| Show talc and soapstone analyses BY RAYMOND B LADOO the present or possible future uses of ground talc as well as the basis of sale and purchase are closely dependent upon physical and chemical properties yet no two talc producers or consumers make the same or even comparable tests adequate uniform tests for all important properties have not even been devised hence the grading of tales is not uniform and is based solely on the ideas of each individual no comparable price quotations from the different producing districts is possible and consumers are dependent almost solely upon the word of the salesman or jobber for the statement of grade on the other hand improperly prepared tales entirely unsuited for certain uses may be sold by irresponsible dealers to the detriment of the whole talc industry the difficulty of comparing tales as now prepared from various districts is f ully fully appreciated but the determination of and agreement upon a series of standard tests would go far toward stabilizing the industry it would form a basis for a comparison of the relative efficiency of various methods of grinding and preparation and would aid a logical extension of uses it is to be hoped that the standardization of tests and grades wil be undertaken the chemical properties of talc are inherent and may not be altered or modified except by selective mining or by chemical processes not practical in the present status of the industry however the determination of chemical properties is not so important por tant for most uses as of physical properties although sometimes certain chemical characteristics are perhaps unduly insisted upon by chemists in some industries physical properties are dependent both upon mineralogical characteristics and upon methods of grinding and sizing and may thus be partly changed or modified by changes in practice there is so little known of the chemistry and physics of the various processes in which talc is used that the purchase and sale of talc is governed more by rule ot of thumb methods dependent upon individual opinion and often prejudice than by accurate standard tests this not only causes unwarranted price fluctuations but injures both the producing and consuming industries by creating false ideas of the suitability of talc for various uses it thus seems important that present tests be standardized dardi zed and made uniform for the whole industry and that new tests devised to fill the existing gaps be conducted physical tests some of the important physical proper mineral Min cral technologist U S bureau of mines ties are 1 size of grain 2 shape of grain 3 hardness 4 color 5 slip 6 specific gravity 7 grit size nature and percentage of oversize 8 absorptive power and 9 behavior under heat 1 size of grain the importance of knowing accurately the average grain size is now generally recognized although a few producers still make no sizing tests the most efficient plants use a series of standard wire screens ranging up to mesh and one uses a mesh screen As the approximate fineness of the talc to be tested is usually known only one screen is customarily to employed although for accurate sizing of non uniform material a series of screens is needed the mesh screen has been generally adopted as the governing screen for most finely ground talc A definite quantity 10 to 20 grams is carefully weighed out and placed on the screen several methods of forcing the talc through the screen are in use it may be shaken or jarred through rubbed through with the fingers or forced through by a fine water spray the first method is the most common but it is very slow for fine material the second method rubbing is only useful as a rough quick test for the rubbing action not only wears out the screen but stretches the wire apart so that the sizing is not accurate the water spray method is not generally known but is one of the best methods in use A nozzle which will fit on an ordinary water faucet designed to produce a very fine spray is used the screen upon which the talc is placed is slowly passed under the spray all the fine material being rapidly washed through the screen is then placed on a warm plate or steam radiator to dry when thoroughly dry the residue is brushed on to a paper and transferred transferred to a balance pan for weighing the spray method although slightly slower than the shaking method is less tedious and is very satisfactory when screens are used it is not sufficient to state merely the screen mesh when reporting tests as different screen makers use different sizes of wire with consequent differences in size of opening to be strictly accurate the size of the screen opening should be expressed in fractions of an inch or in mil millimeters the adoption of a set of standard screens by the whole talc industry would be highly desirable the most accurate method of obtaining grain size is by actual measurement with a microscope equipped with a micrometer eye eve piece the average of a number of measurements gives a close approximation to the average size of particle this 3 method jr is not now used commercially hut but is very useful 2 shape of grain no tests are now made commercially to determine the shape of talc grains although this undoubtedly has a very important bearing upon uses the predominant shape may be round or flat needlelike needle like or lath shaped smooth or irre irregular aular one grain shape may be best tor for paint another for paper and another for or rubber the determination termination Ip of grain shape may be made readily with a microscope and may have a very important application 3 1 hardness the hardness of crude talc is often roughly determined by scratching with the fingernail a knife or a coin but no tests on the hardness or abrasive action of powdered talc are made hardness may be an important factor for some uses for example in lubricants and polish ing agents and as a filler for phonograph records but no tests for the determination of the hardness of ground talc have been devised 4 color A very pure white caloris color is insisted on by many consumers of talc but method of if detecting as yet no standard accurate slight differences in color has been devised practically all talc producers make a rough comparative test by the unaided eye but the human equation enters very strongly into the results are usually compared with a sar sample n ale taken as a standard but differing for each producer and consumer the talc being either placed in little heaps or spread out flat with the finger or a knife on the hand or on blue or white paper flattened heaps on blue paper in but even white light give the best results this method is not satisfactory as no standard by the samples are used in common light and a whole industry and differences in in the human equation produce W accurate deter de alts the importance of is often 0 mi nation of slight color differences