William Hendry on wed 15 may 02
Snail wrote:
> By the way, I assume when you said you were making
> a white underglaze, you meant an uncolored base
> recipe to which you will add stains as colorants.
> White stain would be a really expensive way to get
> a plain white engobe, unless you're doing it for
> the sake of perfect mixing compatibility with a
> palette of other stain-colored engobes, for closely
> controlled effects in painting. For just a general-
> purpose white underglaze, I'd try zircopax, not
> stain.
>
Hi Snail, thanks for all the info. Regarding the above, I actually wanted to
make a white underglaze to give a terrra cotta body a white finish. I am
going to be using translucent glazes stained with some fairly light colors
that would get lost in the "iron" unless I put a white base under them. This
whole situation has arisen because a client (for whom I'm making lots of
tiles) has decided to change their mind about the color scheme they want.
Fine with me, I get paid for change orders. But if I had known before I
would have slipped the tiles when wet. But now they're greenware and I don't
have any experience with engobes that fit greenware. What would you do?
And BTW, I know that even though the term "engobe" gets loosely interchanged
with "slip" there is another slip-like material that called an engobe that
falls somewhere between a true slip and a glaze. Which are you referring to?
Thanks,
William
----- Original Message -----
From: "Snail Scott"
To:
Sent: Wednesday, May 15, 2002 2:09 PM
Subject: Re: 'wanna make an underglaze'
> At 08:53 PM 5/14/02 -0400, you wrote:
> >I wanna make a white underglaze that I'll be firing to c04. I've
searched
> through the clayart archives and elsewhere and found recipes that call for
> stains in what seems to be unusually high amounts...
>
>
> I like my engobes (^6) with about 20-30% stain. (I
> don't go for really 'zowie' colors, though.) And
> for more dilute colors, I mix it with an uncolored
> batch of the same recipe. I add extra frit when I
> add stains, too. For a true underglaze, this may
> not be necessary, but I often use mine as exposed
> surfaces without glaze, and this helps keep the
> texture/appearance the same as the less colored
> areas. Stains can be pretty refractory.
>
> Also, if you apply a clear gloss glaze over your
> underglaze, the color will be more intense than
> without the glaze, so this can affect your choice
> of stain amounts. Always test your underglazes with
> your preferred clear glaze over them, if that's how
> you plan to use them. If you don't have a favorite
> clear, test several, on each of your colors of
> underglaze. Various glaze chemistries will affect
> the underglaze color in different ways.
>
> >...found in the Clayart archive and supposedly from Mason, calls for 20
> parts F4, 10 EPK, 5 Ball, 10 frit, and 40 stain. Converting these recipes
> into percentage terms...calls for almost 99% stain!
>
>
> That Mason recipe you mentioned, (with all the
> ingredients listed in parts, and then 40 of stain)-
> I think that although the base recipe was listed
> in 'parts', the stain was intended as a percentage.
> This is often the case when adding colorants to
> a recipe, regardless of the format of the base
> recipe itself. This will give a very intense color,
> but isn't out of line with many other stain-colored
> recipes. 40% is a popular choice among people who
> do brightly-colored work.
>
>
> >Presuming that this was yet another glaze mystery I didn't understand I
> called Mason where I was told that 10% should be sufficient. That's a
> really BIG difference.
>
>
> 10% is a good percentage for GLAZES. (Did you ask
> about coloring engobes in particular?) Engobes and
> slips/claybodies require much more colorant to
> overcome the opacity of their non-vitrified
> materials, in order to gain the same color range as
> a glaze. The less vitreous/transparent the material,
> the more colorant is needed.
>
> By the way, I assume when you said you were making
> a white underglaze, you meant an uncolored base
> recipe to which you will add stains as colorants.
> White stain would be a really expensive way to get
> a plain white engobe, unless you're doing it for
> the sake of perfect mixing compatibility with a
> palette of other stain-colored engobes, for closely
> controlled effects in painting. For just a general-
> purpose white underglaze, I'd try zircopax, not
> stain.
>
> -Snail
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Snail Scott on wed 15 may 02
At 08:53 PM 5/14/02 -0400, you wrote:
>I wanna make a white underglaze that I'll be firing to c04. I've searched
through the clayart archives and elsewhere and found recipes that call for
stains in what seems to be unusually high amounts...
I like my engobes (^6) with about 20-30% stain. (I
don't go for really 'zowie' colors, though.) And
for more dilute colors, I mix it with an uncolored
batch of the same recipe. I add extra frit when I
add stains, too. For a true underglaze, this may
not be necessary, but I often use mine as exposed
surfaces without glaze, and this helps keep the
texture/appearance the same as the less colored
areas. Stains can be pretty refractory.
