US2142660A - Platinum crucible - Google Patents

Description

Jan. 3, 1939. J. s. sTRElcHER PLATFNUM CRUCIBLE Filed Nov. 17,

1934 Sheets-Sheet 1 F/G. .l

GPA MS CPUC/BLE WEIGHT 1N V EN TOR. annjurez'celr BY Q5 W L A TTRNEYS.

Jan. 3, 1939. J. s. sTRl-:ICHER 2,142,660 Y,

vPLATINUM CRUCIBLE l Filed Nov. -17\, 1934 2 sheets-sheet 2 CPUC/BLE WEIGHT GPA/VIS )IC/G.

. Byww ATTORNEYS.

Patented Jan. 3, 1939 v PLATINUM cnuolns Jol'mm s. stretcher, Newark, N. 1.. signor to' The American Platinum Works, Newark, N. J., a corporation o! New Jersey Application November l'l, 1934, Serial No. '153,418

^ One oi the most important tools of the analytical chemist is the platinum crucible. Since platinum belongs to the relatively rare metals it is a high priced metal. This iact alwayshas been it and still is today the reason'ior making the platinum crucibles with the least possible amount of platinum; the crucibles are made from such thin platinumsheets as to give the crucibles just the strength necessary for their use in the chemical laboratory. 'I'his rule has been applied to the manufacture oi the platinum crucibles for generations; today a tacit agreement exists between the chemists and the manufacturers o! the platinum crucibles which restricts the manufacture of the standard platinum crucible to this rule of thumb: Platinum crucibles together with their covers shall weigh approximately as many grams as theyhold cubic centimeters. Thus a platinum crucible with. aA capacity of 25 cubic centimeters weighs 25 grams (with a tolerance of .5 m) including the cover. The weight of the platinum cover of this crucible is about W2 grams; the weight of the crucible proper is, therefore, only about 20 grams. (Generally the .platinum covers weigh from Mlth to Vath of the aggregate weight of the crucible andv cover.)

This rule of thumb determines and limits absolutely the thickness of the crucible bottom and side wall. The crucibles are made from platinum sheets; these Sheets are cut into circular discs or blanks and are spun over spinning forms made oi' steel or wood; during the spinning operation the circular blanks are pressed with a follower against the steel form; the follower has a iace which nearly covers the bottom of the crucibleshaped steel form; in this manner a crucible bottom is obtained which always has the thickness of the original sheet; the crucible wall becomes slightly and' gradually thinner from the bottom towards the rim ci?v the crucible. Instead of spinning, other processes such as for instance pressing or deep drawing, are also used for forming the circular blanks into 'crucibles of the shape just referred to. Produced according to this rule 45' oi thumb platinum crucibles of the mostwidely used standard sizes have the following dimensions:

lWhen for instance .014" platinum sheet is spun into a 25 cubic centimeter crucible, the thickness of the sheet is reduced, from thebottom towards the rim, from .014" to .010 vor .009"; sheets of a dierent gauge are reduced in thickness to an analogous extent, generally in about the proportion indicated.

Pure platinum as well as the well-known platinum alloys (crucible platinum with .3% indium or rhodium, or platinum-rhodium with 3.5 to 4% rhodium) are treated alike when shaped into crucibles; the platinum-rhodium crucibles sometimes are made from even thinner sheets than the pure platinum crucibles; these alloys vare slightly harder than the crucible platinum.

The'metal of all the platinum crucibles has an extremely fine crystal-grain. This fine crystalgrain ,is attained by the most painstaking hammering of the metal after the spinning process. The metal ot the commercial platinum crucibles has a grain varying in size from .25 to .66 millimeter, or about 18 to 2 grains per square millimeter (measured at the bottom of the crucibles) Chemists as well as manufacturers consider those crucibles as of the best qualities which have the smallest crystal grain.

