layered structures in which flakes, molecular layers, of alumina hydrate and silica interact through hydrogen bonding. It is possible (I am guessing here) that citric acid can disrupt those hydrogen bonds, thereby disaggregating the clay.  All we can say is that empirically, it works.

"I prefer to use citric acid partly because it has a defined composition, but also because it is safe and pleasant to handle – no odour, and comes in the form of easily-dissolved dry crystals like granulated sugar.  Vinegar stinks, and glacial acetic acid is  an aggressive flammable, corrosive liquid with a chokingly acrid smell.

"Calcium sulfate has low solubility, but is not completely insoluble in water - gypsum (calcium sulfate dihydrate) has a solubility of about 2.5g per litre (0.25%)  from 30-100 C. Its solubility is retrograde, meaning that it decreases, rather than increasing, with temperature.  Natural gypsum is an evaporite, a type of rock that often forms by evaporation of lake water in a geological basin with little or no outflow. It can also be produced hydrothermally in hot springs, when water containing sulfuric acid passes through limestone.  

"Calcium citrate is not very soluble either, only in the order of about 0.85g per litre, but the important thing from our point of view is not to get the material into solution but to separate its crystals and make it detach from the glass.

"In other contexts, warm citric acid is used by jewellers and silversmiths as a pickle for dissolving copper oxide (firestain) from silver and gold alloys  after heating / soldering.  It is a safer alternative to the traditional jeweller's pickle of 10% H₂SO₄.

"Citric acid also dissolves rust from iron, without much etching the iron itself, so is good for cleaning rust off tools etc.

"These pictures show a plaster mixing bowl with (presumably) CaSO₄-rich deposit on the surface, cleaned by soaking with 5% citric acid for 4 hours,

and flash from the pate de verre castings with tightly adhering kiln wash, cleaned using 5% citric acid soaked for 4 hours, and vinegar (white wine) soaked for 24 hours.

"I'm not sure about reaction products - I was speculating a lot there, running through hypotheses that I can't support. We don't really have data on the composition of the layers that are stuck to the glass, or a clear idea of why they sometimes stick and sometimes don't (e.g. the differences between transparent and opal glasses in this respect). Maybe this would be a topic to discuss with technical people at Bullseye."

Hope this helps
Best wishes

Chris Jeffree

Subsequent to this work Christopher has done more work and found that Tri-sodium citrate is an even better chemical for cleaning glass of kiln wash and mould material.

Formers

This post is not about the materials that go into the making of glass, but about ways of forming glass once melted or dripped into a space.

Formers are a bit different from moulds.  They are more like the formers used in concrete structures – they are there to resist the movement of the contained materials and give the form or shape desired rather than a natural flow.

These formers can be of anything that can resist the firing temperatures of the process.  Some of the materials are stainless steel, ceramics, fibre board and paper, vermiculite, kiln brick, and I am sure there are others.

Refractory Fibre

Most of these require a separator between themselves and the glass.  The ones which do not are untreated refractory fibre board and fibre paper. 

Most paper is not sufficiently strong to stand on its own. Instead it is used flat and the shape cut out of it.  It can be made in several layers and pinned together to achieve the height desired.  It should be lined in the interior with a thin fibre paper to avoid seeing the layers of the former in the edge of the glass.

For thicker work, fibre board can be used with the shape or form cut from it. Alternatively, it can be used on its side backed up by kiln brick or other material to resist movement. More information on methods and safety are here. 

If hardened, refractory board and paper will need separators between glass and former, just as most other materials will.

Sometimes the fibre board and fibre paper are not heavy enough to resist the flow of the glass.  You can use weights to help resist the movement.  At other times, the glass flows under the fibre and then you need something heavier.  Fortunately, there are a number of refractory materials that can be used.

Other common formers

Vermiculite board is another refractory material that can be cut and shaped much like fibre board.  The vermiculite needs to be covered with kiln wash where it might come into contact with glass or be lined with fibre paper or another separator.

Calcium silicate board can be used in much the same way.  It also needs a separator but does not stand up to such high temperatures as vermiculite.

Ceramics, especially in the form of cut up kiln shelves can be used as straight formers.  They have the advantage, over refractory fibre paper and boards, vermiculite and calcium silicate, of being heavy.  They can resist the movement of thick glass. They need to have a separator and usually a 3mm fibre paper, cut 3mm shorter than the final thickness of the piece, will provide the cushion in the movement that the glass needs.

Kiln brick is an often forgotten former.  The bricks can be cut and formed in many ways, even if not so freely as fibre board and paper.  The bricks do need fibre paper separators to keep the glass from getting into the pores of the brick.

Stainless steel is a common former too.  These are usually formed into an already determined shape and so are not so adaptable as many of the other formers.  Steel contracts much more than glass and needs a cushion of fibre paper, usually 3mm thick to avoid sticking to the glass.

More information on most of these formers can be found here.