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University of Illinois Bulletin

Vol. VI. JULY 14. 1909 No. 39

[Entered February 14, 1902, at Urbana, Illinois, as second-class matter under
Act of Congress of July 16, 1894.]



BULLETIN No. 11

DEPARTMENT OF CERAMICS

C. W. ROLFE. D,r«tor



A Cheap Enamel for
Stoneware

PART II.



By

RAY THOMAS STULL



1908—1909



published fortnightly by the university



A CHEAP ENAMEL FOR STONEWARE.

Part II.

R. T. Stull, Urbana, Illinois.

The last contribution of "A Cheap Enamel for Stone-
ware," in Vol. X, was sent in for publication three days
after the last trials came from the kiln. The most promis-
ing enamels of the work done up to that time were the
members of Series IX, and the best one of the series was
No. 35.

As has been noted, no craze marks were observed on
Xo. 35, when taken from the kiln. After three weeks' time
a craze mark appeared on the bottom of one trial. Crazing
has been going on gradually until now the trials are
completely checkered with craze marks. The author is of
the opinion that the crazing is due to the softness of the
body rather than to the glaze itself, since the trials were
burned at cone 01.

series x.

The maturing temperature of Xo. 35 is altogether too
low for the stoneware body and too high for other .types
of ware, notably those maturing at cone 04. Hence it was
thought advisable to construct a series starting with a
glaze of the same type as Xo. 35, but which should be more
fusible. The RO, A1 2 3 and P 2 ,, which are the same as
those in Xo. 35, are kept constant throughout the series,
the two variable members being Si0 2 and B 2 3 .

Each succeeding member has the B 2 3 decreased one
tenth and the SiQ 2 increased accordingly. Two fritts were



A CHEAP ENAMEL FOR STONEWARE.



made for Series X, which have the following formulae and
batch weights :



Fritt G.



Formula



Batch weights



S571 CaO] f2.3 Si0 2 Bone Ash

I- .357 Al 2 Osi Zinc Oxide

1429 ZnOj L -2857P 2 05 N. C. Kaolin

Flint



30.75

4.03

32.09

33.13



Fritt H.



Formula



Batch weights



.25 K 2 0] f2.125 SiO=

J- .5625 AlsOs-l .250 P2O5

.75 CaO J [1.250 B.Os



Feldspar
Bone Ash
N. C. Kaolin
Boracic Acid



30. SO
17.09
17.83
34.28



Fritt G was calcined in a Hinted sagger at cone 8.
Fritt H was fused in a Hinted crucible and quenched in
water. Both fritts were ground dry to a fine powder ready
for weighing.

Glazes.



Formulae



Batch weights



V

S3 -•


2



u





6
<


6


d




PQ


CuUh


O




'/
O
u

N


-
'5c. ~

u. —



1/ cd


-


36 | .





.6 1


.2


.55


2.3


.2 | 1.0






81.90


4.45


7.07


6.58


37 ! •










2.4


" |' .9


3.06


5.28


73.91


4.24


7.09


6.42


38 I '










2.5


" 1 .8


6.14


10.60


65. S8


4.02


7.11


6.25


39 | '






"




2.6


.7


9.24|l5.94


57.80


3.81


7.13


6.08


40 | '








"


2.7


" 1 .6


12.35l23.31


49.68


3.59


7.15


5.92


41 I '




"




"


2.S


"


.5


15.48l26.71


41.51


3.38


7.17


5 . 7 5


42 I '




"


a


"


2.9




.4


18.63|32.14


33.30


3.16


7.19


5.58


43 1 '




"




"


3.0




.3


21.79137.60


25.04


2.94


7.21


5.42


44 | '




"




"


3.1




.2


24.79|43.09


16.74


2.72


7.23


5.25


45 1 '


'


"


"


"


3.2




.1


28.17|48.60


S.39


2.51


7.25


5.0S


46 | '




" !