Also, if you apply a clear gloss glaze over your
underglaze, the color will be more intense than
without the glaze, so this can affect your choice
of stain amounts. Always test your underglazes with
your preferred clear glaze over them, if that's how
you plan to use them. If you don't have a favorite
clear, test several, on each of your colors of
underglaze. Various glaze chemistries will affect
the underglaze color in different ways.
>...found in the Clayart archive and supposedly from Mason, calls for 20
parts F4, 10 EPK, 5 Ball, 10 frit, and 40 stain. Converting these recipes
into percentage terms...calls for almost 99% stain!
That Mason recipe you mentioned, (with all the
ingredients listed in parts, and then 40 of stain)-
I think that although the base recipe was listed
in 'parts', the stain was intended as a percentage.
This is often the case when adding colorants to
a recipe, regardless of the format of the base
recipe itself. This will give a very intense color,
but isn't out of line with many other stain-colored
recipes. 40% is a popular choice among people who
do brightly-colored work.
>Presuming that this was yet another glaze mystery I didn't understand I
called Mason where I was told that 10% should be sufficient. That's a
really BIG difference.
10% is a good percentage for GLAZES. (Did you ask
about coloring engobes in particular?) Engobes and
slips/claybodies require much more colorant to
overcome the opacity of their non-vitrified
materials, in order to gain the same color range as
a glaze. The less vitreous/transparent the material,
the more colorant is needed.
By the way, I assume when you said you were making
a white underglaze, you meant an uncolored base
recipe to which you will add stains as colorants.
White stain would be a really expensive way to get
a plain white engobe, unless you're doing it for
the sake of perfect mixing compatibility with a
palette of other stain-colored engobes, for closely
controlled effects in painting. For just a general-
purpose white underglaze, I'd try zircopax, not
stain.
-Snail
Snail Scott on thu 16 may 02
At 04:29 PM 5/15/02 -0400, William wrote:
>I actually wanted to
>make a white underglaze to give a terrra cotta body a white finish...I
>would have slipped the tiles when wet. But now they're greenware and I
don't
>have any experience with engobes that fit greenware.
There are many good recipes out there. I won't
recommend one in particular, since I seldom work
at earthenware temperatures, but fortunately
engobes are much more forgiving than glazes, and
generally have quite a wide temperature range and
will fit many types of clay. I will post a few
randomly chosen recipes at the end of this post;
perhaps other folks will offer their tested
favorites.
>And BTW, I know that even though the term "engobe" gets loosely
interchanged
>with "slip" there is another slip-like material that called an engobe that
>falls somewhere between a true slip and a glaze. Which are you referring
to?
A quick scan of the archives will pull up quite
a lot of discussion about the proper use of the
term 'engobe', so I'll skip getting into that in
depth here. In brief, an engobe is a coating for
clay which is not as vitreous as a glaze.
Underglazes are one type of engobe. Some people
consider simple slips (made of clay only) as a
type of engobe, while other people prefer to
reserve the term 'engobe' to describe coatings
which are not just clay, but also include some
other ingredients. These ingredients may include
fluxes to increase vitrification, and also
non-clay or calcined clay components to reduce
shrinkage. This last category is the factor that
you are concerned with for your current project.
By using an engobe instead of a simple slip, you
can choose a recipe which allows for application
to clay in various stages of production. An
engobe that's mostly clay would be suitable for
application to leather-hard ware. An engobe with
less raw clay and more calcined clay may be
better suited for application to dry ware. An
engobe with a lot of calcined material may fit
well on ware that's already been bisqued, etc.
Since you wish to use your engobe as an underglaze,
you will probably wish to choose one which is
relatively nonvitreous, so that it won't much
affect the glazes you'll be applying on top.
Here's a recipe from James Chappell's book:
E-11 White Engobe, for Greenware, ^08-1
14.0 Kaolin
16.0 Tennessee Ball Clay #1
21.0 Calcined Kaolin
14.0 Ferro Frit 3134
5.5 Talc
18.0 Silica
3.0 Zircopax
3.5 Tin Oxide
5.0 Borax
+ CMC
Here's an engobe published by Richard Zakin:
Thendara II, for bone-dry or bisque, to ^3
10 Kaolin
36 Calcined Kaolin
28 Talc
12 Opax
8 Gerstley Borate
2 Barium Carbonate
2 Borax
2 Tin Oxide
Zakin describes this as a good underglaze,
and suitable for stains.
These will be fairly white and opaque, I'd
guess, so if you're planning to add colorant
stains for painting over your white layer,
I'd reduce the zircopax and tin for that batch.
They should do well (as written) for covering
your red terra cotta, though.
Miscellaneous notes: the small amount of gerstley
in the Zakin recipe could be changed to 3134
with no problems. Opax and Zircopax are
interchangeable, and, you may want to replace the
tin if you are planning to use chrome-based
colorants. Recipes with borax should be used
shortly after mixing and not stored, though you
can mix it up dry in a large quantity and only
add water to the amount you need at one time.
-Snail
| |
|
