constitute m aterial having similar properties to that found in naturally-bonded sands, and in
division. This has rendered their exam ination a m atter of some difficulty, but the following may be regarded as com m on constituents of clay : —
(1) Q uartz.— F ree quartz is often present in sufficient quantity seriously to decrease the plasticity of clays. As it does n o t absorb water, however, it decreases the shrinkage which occurs when clays are dried.
(2) Felspars.— Felspars are occasionally present in a sufficiently undecom posed state to admit of their identification under the m icro
scope at high magnification. Orthoclase, which is less liable to decom position than the lime objectionable. On w eathering these sulphides of iron undergo oxidation, with the eventual the w eathering of plagioclase felspars. A lterna
tively it m ay be present as interm ixed chalk or production of dolom ite o r calcium-magnesium carbonate.
recent research has definitely indicated that this is no t m ade up from any one m ineral as com m only supposed. M uch credit is due to C. S. Ross and P. F. K e rr1 fo r the results
35 d 2
obtained in this direction, and their investiga that kaolinite, beidellite, m ontm orillonite and a m ineral resem bling the sericite form of w hite mica are the m ost com m on clay m inerals, and the properties o f the clay are different accord
ing to the type o f m inerals present. F o r example, clays com posed prim arily of beidellite or m ontm orillonite have higher green-bond strength, and possibly better durability characteristics than those com posed of the other clay minerals. This is o f p articular interest in m odern foundry practice, and the tendency for synthetic sand production using bentonite, as this clay contains a high percentage of
Clews.5 Since the action is partly an electro
Passing 270 mesh substance e xtracted from Albany moulding sand 5 morillonite would contain approxim ately 20
per cent, com bined w ater, whilst kaolinite contains 13.9 per cent. M ontm orillonite is dehydrated gradually up to about 550 deg. C., at which tem perature crystallographic changes begin. K aolinite loses its w ater between 400 and 510 deg. C., whilst the hydrated ferric oxides, collectively designated as lim onitic material, lose practically all their m oisture below about 200 deg. C. These observations are extremely valuable when considering the durability o f m oulding sands and it would although the fusing point is fairly satisfactory in the presence of excess carbonaceous m atter
facture o f domestic and industrial earthenw are
and some deposits are capable of service as a bond in m oulding sands w hen m axim um re
fractoriness is no t a principal consideration.
The D evon blue ball clay deposits at N ew ton
lar geological conditions o r are mineralogically
s i m i l a r . I t s h o u l d b e a p p r e c i a t e d , h o w e v e r , t h a t together w ith felspars (albite and oligoclase), and sm aller am ounts o f calcite, quartz, volcanic tonite will prevent retem pering after heating.
O ther properties o f bentonite are as follow
The viscosity, suspending and bond-form ing properties are greatly increased by the a d d i
to be a prom inent constituent o f fuller’s earth.
Since it now appears th at sm ectite an d m ont- m orillonite are identical, it has been suggested that the clays now called fuller’s e a rth could be renam ed and be m ineralogically designated as bentonite. If this be correct, it w ould be of considerable interest to have a com parison with bentonite o f the British fuller’s earth deposits in Surrey, K ent and Bedfordshire.
A t present the only hom e-m ined colloidal clay which can be used instead of bentonite is that m arketed under the nam e o f Colbond.
This has a sim ilar chem ical analysis as shown by T able II, but it is not so plastic nor does it absorb so m uch water. A ctual tests in this connection conducted by the author indicate that for increasing the bond strength of m ould
ing sands about double the quantity o f C olbond is required over the best bentonite to obtain a certain strength figure. Tests conducted by Sheehan,10 on the other hand, showed that the green-bond strength o f bentonite and C olbond were in the ratio o f 5:3. T he fusion p o int of the latter m aterial is higher than bentonite, being between 1,480 and 1,500 deg. C.
Fireclays
As the nam e implies, these are clays which have a notable resistance to heat and it is not custom ary to include m aterials under this term unless they have a fusing tem perature above 1,580 deg. C. Fireclays are usually associated with the coal m easures and are found in seams or beds varying from a few inches to about 5 ft. in thickness and as mined are in the form of irregularly shaped lum ps o f various sizes.
Some of the best refractory clays are under
clays o f coal seams but it by no means follows that all fireclays are found in this position as, for example, the “ pocket clays ” o f D erbyshire and Staffordshire. Typical chemical analyses of British and A m erican fireclays are given in Table II. The m ajority o f the British materials are of the siliceous type, i.e., they contain m ore silica than is required to form kaolinite with the alum ina present. The percentage o f silica varies from about 45 to 80, the clay then pass
ing with further increase o f silica into a quartzose rock, which when mixed w ith clay often assumes the well-known m aterial ganister.
On the whole, the Scottish clays are decidedly less siliceous than the English or Welsh. N o British fireclays have a refractoriness greater than 1,770 deg. C. and m ost of them are around 1,670 to 1,710 deg. C., although some have only a fusing point of around 1,580 deg. C. or less.
The specific gravity is about 2.6.
The addition of fireclay to m oulding sands is not uncom m on in some districts, particularly for the founding of steel castings.
In Ennos and Scott’s11 report on th e fireclay resources of G reat Britain, the general survey o f the chemical analyses and refractory tests gives the following im portant conclusions: —
(1) T he greater the ratio o f alum ina to silica and the greater the percentage of com bined water, the higher is the refractory quality o f the clay.
