Richard Selfridge on mon 20 apr 98
Rachel,the first bit of advice is that pin holing is historically what you
often get with majolica. Recently we saw wonderful pieces in the new Getty,
L.A.County Museum of Art, Hammer, San Diego, also Gardiner Museum,
everywhere we have gone majolica pieces have serious pinholes. Maybe we
need to get used to it. Yes, I know people think it is a "flaw" and who
wants to "explain" it to everyone. So... What follows is most of what we
have learned about "pinholes". First it is wise to distinguish between a
pinhole and a healed blowout which has left a white spot in the colored
decoration of a majolica piece. The first is more likely caused by a void
in the glaze surface that can be traced to the application of the glaze
over a hole in the surface of the pot. The second is usually caused by
outgasing from, sometimes the glaze, but more usually the clay body. The
first is a somewhat mechanical problem and the latter is more
firing/chemical. Some of the remedies overlap in terms of the workshop
practice that solves the problems.
1. Avoid rough surfaces which leave the body open especially if the body
contains grog. When you trim, smooth the opened trimed surface with a
rubber kidney.
2. After bisquing, sand and wash the surface before dipping in the glaze.
The washing is a rinse that doesn't soak the piece but fills the voids with
water which mediates the air compression effect that occurs when you dip
the piece in the glaze. The surface should just be changing back to its dry
color when you dip.(about a half hour).Instead of the glaze compressing the
air in the voids and then blowing back out as a pinhole, it tends to fill
them with a smooth coat of glaze.
3. After the glaze has dried, rub gently the pinhole spots that remain to
fill these holes with surronding glaze. If you do it later the same day it
will not raise much dust. This is the time you would be scraping double
glazed "drips" off as well. By filling these "holes" in the raw glazed
surface we are discouraging the pathways of the escaping out gases. If you
examine the fired pinhole it is a circular crawl around the open hole in
the body. Having some glaze in there discourages this minicrawling.
This about covers the mechanical first part except to say that you can rub
majolica glaze into any pinholes that appear after firing and refire the
piece. We refire in this case in the next bisque firing which will melt
the filled in glaze without making the deco run and change.
Now for the second part which is about firing and outgassing.
1. Avoid Talc or any other gas producing body fluxes in the clay body. We
use frit 3124 as a body flux. Others such as 3195 seem to produce more out
gassing and Talc is flatulent in the extreme.
2. Bisque and glaze fire SLOWLY especially at the top end. To the extent
that you can get most of the gas out of the body before you glaze it the
chemical and firing pinholes will be almost entirely eliminated. A lot of
this gas comes from lignite and sulfer which are common impurities in
terracotta clay. We bisque to 06 and try to slow the firing so the the last
three cones go down about 45 min. per cone. This is to let the gasses out
of the whole piece, not just the rims and handles. If you stack pieces, the
centers will surely be underdone and full of gas in a fast bisque. You can
see this by looking at the pictures of many majolica platters in books and
mags. The centers will be full of white spots while the rims will be free
from them. Some people like this as a "decorative" motif, but for me it
looks like it spoils more pieces than it inhances.
3. It helps to realize that the decorative glaze painted on the more
refractory (it contains the more refractory zirconium opacifier) base
majolica melts first and that the gas will form a blister which then pops
and leaves a white spot where it has carried the colored glaze away. If
cooled too quickly these often leave pinholes with voids to the body. By
firing slowly to 04 with a soak of 45 min. per cone at the top most of
these "blisters" will heal over and cover the hole to the body.
So then how important is all this pinhole problem? With the large
figurative pieces that we do when we see a large pinhole zit or a bunch of
them on the face of a pretty figure, we hear the siren of the acne
ambulance and rush them to the refire dermatologist. Rub the base glaze in
the hole, dab with the color on top and refire to cone 06, and hope for the
best. Others who are not as obsessive might use "room temperature" glazes
acrylic paint or nail polish to fill in these holes, but only on pieces
that are decorative and not meant for the dishwasher or the oven.
Hope this helps, look at a more complete discussion of our process on our
web site under "Short Stories" at http://www.compusmart.ab.ca/selfridg
Richard Selfridge
----------------------------Original message----------------------------
Hi folks,
I have a question about majolica. I am really enjoying the effects of
majolica but having a heck of a time with pinholes. If anyone has any
thoughts on remedying this problem I would be greatful.
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