I'here are, according to my experience and research, three main causes of the destruction oi platinum crucibles during their use in chemical laboratories:

1. When the crucibles are used for holding molten masses of alkali carbonate or potassium bisulfate, the thin, hot and therefore extremely soft and pliable crucible metal sags under the weight of the melt. The crucibles are considerably distended along the bottom section and as farup as the melt is lling the crucible; the crys- .tals of the crucible wall are stretched; the intercrystalline spaces are enlarged. When the melts cool slowly within the crucibles or when they are quenched, the melts expand slightly and cause a secondary stretching of the metal. Both of these actions change the shape of the crucibles very rapidly. Wooden or other forms are orten used to iron out these bulges; this procedure l very often causes. cracks within the metal. The alkali carbonate melts also 'contribute towards the destruction of the platinum crucibles chemically. These melts corrode platinum slightly; but this corrosion takes place especially along the edge 2. Platinum crucibles are most frequently heated with gas burners (Meker burners, etc.); the crucibles are heated in such a way that they are completely enveloped by the blue coneof the gas llame. This blue conev of the gas flame contains hydrogen (the amount of hydrogen in this blue cone is regulated by the law of the water gas reaction). Wherever this blue cone of any gas ilame touches the platinum crucible, hydrogen dliuses through the platinum and causesreducing processes within the melts; metal oxides are reduced to the metals; these metals alloy withthe platinum very readily and destroy the crucibles rapidly.

3. Every platinum crucible is handed to the chemist in the hard (cold worked) and highly polished state. Heating the crucibles for the iirst time (in the gas burner or in an electrically heated muille; the temperature attained is seldom higher than llElllo C.) causes instant recrystallization oi the metal. The distorted grain structure, attained by forming and hammering the crucible, is replaced by new, small and normal grains (an cquiaxed grain, Whose diameter is approximately the same in all directions). The strain hardening eilect is completely lost with thisv change of structure; the metal becomes very soft and ductile. Dining the use of these crucibles it very often happens that, at restricted areas or places, an exaggerated grain growth takes place and an abnormally large grain is produced. Such exaggerated grain growth is 'the cause of many cracks in platinum crucibles.

When, in the chemicai laboratory, platinum ware is destroyed, these three causes of destruction are not always equally active; the conditions which cause such .i allures vary with each laboratory; they are very seldom recognized and identified individually. Therefore failures of platinum. ware are very seldom traced to their true source.l .In chemical literature they are, mostly recorded as interesting phenomena, essentially connected with the nature of platinum metal and therefore unavoidable. But another notion obscures the underlying facts. The platinum cruclble is made today, with only minor changes, according to a tradition nearly 75 years old. Neither the process of making the cruclble, nor the hud of-crucible produced by this process is considered to have any bearing upon the failures of the crucibles when in use. The platinum crucible of yesterday and of today is really taken from the hands of the manufacturer like a ,constant of nature and in case any faults are found, the. blame is put primarily upon the chemical quality or purity of platinum. But since platinum is generally refined to a much higher degree than any other commercial metal ever was rened, the chemical quality of platinum is least to blame for any failures. v

Through an extensive research I have found that when the platinum Crucible is varied in its physical structure the metal shows Widely varying behavior under the conditions under which platinum crucibles are generally used. In producing such variations Vin the structure of the metal it was necessary to stop giving primary importance to the purely commercial viewpoint of the price of the metal; it was necessaryI to follow a line of development basedv solely upon the properties of platinum metal itself.V These tests have enabled me to devise my present invention, showing a. way of making a platinum crucible which combines with a least amount of platinum the highest resistance to corrosion, which reduces the dif fusion of the hydrogen of theilame through the crucible walls to a minimum and which overcomes the possibility of the changes in the shapes of the crucibles during the fusion of alkali carbonate.