of great for color is not only an index inde of indicator ww value but also it is often an is poor color c proper methods of grinding be way but TO not always due to impurities 10 due to insufficient or improper grandin gr grinding indin WAS in a talc ta color one instance a yellow ebani attributed to iron but a microscopic would grinding lindine rin dine that finer proved co or objectionable objection abl eliminate most of the the talce irn proves e finer grinding of many fa standard gnei color A uniform and be dethe determination of color should and adopted the applied to 5 slip slip is a term grod of 0 relative smoothness or greasiness uses ine tor for so talc good slip is desirable able for objection and unimportant or even deter roughly rouga ri others slip is ordinarily tap between beaw talc mined by rubbing the this xio nand hand fingers or in the palm or of the io blue in test is very rough and is of little of jalcs the accurate comparison of uni uniform forin testing of slip would be very desirable 6 specific gravity the specific gravity of pure crude talc varies from to but the apparent specific gravity of ground talc as indicated by its behavior in water oil or other liquids varies between much wider limits this is due to the facts 1 that other heavier minerals often occur with talc and are ground with it 2 that grains of different shapes have different rates of settling in liquids 3 that very finely ground talc has colloidal properties which tend to keep it in suspension in liquids some paint manufacturers desire a heavy talc which will sink in water and others other demand a flaky or fibrous talc which has a tendency to stay in suspension and not cake down solidly some of these differences are due to differences in specific gravity and some to shape and size of grain surface tension etc it would be very desirable si therefore to devise uniform method ads of determining not only the specific gravity but bui also the other properties that affect the behavior of talc both as a dry filler and in liquids no such tests are now in use 7 grit size nature and percentage of oversize the presence of grit or oversize size in ground talc is objectionable for most uses sometimes the grit is a siliceous or other impurity and sometimes it is merely insufficiently ground talc in some products product siliceous impurities would be very harmful whereas oversize talc would not be objectionable tio for example in lubricants ir other products oversize of any nature and siliceous impurities even if finely ground would be injurious for example in paper manufacture it is therefore important t to know the size nature and amount of grit or oversize present in ground talc the screen tests described may be used to determine the amount of oversize but the nature and size can only be determined under a microscope if the grit is siliceous and d not oversize talc its presence can be detected by gritting britting grit ting between the teeth or by rubbing between the finger nails this method is a crude one and does not determine the nature size nor relative amount present another method in use is to flatten out ut a small pile of talc so as to present a smooth surface by pressing or rubbing with a PIL piece lee of plate glass this method will detect only coarse impurities present at or close ose to the surface of the pile there is then men no test or series of tests now in use watch enable accurate and uniform determinations mi li nations of this property to be made A careful microscopic examination is of great assistance assistance but is not wholly satisfactory for this Pur purpose pos 8 absorptive power the absorptive power of ground tales is a property which laas 8 never ver been determined commercially but ut in many uses of talc it is probably of greater importance than has been suspected in the manufacture of paints the oil absorption of the various paint materials is ol of great importance but even for this use talc tala producers do not determine the absorption it is probable that when talc is used ir the manufacture of paper the absorptive power of talc has an important effect upon retention when talc is used as a deodorant a or greasing degreasing de agent it is certain that absorptive power is very important por tant it is possible that the oil absorption test of the paint chemist might be used to determinate deter minte the absorptive power of talc in general for various uses but this subject is not well understood and is worthy of careful consideration 9 behavior under heat massive talc and soapstone have long been used as both heat resisting and heat retaining agents and ground talc has been used in pipe covering compounds and fireproof paint the last use is growing in importance and the value rif of talc as a fireproofing material is becoming better known when used as a ceramic material in the production of china and porcelain its behavior under high heat is ig of great importance some tales contain mechanically combined water others contain lime dolomite iron or other impurities which may alter the color destroy the slip or cause the talc to swell under heat pure tales vary in melting point and in behavior under high heat in ceramic compounds standard and uniform tests for the behavior of talc under heat should be devised pure talc contains theoretically silica magnesia and water ha but talc of exactly this composition is practically never found in nature impurities often found are A ALA 10 iron oxide feo or feo deos iron sulphide or pyrite fes lime cao usually as calcite caco an excess of magnesia as dolomite caeg COs an excess of silica as quartz and sometimes manganese oxide mno for some uses none of these are objectionable but for most purposes several or all are harmful silica and are inert but may form grit iron oxide and sometimes iron sulp sulphide bide in pa injure the color manganese oxide is not often found but when present may discolor the talc the presence of small amounts of lime or doumite for many uses is not objectionable some paper chemists object to lime on the ground that it causes foaming and tends to neutralize the sulphite solution but other paper manufacturers do not object to carbonates in fact it is stated that in some paper mills ground limestone or calcium carbonate is used exclusively as a filler the presence of some of these impurities may be easily detected under the microscope hut but the relative percentages may only be determined by chemical analysis simple quantitative tests for iron and for carbon ates are usually sufficient and a complete chemical analysis is unnecessary in addition to a chemical analysis the determination of chemical behavior in different solutions for example the resistance to acids and kalies alkalis al and the determination of such physicochemical chemical properties as colloidal suspension in water oil etc are often important |