(2) Basic oxides, such as ferrous oxide, lime, magnesia and alkalis, lower the refrac
toriness, but their influence as fluxes does not appear to be nearly so m arked as that of silica.
(3) It is impossible, taking the whole series of fireclays, to find any relation between the refractoriness and the basic oxide fluxes, since the influence of the latter is often completely m asked by the effect due to varying silica.
Only when com paring clays of similar alum ina
Fi g. 1.— Dia g r a m s h o w i n g t h e Re l a t io n BETW EEN TH E C O M B IN ED W A T E R AND Fu s i o n Po i n t o f Fi r e c l a y s. ( En n o s a n d Sc o t t.)
and silica content is the effect of basic oxide fluxes apparent.
(4) W ith one exception, all the fireclays in
vestigated which contain less than 50 per cent, silica, over 30 per cent, alum ina, and m ore than 8 per cent, o f com bined w ater, soften at o r above 1,670 deg. C. The am ount of basic flux present in the^e highly refractory clays m ay vary w ithin limits, but where the percentage o f silica is com paratively low (48 per cent.), the total o f lime, magnesia and alkalis m ay be as high as 5.8 per cent. This does no t preclude the existence of good re
fractory fireclays with less alum ina and m ore silica (50 to 60 per cent.), but then m uch de
pends on the am ount and nature of the alum ina and basic fluxes present.
(5) W hen the percentage of silica rises to between 60 and 80 per cent., with a corre-39
spondiog fall in the alum ina, the fireclay softens below 1,670 deg. C., even though the am ount of basic oxide flux is com paratively small.
(6) T he am ount o f alkali which a high- grade fireclay can carry seems to depend on the silica and alum ina content, as shown b elo w : —
P er cent.
S i0 2.
P er cent.
A120 3.
P er cent.
to ta l alkalies (K 20 and
N a 20).
Softening point, deg. C.
5 7.4 2 6 .4 2 .6 0 1,670 to 1,690
53.3 2 8 .0 3 .2 0 1,670 to 1,690
4 7.9 30.05 3 .99 1,670 to 1,690
4 7.5 33.1 4 .5 1,670 to 1,690
(7) T here is very little concordance be
tween refractoriness and chem ical com posi
tio n in fireclays o f lower quality, and it is no t generally possible to grade these by m eans of chem ical analysis w ith any degree of accuracy.
P robably th e sim plest and m ost useful chem ical m ethod of estim ating the refracto ri
ness of fireclays is to determ ine the com bined water. This constituent increases w ith rise in refractoriness, and it m ay be easily and rapidly determ ined on a sample of clay, no chem ical separations being involved in the process.
W ith som e exceptions the rule appears to hold th at for clays with com bined water greater than
9 per cent, the cone value to com bined w ater ratio is very nearly three. W hen the com bined w ater is between 7 and 9 per cent, the ratio approxim ates to 3.5; while fo r com bined w ater less th a n 7 per cent, the ratio rises to fo u r or m ore. This relation
ship is quite em pirical and fa r fro m accurate, bu t it gives the quickest chem ical m eans of distinguishing between good and bad clays.
Fig. 1 reproduces a diagram fro m E nnos and Scott’s w ork showing the relation betw een the com bined w ater a n d softening p o in t o f fireclays.
These results should be o f considerable fu tu re value fo r assessing th e quality o f clay bonding m aterials, p articularly those used fo r synthetic sand production, and they m ay quite possibly be adapted to th e study o f naturally -b o n d ed m oulding sands in general.
R E F E R E N C E S .
1 C. S. R o ss a n d P . F . K e rr. “ T h e C lay M inerals a n d T h e ir I d e n tity .” J n l. Sed. P e t., 1931, vol. 1, p p . 55— 65.
a P . E . G rim , R . H . B ra y a n d W . F . B ra d ley . " T h e C o n s titu tio n o f B o n d C lays an d I t s In flu e n ce o n B o n d in g P r o p e r tie s .”
T ra n s. A .F .A ., vol. 7, 1936, p . 211.
• H . R le s. " G eology a n d C lay R esea rc h .” B u lle tin o f A m erican C eram ic Soc., v o l. 14, N o, 9, 1935.
4 J . W . M ellor. T ra n s. Cer. Soc., v o l. 13, 1914, p . 8 3 , v o l. 16>
1917, p. 73.
‘ H . R ies. " C lays, T h e ir O ccurrence, P ro p e rtie s a n d U se s.”
N ew Y o rk , 1914, p . 127.
• F . H . Clews. “ S e ttlin g a n d A n ti-s e ttlin g P ro p e rtie s o f C lays.” “ F o u n d ry T ra d e J o u r n a l,” D ec. 5 , 1935, p . 422.
’ Senfft. “ T h o n su b stan z e n .” B erlin , 1879.
• J . G. A . S k erl. P ro c. I.B .F ., 1930— 31, vol. 24, p . 195.
’ F . K e rr. A m er. M in eralo g ist, 1932.
’• J . ,T. S h eeh a n . P ro c. I.B .F ., vol. 27, 1933— 34, p . 229.
11 F . R . E n n o s a n d A. S c o tt. S pecial R e p o r t o n th e M in e ra l .R esources of G rea t B rita in . Vol. 28. H .M . S ta tio n e ry O fflce.
L o n d o n , 1924, p . 69.
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