My researches showed that platinum can be best studied in its chemical and physical behavior lwhen shaped into a large number of crucibles 4of the same capacity but with different bottom and wall thickness Aand when in such crucibles alkali carbonate melts are made under varying but always exactly determinable conditions. These tests and their results are of special advantage, as they correspond absolutely to the practical case in the chemical laboratory. In addition thereto, special tests have mownfthat alkali carbonate melts corrode platinum only insofar as they are dissociated to alkali hydroxide or alkali oxide and when oxygen, air or such gases as produce oxygen through dissociation (for instance carbon dioxide) are present. Soda ash and potash start to dissociate into the oxides at 300 C.; the higher the temperature, the more alkali oxide is produced within the melts (potassium carbonate is decomposed to a higher degree than sodium carbonate at the same temperatures), the greater is the corrosion, therefore, of the plati num crucibles by the alkali carbonate melts. The alkali carbonate melts act upon the platinum. under oxidizing conditions, not only in a purely chemical Way; as tests have shown, the process of corrosion is also activated by the platinum itself and especially the platinum oxides which are formed during the initial corrosion and which are oating within the melt. Platinum is never attacked or corroded by alkali carbonate melts in a purely reducing atmosphere; on the contrary, in such systems hydrogen reduces to the alkali metals, thetraces of alkali oxide formed above 800 C.; these metals are then deposited upon the inner surface of the platinum crucibles; these alkali metal deposits alloy with the platinum. Such melts, made under strictly reducing conditions, are always brilliantly White. l the alkali carbonate melts are made in a Inutile which is electrically heated and Where there is always an excess of air, fusion proceeds under purely oxidizing conditions. Tests have shown that this is the only way in which it is'possible to obtain exact corrosion data for platinum at diierent temperatures. 'I'he melts resulting from such fusions are slightly colored (brown) by the platinum oxide which is'formed by the corrosion of platinum; since the corrosion rate increases with the length of time and with higher temperatures thecolor of the melts is also intensied when the fusionsare carried on for a longer time. When a gas burner is used for heating the platinum crucibles during these fusions, corrosion of the platinum also takes place, but the rate of corrosion is quite different from the above case; the melts `corrode the platinum in a minor degree. Special tests have shown that'the corrosion depends upon the rate with which )the hydrogen within the flame diluses through the platinum into the melt and upon the degree with which the ame enveloping the crucible -blocks the air from entering the melt freely. These are the two factors which cause the reduced corrosion of the platinum when.

the crucibles are heated with a gas flame.

Applyingthe testing method. explained above, to platinum crucibles with diilerent bottom and wall thiclmess, there were ascertained a series of new and very important facts Awhich had never Vbeen taken into account in the manufacture of corosion decreases with the increase in the thickne s of the platinum sheet and with the increase in )the size of the crystal grain.

f2. The corrosion is leastin crucibles made from platinum sheets of a'thickness of .020" and over.

3. When the platinum crucibles are heated with a gas burner (Meker burner, etc.) the corrosion of platinum decreases wit thickness of the crucible. 'I'he fusion proceeds under reducing conditions in crucibles made from.

sheets .018" thick and thinner. The reducing action within such thin-walled crucibles is'caused by the diusion of the hydrogen ofthe llame through the platinum into the melt.

4. Platinum sheets with a thickness of .020

and more act practically as an impervious shield against the hydrogen of the flame; within platinum crucibles made from sheets .020" @and thicker, the fusion of alkali carbonate proceeds under oxidizing conditions similar to those obtaining with an electrically heated muiile.

5. During thefusion of alkali carbonate, platinum crucibles made from sheets .020" thick and more will sag'but slightly or not at all lunder the weight of the melts. Thin walled' platinum crucibles (thinner than .020") sag very quickly and easily under the weight of the melts.

'I'hese tests and their results show that the thin walled crucible which is made from sheets thinner than .020" (this includes all the commercial standard platinum crucibles, as they are made from sheets varying in thickness from .010 ,-to .0175", according to the volume by weight rule), with the extremely iine crystal structure y is in a physical state where it is most susceptible to destruction in the regular routine work of the chemical laboratory. These thin walled crucibles show the least resistance to the difusion of the hydrogen of the flame. extremely ne crystal grain, they develop the highest catalytic activity when theyare heated under oxidizing conditions; therefore, they show the highest corrosion rate with the alakali carbonate melts. 'The thin and finely' crystalline platinum sheets are extremely soit when in the hot state; thereforathin walled crucibles will sag and change their shape quickly under the weight of the melts. l

The tests mentioned above indicate that the lack of stability of the standard platinum crucibles is overcome completely when the crucibles (of whatever capacity) are made from sheets of a thickness of at least .020".or more. My invention vis based on these tests, and enables me to produce platinum crucibles `far superior to the standard crucibles now employed, as to durability, permanence of shape, and resistance to deleterious influences such as corrosion by the melt contained in the crucible or impairment by the action of the hydrogen of a llame used for heating the crucible.