"


" 1


3.3


"


.0


31.39l54.16




2.28


7.27


4.90



A CHEAP ENAMEL FOR STONEWARE. 5

These glazes were applied to bone dry stoneware
crocks but flaked so badly on drying that the series was
abandoned. A few subsequent dips of the two extremes
of this series (Nos. 36 and 4G ) worked very satisfactorily
on biscuit. No further investigation was made into the
working qualities of the members of this series on biscuit
since the work is mainly along the line of single fire
enamels.

SERIES XI.

Series X was modified in series XI to overcome flak-
ing. In this series the same formulae are used as in
scries X, the difference being that in series XI the raw
clay is increased from 0.1 to 0.15 and Tennessee ball clay
No. •"> is used in place of Georgia kaolin. This required
the making of two new frit ts (I and J), since the excess
clay above <U in series X was fritted.

Fritt I.

Formula Batch weights

.5 K-O] f 3.0 SiO- Brandywine Feldspar 44.2S

.5 AUCM Zinc Oxide 6.44

.5 ZnO J ( 2.5 B:0;; Boracic Acid 49.28
Combined weight 494.

Fritt J.

Formula Batch weights

("1.3J3 SiO, Bone Ash 45.0o

1.0 CaO .Zy 3 AUCM X. C. Kaolin 37.50

[ .3^ P=0., Flint 17.44

The first one was drop fritted while the latter was
calcined to cone 8 in a fiinted sagger. Both were reduced
to powder and used in this form in weighing up the
glazes.



A CHEAP ENAMEL FOR STONEWARE.



Batch Weights Series XI (Having the same succes-
sive formulae as Series X).



N

03

5


to

T3 in
V.-0


Fritt I


Fritt J


."2
O

o
a

N


Tenn. Ball
Clay No. 3


E


47




53.89
48.63


35.56
35.66




10.55
10.58




48


3.05


0.44


1.64


49


6.11


43.35


35.75


0.89


10.61


3.29


50


9.19


38.03


35 . 85


1.34


10.64


4.95


51


12.28


32.69


35.95


1.79


10.67


6.62


52


15.40


27.32


36.05


2 ^4


10.70


8.29


53


18.53


21.91


36.15


2.70


10.73


9.98


54


21.68


16.48


36.25


3.15


10.76


11.68


55


24.85


11.02


36.35


3.61


10-79


13.38


56


28 . 03


5.52


36.46


4.08


10.82


15.09


57


31.23




36.56


4.54


10.85


16.82



Three burns were made of the members of this series.
The first four members were burned to cone 04, the first
six to cone 01, and all were burned to cone 6.



Cone 04 Burn.

No. 47 in this burn appeared to be well matured and
glossy. It was not quite so opaque nor light colored as
No. 35 (see p. 239 and p. 241, Vol. X, Trans. A.C.S.). A
few small bubbles appear on the exposed rim. The glaze
crazes some on the soft buff body.

No. 4S is the best glaze of the burn, though it has a
very slight tendency to pull away from the rim. It is
nearly as opaque and light in color as No. 35. It crazes
some.

No. 49 (having the same formula as No. 35) is under-
fired, being a little dull in texture. It is opaque and
nearly white, crazes.

No. 50 is decidedly underfilled, having the appearance
of highly vitrified porcelain. Badly crazed. In two places
the glaze has loosened and stands out from the body.



A CHEAP ENAMEL FOR STONEWARE.



Gone 01 Burn.

No. 47 is glossy, but the opacity has decreased, giving
a translucent glaze. Bubbling has also increased, crazing
decreased.

The opacity of No. 48 has also diminished. The glaze
in this burn is otherwise good.

No. 49 is opaque but not quite so light colored as No.
35. A slight tendency to flake appears. The glaze shows
a few craze marks.

No. 50 is more opaque than 49 though not quite so
glossy. Crazes badly.

Nos. 51 and 52 are underfired and crazed.

Cone 6 Burn.

In this burn all glazes show small bubbles up to No.
55. No. 47 is bubbled the worst, having a frothlike ap-
pearance. Bubbling decreases towards No. 55. No. 55
has a pitted appearance. These glazes are undoubtedly
overtired.

