Cleaning Kiln Wash from Glass without Etching

 This is a note from Christopher Jeffree on a piece of research he did on the effects of three chemicals to remove kiln wash and investment residue from glass.  These are the common vinegar soak, my preferred citric acid soak and a tri-sodium citrate soak.  

This latter is a neutralised citric acid. It is widely used in the food, and engineering industries. It is an anti-oxidant. It is used to remove limescale also. Clearly it is an all around useful chemical.  It is edible, widely available, and cheap.

Christopher informs me that "One interesting application for it is for retarding the setting of gypsum plaster, so it is sold by plasterers and building merchants."  It is also available through Amazon, Ebay and sellers of food making supplies.  Typically, it is sold as tri-sodium citrate dihydrate.

Without more introduction, here is Christopher's research and conclusions.

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Which etches glass more – 6% vinegar or 6% citric acid? To cut a long story short, a quick experiment shows that it depends on the glass.

·         Both acids etch opal glasses, especially some reds, oranges and yellows, when soaked for 48h, but citric acid etches the same colours more in the same time.

·         Most transparent colours and clears are very resistant to etching, even when exposed for much longer times.

·         The neutralized form of citric acid, tri-sodium citrate, is just as effective as citric acid for cleaning glass of mould material and kiln wash but does not etch either transparents or opals during extended soaks of several days.

·         Bottom line:  to avoid glass etching, long soaks should be carried out in trisodium citrate, not in vinegar or citric acid

 

Samples containing mainly opal yellows and oranges.

Samples containing mainly opal blues and greens. Due to a slight difference in angle of illumination, the etch pits appear bright in this set of sample, but dark in the yellow set above.

 

©Chris Jeffree, December 2021

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.