' Thel superior results obtained by my invention appear very clearly from the data shown inthe accompanying drawings, in which Figs. 1 and 2 are diagrams illustrating the corrosion conditions observed when crucibles ofv a predetermined capacity (here assumedv as 25 cubic centimeters) are made from platinum sheets of different thicknesses and of different character and exposed to a decrease in the wallv As they have an v the action of two diilerent melts; Fig. 1 represents the results obtained when an electrically heated mume is employed, while Fig. 2 illustrates those observed when the crucibles are heated with a Meker gas-burner; Fig. 3 is a vertical section oi a crucible made according to my invention from,V

finely-crystalline platinum; and Fig. 4 is a simi` lar view of a platinum crucible the metal of which is of the so-called single crystal character.

According to extensive tests, the best resultsI are obtained with my invention by using platinum sheets .025" thick. For the same volume or capacity, the new crucibles embodying my invention are heavier than standard crucibles such as now employed, as will be evident from the following table giving the approximate weight of platinum crucibles (crucible alone, without the cover) made from platinum sheet .025 thick:

New crucible, Standard Incxeo Volum .c25/'wan crucibles in weight Cubic cmimeten Grams Grams Gram l0 22 8 14 l5 28 12 i6 zo a4 1e 1s 25 40 20 Z) 30 46 24 22 40 54. 32 22 50 62 40 22 so 7o so eo taken as .025", the line A indicates that the the portion of the line A from the point A' upward (thicknessl .020" or over) indicates the range of thickness to be employed according to my invention. As will be explained below, the pointl A vpoints of said line below A'.

maybe termed a critical point ofthe line A, in

that conditions are materially diierent for points of the line A above A' from the conditions for With an increase in weight from 14 to 22 grams of platinum for the standard sizes of the chemical laboratory crucible, the main and most powerful factors tending to destroy platinumcrucibles are entirely eliminated and overcome, namely on the .one hand, the diffusion of hydrogen into the melts when gas burners are used and the subsequent destruction of the crucible through chemical action; and on the otherhand. the sagging of the crucibles and their subsequent destruction throughmechanical causes. With the new thick-Wall crucibles of my invention the fusion of alkali carbonates proceeds under oxidizing conditions even when gas burners are used for heating f the crucibles. The modern textbooks of quantitative analytical chemistry advise the use of a small amount of niter together with the alkali carbonates in order to i'nsure positively an to platinum and causes a platinum sponge deposit upon the inside surface of the thin walled crucibles. From such crucibles the cold melts can be separated only with diiiiculty; applying mechanical ways toseparate the melts from such crucibles destroys the thin walled crucibles very quickly. The new thick walled crucible is least corroded by the alkali carbonate melts; the inner surface of the crucibles remains smooth for a very long time; the melts are very evenly heatedv within such crucibles;A the cold melts are most easily separated from these4 crucibles. 'Ihese improvements are attained with all the thick Wall crucibles made from C. P. platinum, Crucible platinum? as well as from the platinum-rhodium alloy containing up to 4% Rh.

As an example, I may produce platinum crucibles embodying my invention, in the 25 cubic centimeter size, as follows: Crucible platinum is rolled to a thickness of 0.120" and annealed at 800 C. for `three hours. This platinum sheet is then rolled down to sheets of different thicknesses, each sheet being of uniform thickness, and the several sheets varying in thickness from .100" to .004". From-each sheet I cut two circular pieces or blanks; about 66 millimeters in diameter, and anneal these for 15 hours at 800 C.

. Thereupon all these blanks are spun to i'orm the -ist (2.l (the results being-shown'in Fig. 2).

shells or bodies of crucibles of 25 cubicl centimeter capacity, and such shells or bodies are iinished by hammering or other suitable operation. One crucible of each pair is recrystallized by heating to 1000 C. and then kept in the resulting finely crystalline state. The other crucible of each pair is transformedinto a single-crystal crucible according to a method described below. When I speak of the thickness of the crucible wall, I mean that of the crucible bottom;l the side wall vis usually of somewhat smaller thickness, as has been stated above.

