Limescale (consisting of mainly calcium carbonate, plus calcium
sulfate, barium sulfate, calcium phosphate, magnesium hydroxide,
zinc phosphate, iron hydroxides and silica, dependent on the
geographical area) is a problem in heated water systems wherever
water is obtained from limestone or chalk countryside. It
is formed primarily because the solubility of calcium carbonate
decreases with increasing temperature. Limescale is only a
problem if calcium carbonate deposits calcite
crystals, which may form directly or subsequent to metastable
hexagonal and fibrous vaterite crystal formation. Orthorhombic
aragonite crystals have a higher density and, although intrinsically
harder, are less prone to form hard scale, but are only about
one kJ mol-1 less stable 
and favored at higher temperatures.
Properties of crystalline polymorphs of CaCO3
Density, g cm-3
The mechanism by which the magnetic field produces its effect seems down to the presence of disordered hydrated CaCO3 aggregates ,a forming liquid emulsions which may be affected by the magnetic field and so convert into different prenucleation clusters and hence different structures on crystallization .
Once formed, crystals
are kinetically (if not thermodynamically) stable for hundreds
of hours. By drawing water through a static magnetic field
(B ~0.1 T , B~10 T m-1, it has
been shown that the initial amount of aragonite formed is
significantly increased over calcite in samples with and without
the presence of dissolved iron ,
although this aragonite eventually changes to calcite .
A separate experiment has shown that drawing a pure solution of calcium carbonate and bicarbonate through static magnets (0.16 T) for 5-30 min increases the precipitate formed on degassing the excess dissolved carbon dioxide . Magnetic treatment has been shown to affect the crystal surfaces of both aragonite and calcite .
The direction and variation of the magnetic field has also
been shown to be important ,
with crystal size decreasing with increasing magnetic field
. A different
group has showed agreement in a recent study where under similar
conditions (B = 0.5 T, flow rate = 0.1
m s-1) the magnetic field produces mainly a mixture
of aragonite (44%) and vaterite (42%) whereas without it well-crystallized
calcite (34%) is formed with little aragonite (14%) .
It has been proposed that the smaller water cluster size,
being more reactive, hydrates the calcium and carbonate (particularly;
see a recent supporting study )
ions more effectively and so encourages aragonite nucleation
the magnetic field may cause a surface and/or orientation
effects on the growing crystals .
It is possible, however, that the major effect is the magnetically
induced competitive formation of hydrated silica in suitable
solutions that then absorbs calcium ions . The pipe material and its surface were also found to be important .
There are many devices on the market for the magnetic treatment
of water for the removal of such limescale. The sales success of
these devices would seem to indicate that some work as promoted,
at least under some circumstances.
Magnetic treatment of water is
claimed to cause four effects: 
- Reduction in the amount of limescale formed.
- Production of a less tenacious limescale due to a change in
the crystal morphology.
- Removal of existing scale (3 - 6 months).
- Retention of anti-scaling properties for hours following treatment.
Many tests mainly utilizing single pass systems, however,
have proved negative .
Recirculatory systems, with prolonged magnetic exposure, give
more supportive results. Rapid movement (1200 rpm) in a strong
magnetic field (4.75 T) had a significant effect compared
with the movement or field alone .
A smaller number of larger crystals causes the effect as nucleation
is suppressed and crystal growth is enhanced. It is possible
that the effect is due to magnetically enhanced corrosion
promoting the release of Fe2+ that, even at ppm
reduces calcite but has no effect on aragonite production;
inhibiting the thermodynamic transformation of aragonite into
calcite. (Fe2+ may be used as a threshold inhibitor
by industry). However the amount of dissolution, required
by this theory, has not been found. Magnetic treatment devices
that create additional turbulence may enhance anti-scaling
effect , perhaps
by encouraging precipitation in the bulk (due to a combination
of magnetically modifies hydration and the shifting of charged
particles in the magnetic field )
rather than by deposition. Another contributing factor may
be the lowering of the surface tension, multiple passes producing
increased lowering up to about 8% .
A potential (= v.B.L
where L is the distance between detecting electrodes) is generated
when a dilute electrolyte flows (v) through a transverse magnetic
field (B, greatest when the flow is orthogonal to the magnetic
field). This may increase colloidal coagulation. A recent
well-controlled study has shown that scaling can be reduced
by a few percent by even one pass though a simple magnetic
device but that it is difficult to increase this effect to
more than about 20% even with extensive recirculation .
This study also showed an optimum in the flow rate as at too
high a flow rate the magnetic field was encountered only briefly,
an effect recently confirmed .
Recently, the presence of dissolved oxygen has been shown
important for the production of the magnetic effect for forming
aragonite rather than calcite , and for initiating scaling . It may be assumed that many of the studies described on this
page did not control for oxygen content, so their effects
may have been moderated by the varying dissolved oxygen contents.
There are 'electronic' devices, related to the above purely
magnetic devices, that use weak electromagnetic signals utilizing
a coil wound around a pipe. A square-wave pattern is often
used as it effectively contains many frequencies from a few
Hz to 100 kHz .
This changes the magnetic field in a manner similar to a number
of rapid passes past a very weak static magnet. However the
changing electric field will also
contribute to the effect, as shown using a pulsed electric
field . Recently,13.56 MHz at 2 V has been found to work well .
a These sloppy objects are known as DOLLOPs (dynamically-ordered liquid-like oxyanion polymers) .