Muriatic acid is a
common name for hydrochloric acid. Let’s look at what
is being cleaned off first.
The main components of
kiln wash are hydrated aluminia, kaolin, and colouring. Colouring burns away, hydrated
aluminum is inert at kilnforming temperatures, Kaolin begins a non-reversable change from
hexagonal plates to a crystalline form at about 600C/1100F and completes it by
900C/1650F. Now consider the characteristics of each element.
Aluminium Oxide
Aluminium oxide is widely used for its
hardness and strength. It is only slightly softer than diamond. In its hydrated
form it is a separator between glass and supporting structures. It has
excellent refractory characteristics with a melting point of 2,072 °C/3,762 °F.
But it is insoluble in water and all solvents. It is largely impervious to
acids.
Kaolin
|
Kaolinite structure, showing the interlayer hydrogen bonds in white. Source: Wikipedia |
Compared with other clay minerals, kaolinite is chemically
and structurally simple. It consists of layers, each bound together by
shared oxygen ions. The layers are bonded via hydrogen bonding between
oxygen on the outer face of one sheet and the other. … The close hydrogen
bonding between layers also hinders water molecules from infiltrating between
layers, accounting for kaolinite's non-swelling character.
When moistened, the tiny plate-like crystals of kaolinite
acquire a layer of water molecules that cause crystals to adhere to each other
and give kaolin clay its cohesiveness. The bonds are weak enough to allow the
plates to slip past each other when the clay is being moulded, but strong enough
to hold the plates in place and allow the moulded clay to retain its shape. Source: https://en.wikipedia.org/wiki/Kaolinite
It is this slipperiness that makes it a good carrier of
the aluminium hydrate. However, kaolin
begins a non-reversable change from hexagonal plates to a crystalline form at
about 600C/1100F and completes it by 900C/1650F. It is the crystalline form that
sticks to glass. So, it is the clay (kaolin) that needs to be removed from the
glass.
Hydrochloric acid as a cleaner of kiln wash
Glass is almost
impervious when it has a minimum of modifiers. Glass which
has a minimum amount of [modifiers] and is almost entirely SiO2 is remarkably
chemically inert and reacts only with very strong alkaline (bases) materials. Source: https://www.quora.com/How-come-hydrochloric-acid-does-not-burn-through-the-glass-bottle-that-its-stored-in
Note that coloured and fusing glass have
a significant level of sodium and potassium modifiers. This means that fusing
glass is subject to attack by hydrochloric acid.
Safety
notes on hydrochloric acid
Being a strong acid, hydrochloric acid is corrosive to
living tissue and to many materials, but not to rubber. Typically, rubber
protective gloves and related protective gear are used when handling
concentrated solutions. Solutions of less than 25% cause skin irritation,
serious eye irritation and respiratory irritation. Over 25% causes severe skin
burns and eye damage. It is also a precursor of many illegal drugs. Serious
safety gear is required to handle even 10% solutions.
Even then:
“Clays are not truly soluble in HCl
acid, [but] exposure to HCl acid does affect the structure of clay minerals. Hydrochloric acid cleans clay minerals by removing free iron
oxide from the surface. … The dissolution of kaolinite clay in hydrochloric acid solutions
has been carried out in the presence of fluoride ions. Leaching in the presence
of fluoride ions activates the clay for leaching, making higher extractions
possible at lower roasting and leaching temperatures. Acetic acid [vinegar] is less effective.” Source: Stability of Clay Minerals in Acid,
by D E Simon and M S Anderson. https://onepetro.org/SPEFD/proceedings-abstract/90FD/All-90FD/SPE-19422-MS/68436
This piece of research shows that
hydrochloric acid is most effective in combination with fluoride and heat.
Other reported research from Researchgate shows:
“Kaolin and other clays are
partly soluble in acidic solutions (organic or inorganic acids in water) but
the … solubility is never complete. Increasing the acid content doesn't …
increase the solubility.” Philip
G Jessop, Queen's
University.
“Potassium
hydroxide … will get
kaolinite dissolved with a white residue for selective leaching. … The most aggressive
solvent is hydrofluoric acid which "kills" almost all silicates [including
kaolin]. … For the kaolinite group … use hydrazine as solvent.” Harald G. Dill, Leibniz Universität Hannover.
Hydrazine is highly toxic unless handled in solution. Hydrofluoric acid may dissolve
the kaolin, but it also dissolves the minerals in glass. Both these chemicals are extremely
dangerous.
Conclusion
It is
not advisable to use hydrochloric (muriatic) acid as a cleaner of the kaolin in
kiln wash from glass.
There
are other much safer methods which use a chelating action rather than
attempting to dissolve the almost insoluble kaolin. These are citric acid for
brief (less that 24 hours) soaking, or trisodium citrate for longer periods.