'Ihe crucibl of both types or series are subjected'to corr on tests as follows: Each test is made for a period of one hour. In one series of tests, I produce within the-crucible (of 25 cubic centimeter capacity) a melt of 20 grams of soda (NanCOs), in another series of tests the melt consists of 20 grams of a mixture of 4 parts of soda -ash (NaaCOa) and 5 parts of potash (KzCOs). 'I'he fusion of the substance from which the meltv is 'formed may be accomplished in an electrically heated muiile at 1000 C. (Fig. 1 illustrates the results of this treatment), or with the aidof a Meker blast gas burner at 1050 In the drawings, the lines B, C, D, E, B', C', and E' indicate the corrosion losses (due to the action oi' the hot melt on the platinum) with diierent melts,

diierent heating agents, and diiIerent weights and'characters of the platinum crucible.; For

.each of these lines, the abscissae indicate crucible weights in grams, and the ordinates,.c9rrosion losses in milligrams. Unes B, C, D, E relate to fusion in an electrically-heated muiile, and lines B'. C', 1'."A toheatixigV with the aid of ,a Meker blast gas burner. Lines lB, C, B', `C' relate to the corrosive action of a mixture oi soda and potash of the composition mentioned above, while lines D, E, E relate to the corrosive action of a soda melt. The light lines B, D, B' illustrate corrosion in the case oi' crucibles in the iinely crystalline state, while the heavy lines C, E, C', E' indicate the degree of corrosion in the case of crucibles of the single-crystal type more fully described below.

Fig. 1 shows clearly that with a crucible made from a platinum sheet .020" thick (point A' of line A, indicating a crucible weight of 30 grams), the corrosion rate approaches its lowest and approximately constant value, in the case of an' electrically heated muille. 'I'he corrosion rate reaches its minimum when the sheet is .025" thick (crucible weight 40 grams). Thin-Walled crucibles have the highest corrosion rates on account of the greater chemical activity of the small crystal grains. Under otherwise like conditions, the corrosion rate is generally smaller when the crucibles are of the single crystal type, as compared with crucibles of the finely crystalline type.

Fig. 2 shows that when the crucibles are heated with a Meker burner, the corrosion rate approaches its peak with the crucible made from a platinum sheet .020" thick. With this particular mode of heating, the ythin walled crucibles (less than .020" thickness) show the lowest corromon rates, on account oi the high rate of dirfusion of the hydrogen of the ilame through the crucible walls. Comparison of the lines B and C' shows that in this case also the single-crystal l crucibles arecorroded less than the iinely crystalline crucibles.

The third main cause for the destruction of the standard platinum crucible is exaggerated grain growth oi.' the metal, caused by secondary recrystallization of the pure metal or the crucible platinum (chemists call it mostly recrystallization). This harmful, locally restricted exaggerated grain growth of platinumis caused, as tests have shown. when the recrystallized metal is in some places slightly bent or hammered and after such locally restricted mechanical treatment again heated to 1000 C. and above. Such mechanical treatment causes critical and local strainsor strain gradients within such metal which is already recrystallized and which has been transformed through such recrystallization into an extremelyiine grain; these strains are suddenly released when the crucibles are heated again to 1000 C. and above; they cause the small grains to coalesce locally to extremely large grains. Platinum-.crucibles with nests of such large grains develop cracks within such nests or at the boundaries where large and small grains meet. The changes in shape, caused by the alkali carbonate melts when contained in the standard, thin-walled and finely crystallin'e crucibles always inducethe chemist, after each fusion, to apply' to the crucible ythe above characterized mechanical treatment, fto reshape the'same; but such "reshaping operations result, sooner or later, in exaggerated grain growth and ultimately in the destruction .or the crucible. As has been stated above, platinum crucibles with a bottom thickness of .020 and more, especially those from .025"` upward, do not change their shape under the weight oi.' the melts; therefore, they never needfreshaping after the fusions.

By the use of my improved thick-walled crucibles, v

therefore, the third cause of destruction of the arranco' l't'is, however, possible to overcome this third cause for the destruction ofthe platinum crucibles entirely, by transforming the ne crystalline be improved to such a degree that the crystals in sheets and wires will grow to sizes up to 1- centimeter. and even more in length and width. When this method is applied to the finely crystalline platinum crucibles, the metal of the crucibles is transformed in itsA entirety into an aggregate of such single crystals which have a thickness equal to that of the orucible wall and a' length and width many times the thickness of the cruclble wall; the whole cruclble consists ofI a few tightly packed, large single crystals. These large crystalsfare all of the lsame order i of magnitude, that is to say, the cruclble contains no small crystals. The term uniform-large crystals as used in the appended'claims, is to be understood as stating that the cruclble consists exclusively of such large crystals. These single crystal crucibles have features and charactera istics analogous to those of the Well-known single d 2. This hard-worked cruclble is quickly heated throughout to 1200" CA. and above, preferably to l600 C. and above. The orucible is kept at these temperatures from l/2 to 2 hours. The metal recrystallizes instantly to a line and normal'grain. The metal is then in the dead soft state.