No. 56 is well matured and shows no bad defects. Its
whiteness is a little better than that of a good bristol.

No. 57 is a little whiter than 5G though not brilliant
enough to be a good glaze. It would be better at cone 7.

Conclusions on Series XI.

The glazes of this series are capable of producing glossy
opaque enamels when given the right heat treatment. If
a little underfired, they are dry and dull. When slightly
overflred, they lose opacity and begin to develop bubbling.
The range of temperature is very short, not over 1^ cones
at most. For this reason they are not commercially valu-
able.



8 A CHEAP ENAMEL FOR STONEWARE.

The writer's practical experience in glazes in which
magnesite was used, brought out the fact that MgO pos-
sessed strong opacifying properties. Tin enamels con-
taining 0.25 MgO required less tin oxide in order to pro-
duce the same degree of opacity than those containing no
MgO. It was also observed in testing feldspars that soda
feldspar of a pure grade fused to a whiter and more opaque
bead than potash spar. The crystalline form of soda spar
generally occurs as a porcelain white mineral called
Albite; potash spar, or Orthoclase, on the other hand, is
more translucent and dark in color.

If it is true concerning bases belonging to the same
chemical group or family that those of low molecular
weight possess a lower degree of solubility in a silicate
fusion and a higher velocity of crystallization than bases
of high molecular weight, then less molecular quantities
of Na 2 and MgO would be required to produce "super-
saturation" and "incipient crystallization" than the bases
of their corresponding groups having higher molecular
weights, namely K 2 and CaO.

Acting upon the above suggestions, series XII and
XIII were constructed. It will be noted that the members
of these series are glazes of the bristol type, in which
Na,0 is substituted for K 2 and MgO replaces CaO. In
these two series the RO is kept constant.

SERIES XII.



Formulae


Batch wciglits


o

N

-

3


q
1


o
be


C

N


6

<


5




v be


u
"y.

u
N


- _■
HU


-


58
59
60
61
62
63


1 f/z


f/z


f/z


.f/z

"


2.2

2.5
2.8
3.1
3.4
3.7


I 68.37
63.88
59.94

56.45
53.34

| 50.56


10.96
10.24
9.60
9.05
8.55
S.10


10.57) 10.10

9.87] 9.4:i
9.26| 8.85
8.721 S.33
8.25| 7.S8
7.82) 7.47


6. 58
12.35
17.45
21. 9S
26.05



A CHEAP ENAMEL FOR STONEWARE.



SERIES XIII.



Formulae




B


atch zveights
















ct




"y.

o


Ball

No. 1




N

.-t

O





O

■j.


Z
N


Z
<


o

■jf.


— —

O u


-5S


o
N


Tenn
Clay


g


64


■ 3/


• 3/3


■3/3


.5


2.3/


64.06


10.27


9.90


15.77




(15




"


••


*'


2.6/3


6(1.10


9.63


9.29


14.79


6.19


lili




"


••


"


2.9/ 3


56.59


9.07


S.75


13.93


11.66


67


"


•'


"




3.2/


53.48


8.57


8.26


13.16


16.53


68




"


"


'•


3.5/


50.68


S.12


7.83


12.48


20.89


69






"


"


3.8/


48.17


7.72


7.44


11.86


24.S1



These glazes were applied to bone dry stoneware trials
and fired to cone 8. They were taken from the kiln while
still quite hot. None of them show any signs of crazing.
There is very little difference in the whiteness of the mem-
bers of the two series. All are as opaque and white as a
good tin enamel. Nos. 58 and 04 are a little dull. Glos
siness increases with increase in Si0 2 . The glossiness of
Xos. CO to 63 and (Hi to 69 are very good and in this re-
spect will compare favorably with the majority of stone-
ware glazes on the market and are far superior to them
in whiteness. All five pieces of No. (52 are especially good.
All members of Series XII have a slight tendency to bead
where the glaze is too thick. The glazes do not show any
tendency to pull away from the corners and edges. The
small beading tendency can be overcome by the addition
of borax.

