More Information on Citric acid

Vinegar is not recommended for cleaning of glass, and especially not to soak glass in to remove kiln wash or investment materials.

The reasons for avoiding vinegar are that

·        Dilute vinegar - as culinary vinegar is - attacks glass, giving a mild etch to the surface similar to devitrification.  Concentrated vinegar – oddly - does not attack glass as strongly.

·        It is of variable quality – due to uncontrolled strength, various culinary additives, etc.,

·        It has a strong odour, and

·        It takes a long time to work.

Citric Acid

However, there is an acid which works very well to remove investment materials and kiln wash without affecting the glass.  It is the humble citric acid.

Citric acid is a weak organic acid that has the chemical formula C₆H₈O₇. It occurs naturally in citrus fruits, although that is not the best source for cleaning purposes.

More than two million tons of citric acid are manufactured every year. It is used widely as anacidifer, as a flavouring agent and chelating agent. It is the last that is of most interest to kilnformers.

Chemical characteristics

A citrate is a derivative of citric acid. There are many formulations. Two examples are a salt that is named trisodium citrate (also known as sodium citrate); and an ester called triethyl citrate. We are more interested in the first as it is cheap and widely available.

The citrate ion forms complexes with metallic cations.  It forms complexes even with alkali metal cations. This makes citric acid an excellent chelating agent, especially of interest in removing kiln wash and refractory materials from glass.

Chelation 

This is a type of bonding of ions and molecules to metal ions. The agents are usually organic compounds. Chelation is useful in applications such as providing nutritional supplements, in chelation therapy to remove toxic metals from the body, in MRI scanning, in chemical water treatment to assist in the removal of metals, and in fertilisers, among other things. 

Citric Acid as a Cleaning and chelating agent

Citric acid is an excellent chelating agent, binding metals by making them soluble. Among many cleaning uses are:

    to remove and discourage the build-up of lime scale, from boilers and evaporators. 

    to treat water by chelating the metals in hard water, cleaners produce foam and work better without need for water softening. Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. 

    A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. 

    Citric acid can be used in shampoo to wash out wax and colouring from the hair. 

    In industry, it is used to dissolve rust from steel and to form a coating on stainless steels to resist corrosion.

Its use in kiln forming is to make use of the chelation properties when dealing with kiln wash and investment material residues.  Aluminium hydrate is the main ingredient of all kiln washes.  When it becomes bound to glass, it is impervious to almost all chemicals.  The chelating property of citric acid enables the bond between the glass and the kiln wash to be broken by incorporating the molecules within its own, making a colloidal solution.  This process is approximately 6 times faster than any vinegar solution and without the odour and etching risks.

A sample of the affected glass followed by 4 hours in citric acid and 24 hours in vinegar.
Credit: Christopher Jeffree

A 5% solution made up with 50gm of granular citric acid in 1 litre of water is all the strength that is required. The affected glass can be soaked in this solution for the time required to complete the chelation without the risk of etching, and without needing ventilation to remove smells.  Unless you are using a lot of cleaner, it is better to make up much smaller amounts as mould can grow on this organic solution.

A 5% solution made up of 50gms citric acid in 500ml of water and 500ml of isopropyl alcohol makes an inexpensive and effective glass cleaner. However, if left for a length of time, it becomes sticky.  Apply the solution, scrub the glass and immediately wipe off the solution.  Then polish the glass dry. The alcohol in the solution makes keeping large quantites possible. 

This post was compiled with the assistance of Wikipedia, Christopher Jeffree and my own experience.

Although this post remains valid, there is another chemical for long soaks to remove mould material or kilnwash.