Ivor and Olive Lewis on wed 22 feb 06
Dear Randall Moody .
To address your wish someone has to tell us what causes the reticulation =
we term "Orange Peel" on a salt glazed surface. Until we can describe =
and explain the physical and chemical changes that take place when salt =
reacts with surface clay on a pot it is going to be very difficult to =
imitate the process.
Hank Murrow on wed 22 feb 06
On Feb 21, 2006, at 10:58 PM, Ivor and Olive Lewis wrote:
> Dear Randall Moody .
> To address your wish someone has to tell us what causes the
> reticulation we term "Orange Peel" on a salt glazed surface. Until we
> can describe and explain the physical and chemical changes that take
> place when salt reacts with surface clay on a pot it is going to be
> very difficult to imitate the process.
Well, Ivor, let me try. "Reticulation', as you term it is (I believe) a
result of the tendency of high alumina melts to bead up. You may see
this in certain high alumina shinos(including mine), and salt-glaze
formed from the reaction of sodium vapor with a body high in alumina.
Conversely, the reaction of sodium vapor with high silica
bodies(porcelains, etc.) will result in noticeably less reticulation
(orange peel). the reason for this is the tendency of silica to form
melts that are 'wetter' than the high alumina ones.
These are effects I have noticed regularly in my work, exploiting the
tendencies to produce the 'look' that I desire.
Cheers, Hank in Eugene
Ivor and Olive Lewis on fri 24 feb 06
Dear Hank Murrow,=20
Can't say anything about Shino Glazes. Never made nor fired one. But I =
will go along with your suggestion that it is a function of Viscosity =
and Surface tension, to which I would add Temperature. (See tables in F. =
Hamer, " The Potter's Dictionary...." p 310 "Table of oxide influence =
upon viscosity" and p 287 "Surface Tension"
In the case of Alumina rich melts the cohesive forces within a melt are =
greater than the adhesive forces between the melt and the substrate.This =
seems to imply high surface tension and low viscosity. So the coating =
pulls away from the substrate and forms spheres . If viscosity remains =
high fragments will remain blocky and not spherulise
In the case of Silica rich melts the adhesive force is greater than the =
cohesive force within the melt. This seems to imply low viscosity and =
low surface tension. so the coating can wet and flow over the surface =
and beads flatten out to show a low meniscus.
( I am using your information here. Excess silica, so I understand can =
raise the melting point of a glaze. My test work on reactions between =
Sodium chloride and various raw clay minerals showed that Sodium =
chloride did not react with pure 200 mesh Silica at Cone 8/9).
It is probable that Salt Glaze Reticulation is a function of gas =
evolution and cooling rate.
Randall Moody on fri 24 feb 06
Great information and thanks! (Even if it is a long way for "no".) :)