In Scries XIII in which the A1 2 3 is increased the
beading has increased a little. The glazes do not appear
to be quite so fusible. Otherwise they are good.

In order to compare the opacifying effect of MgO with
that of ZnO, the following series was prepared by the class
in glaze making in their regular laboratory work. In this
series I Series XIV i a glaze of the Bristol type was selected
for the first member. In each succeeding one 0.1 MgO
replaces 0.1 ZnO, all other members remaining constant.



10



A CHEAP ENAMEL FOR STONEWARE.



SERIES XIV.



Formulae


Batch weights




















— ,




u
























<u


T3






















re .






















re


be


PQ o


C "5


'S


















A a


c


.fc


rt ¥•


o




4J
N
03


O


o

re


O


o

be


6


6


C3 c/1

re -3

O *0J


.a


5.3


'I to

1- JS


u


■g


3


W


U


N


§


<


c/5


D^fc


^


HU


OS


N


E


H-l


.3


.2


.5


.0


.5


4.0


1G7.1


20. 0|


51.6




| 40.5


128.0


H-2


"




.4


.1


"


"


"


"


"


8.4


32.4


"


H-3


"




.3


.2


"


"


"


" 1


"


16.8


24.3


"


H-4


"




.2


.3


"


"


"


" 1


"


25.2


16.2


"


H-5


"


"


.1


.4


"


"


"


" !


"


33.6


8.1


"


H-6


"


"


.0


.5


"


"


"


" 1


"


42.0




"



These glazes were applied to small bone dry stone-
ware crocks and fired to corie 7 in 25 hours.

There is but little difference in the appearance of the
members of this series. If anything, H-6 is a little the
whitest and smoothest in texture. No bad defects appear.
The conclusion which we may draw from the appearance
of the trials is that MgO is at least equally as good an
opacifier as the same molecular quantity of ZnO.

DISCUSSION.

Mr. Stull: Referring to the trials will say that I
have brought only two, No. 35 representing the most
promising enamel of last year's work and No. 62 which
is the most promising one to date. You will notice that
No. 35 is a sort of ivory color and matures altogether too
low for stoneware. The trials were nested, and happened
to touch at this point (indicating) where a bubble formed,
hence this bubble near the rim is not really a defect of the
glaze in itself. You will notice that the enamel is badly
crazed, and has a little tendency to flake though not bad
enough to be serious. No. 62 is a member from Series
XII. You will notice in the bottom where the glaze is
thick, it beaded in one spot and partially healed over
again. Those who have had experience in Bristol glazes
will agree with me that the beading is not bad, since no
"dope" was added to these glazes to prevent it.



A CHEAP ENAMEL FOR STONEWARE. 11

There is a great field open for investigation to de-
termine the opacifying effects of different materials. We
have evidence that magnesia is a good opacifier; a good
grade of magnesite when properly used in conjunction
with zinc oxide will give whiter and glossier effects than
the regular type of Bristol glaze.

Mr. Purdy: Those of us who have had experience
along this line note with interest the discussion of the
opacifying effect of Na 2 as against K 2 0, and Mgo as
against CaO, in conjunction with zinc.

Mr. Jackson : In regard to the trial passed around,
I notice two temperatures mentioned, for instance, cone
01 and cone 8. I am not very familiar with the stoneware
business, but I would like to ask whether it is customary
to have two fires, a biscuit and glost? Do I understand
that the piece going around had a previous biscuit fire?

Mr. Stull: No, sir.

Mr. Purdy: There is only one part of the country
that I know of where they use two fires, and that is in the
West where they have clays of enormous shrinkage.

Mr. Jackson: Is it practicable and customary to
make stoneware as low as 01?

Mr. Purdy: It is not customary; it might be prac-
ticable.

Mr. Stull: I am sure there is no true stoneware made
in this country as low as 01 ; but I am sure that terra cotta
is made even as low as cone 05 where a slip and enamel
are applied to the bone dry ware and matured in one
burn, which is a much more difficult problem.

Mr. Purdy: Bristol glaze type?

Mr. 8tv.ll: No. sir; an underglaze coating or slip,
and white tin enamel or glaze applied over it and all ma-
tured in one burn. If it can be done on large pieces, I do
not see why it could not be done on small pieces.

Mr. Purdy : Is this over-spray a raw leadless glaze?

Mr. Stull: No sir, a raw lead glaze not a leadless
glaze.



12 A CHEAP EXAMEL FOR STONEWARE.

Mr. Purely: That is another question. I do not call
that a Bristol glaze. We must be careful to keep Bristol
glaze apart from lead glazes because it has different prop-
erties.

Mr, Jackson : The point I was getting at is how
large a field the one-fire proposition covers so far as a
stoneware body is concerned. I do not call any glaze
carrying boric acid a Bristol glaze.

Mr. Purdy: You are wrong.

Mr. Jackson : That is the idea I had. Is the boric
acid fritted?

Mr. Stall: Yes, sir, it is in series ten and eleven.

Mr. Jackson : I would like to ask Prof. Orton if I
am right in what I said. When a glaze carries boric acid
fritted in it, can it properly be called a Bristol glaze?

Mr. Orion: It does not meet the idea of a Bristol
glaze that I have been teaching. A Bristol glaze is sup-
posed to be a raw glaze. However, this glaze under dis-
cussion might be a type which we could afford to use in
place of the old raw Bristol glaze. I do not know about
that.

Mr. Jackson : That is as I understood it. The glaze
at the top of the list on the blackboard is of the Bristol
glaze type as I understand it.

Mr. Orton : It is a little too high in silica for a
typical Bristol glaze.

Mr. Stall: I am afraid a great many are of the
opinion that this work is along the line of Bristol glazes.
That is not the line of inquiry, but a cheap enamel for
stoneware as indicated by the title on the program. The
main object of the investigation is to produce a cheap
enamel for the lower grades of ware which will be a fairly
good substitute for tin enamel ; not only for stoneware
but for other types of ware maturing at lower tempera-
tures. That does not confine the investigator to one type
of glaze. It is a broad field to work in. I am sorry 1
have not been able to do more work in this line of experi-
mentation.



A CHEAP ENAMEL FOR STONEWARE. 13

Mr. Fuller: What do you mean by "dope" to keep
the glaze from crawling?

Mr. Stutt: Nearly all stoneware glazes have a ten-
dency to crawl or bead because they are highly viscous.
In order to prevent this crawling it is customary for pot-
ters to use such materials as plaster of paris, borax, car-
bonate of soda, or some other soluble salt in their glazes.
Exactly what action these soluble salts have upon the
glaze has not been definitely determined. My opinion is
that in the case of borax and carbonate of soda, these ma-
terials not only serve to hold the glaze to the bone dry
body after dipping but also fuse in the early stage of burn-
ing forming a sticky matrix which prevents the glaze from
curling up, which precedes beading.

Mr. Stephani: I would like to say that I have had
some experience in the terra cotta line with beading and
crawling, and my impression is that there are two kinds.
We get one by getting the slip or glaze too heavy on the
ware and the consequence is one <>r the other cracks in
drying before it goes into the kiln, and when the glaze
begins to fuse it beads and crawls; and in the other case
we get an effect which does not appear before it goes into
the kiln. I think manufacturers, as a rule, for the first
kind of beading, use a material something like calcium
sulphate, and some use glue to hold the glaze together in
drying, making it tougher. For the second kind, the kind
happening in the kiln and which does not show up in the
drying, boracic acid is used; or, in a terra cotta glaze the
ZnO is reduced with MgO. The latter fault is due to im-
perfect glaze, while the former can be overcome mechanic-
ally, i. e. by applying the underslip or glaze thinner.

Mr. Fackt : May I ask Mr. Stephani whether he has
used Cornwall stone in place of the feldspar to prevent
the crawling and cracking which he speaks of as happen-
ing before the ware goes into the kiln?



14 A CHEAP EXAMEL FOR STOXEWARE.

Mr. Stephani: I never used Cornwall stone in any
glaze; but I find the best way to overcome that effect is
by remedying the underslip, or putting less of it on the
ware.

Mr. Miller: Mr. Stull tells us he finds an improve-
ment in color by substituting magnesium for tin.

Mr. Purely: The substitution of magnesium for what?

Mr. Stull: An improvement in color by substituting
magnesium oxide for zinc oxide. It does make a little
difference in the fusibility. Glazes containing MgO are
not quite so fusible as glazes containing ZnO, other things
being equal. Dr. Sinionis, in his work on cones, in which
he used magnesium oxide, points out that the action of
the MgO in the cones is to hold the cones up to a certain
point when the fusion is comparatively sudden. It is
claimed for that reason that they are better indicators
of temperature than the regular Seger cones. We are
getting more light all the time as to the action of MgO
in ceramic mixtures. In fact, we do not yet know all of
its possibilities. The magnesite which I used was rather
a pure variety obtained from the Foote Mineral Company.
I am told that a good variety of the Grecian magnesite is
delivered in New York for about twenty-four dollars a
ton. If it can be used successfully it will cheapen enamels
wonderfully and fill a long felt want for the lower grades
of ware.

Mr. Frink : One thing has occurred to me in regard
to this discussion of magnesia. Magnesia is used in glass
to some extent, usually being found as an impurity in
the lime, and experiments have been made which show
that the viscosity is very much reduced thereby.

Mr. Purely: The viscosity is reduced?

Mr. Frink: Yes. That bears out what the gentle-
man has said. When the temperature rises to a certain
point, liquefaction is reached more rapidly. Does not the
similar action of borax account for the beneficial results
of this material in the case of crawling or beading, be-
cause of the filling of the interstices with liquid?



A CHEAP ENAMEL FOR STONEWARE. 15

Mr. Purdy. Was it used in a small pot or a big
batch?

Mr. Frink: I have experimented with it in tank of
capacity of 600 tons, and also with quantities of fifty or
sixty pounds.

Mr. Purdy: The viscosity seemed to be the same in
both instances?

Mr. Frink : Yes.

Mr. Purdy: What was the effect of magnesia on de-
vitrification?

Mr. Frink: That seems to vary according to con-
ditions, under which it is melted. If melted under a high
reducing flame devitrification takes place slowly; but
under a high oxidizing flame it takes place rapidly.

Mr. Purdy: Is that peculiar of your magnesium
glasses or is it true also of lime glasses?

Mr. Frink: Yes; it is also true as to lime glasses;
but with the magnesium glasses it is particularly so and
I attribute it to the more liquid condition. That is my
theory.

Mr. Purdy: We have to have equivalent of devitri-
fication to have opacity.

Mr. Frink : Yes; and I imagine if you put that under
oxidizing conditions you will get that more rapidly.

Mr. Purdy: I will ask Mr. Stull if magnesium in the
form of light, fluffy oxide has the same effect as when in-
troduced as magnesite?

Mr. Stull: I have never used the light commercial
magnesia only in a fritt on one or two occasions, and have
never made any comparisons between it and the mineral
magnesite. Commercial magnesia is comparatively ex-
pensive. We can obtain magnesite which will analyze
very high in MgC0 3 , with very little impurity which would
cause discoloration. There is a deposit in California
which is claimed by some to be better than the Grecian,
but the cost of mining it and shipping it east is greater.
By a little sorting, a comparatively pure grade can be
had as low as thirty to fortv dollars a ton.



16 A CHEAP ENAMEL FOB STONEWARE.

Mr. Purdy: I asked that question in the interest of
Hottinger, Barringer, and Hull.

Mr. Stall: What is the effect of magnesia upon iron
color in glass? In series twelve and thirteen Tennessee
ball clay was used, yet the glazes do not show any trace
of cream color.

Mr. Frink : It is so contradictory in action that I
cannot answer the question definitely. We sometimes get
one effect and sometimes another. I think this is a mat-


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