3. This platinum cruclble (recrystallized completely and transformed into the dead soft state) is now hammered lightly with a highly polished steel-hammer all over its surface. The reduction in thickness resulting from this hammering treatment sh preferably not exceed 10%;` I have found t best results are-obtained when such reduction in thickness amounts to about 3%. The best way to attain such a mechanical treatment is to take care never to apply the hammer twice to the same spot. Through such mechanical treatment critical strain gradients are created all over and within the dead soft metal.

e. This still finely crystalline, `but dead soft and very lightly hammered orucible is quickly heated to l200 C. and more, preferably to 1600 C. and more and is kept at these high tem- Special tests have shown that with the same metal and the same wall thickness the sinslecrystal crucibles are thebetter, the nner the crystal grain of the original orucible is. According to lthese tests the ilnestgrain of the metal is obtained when the platinum sheetused for the shaping of -the orucible shells is reduced im thickness at least 75%, when rolled to the special size (best results are obtained with a 'reduction of 85% and even more). This reduction in thickness is generally performed in two or more stages with intermediate annealing, and these intermediate annealing operations are each conducted .800 C.- and for a time of from 1 to 15 hours.

These working conditions should also be observed in the method of producing the orucible shells by the spinning process and to the necessary intermediate and nal annealings before the shells are hammered. The method of producing the single crystal crucibles can be applied to the commercial cruclble platinum (platinum .with .3% iridium or rhodium), and especially to thev different kinds of the C. P. (chemically pure) platinum. The size of the single crystals increases with the purity of the metal. This method of producing single-crystal crucibles isv not applicable, however, to an alloy containing platinum and more than 2% of rhodium. The method of producing single-crystal crucibles can be applied to all the cruclble sizes, to the thin-walled as well as to the ,thick-walled crucibles. The best results are obtained with the thick-walled crucibles, which are made from sheets ,020" to .060" thick, especially those .025" thick. Tests have shown that the critical strain gradients are most easily attained with these thick-walled crucibles through the hammering process explained above. With a sheet .025" thick, single crystals with an average length and width of more than six times the wall thickness are easily attained when orucible platinum is used; when C. P. platinum is used, "single crystals measuring in length and width up to 20times the diameter of the cruclble wall (made from the .02.5" sheet) are obtained and even larger crystals. In any event, I prefer that both the length and the width of these "single crystals should be at least three times the thickness of the cruclble wall. With the thin walled crucibles the slightest strokes with the lightest hammer cause reductions' and strains within the dead soft metal which exceed the critical range. When the sheets are thicker than .060" they are dimcult to work; very strong strokes have to be 1applied to them; these strong strokes cause at the surface of the metal a high reduction whereas the core of the metal is hardly inuenced by the mechanical treatment.

Platinum metal when transformed into the single crystal structure changes its qualities considerably. The `finely crystalline cruclble always has rather a dull tone in its color when in the highly polished state; the metal of the singlecrystal cruclble has an extremely high brilliancy; th metal attains this state of high brilliancy with the first secondary recrystallization, when it is annealed at 1600 C. This difference in color is most easily observed with artificial light.y Since the brilliancy of the single-crystal orucible is its original and natural condition, it is never necessary to polish the surface of such a cruclble. The standard platinum crucibles such as made prior to my invention, are polished with iron oxide or chromium oxide; .small amounts of these substances always stick to the surface of the metal:-

the single-crystal crucible has an absolutely clean surface. The natural brilliancy of the singlecrystal crucible is very stable; only the most severe corrosive action can destroy this brilliancy.

'The forces which cause recrystallization and exaggerated grain growth in the fine crystalline crucible are completely deadened within the single-crystal crucible; the single-crystal crucible is' therefore absolutely stable in this respect. Therefore repeated rehammering and subsequent reannealng is never harmful to the single-crystal crucibles (as it is to the thin-walled, standard and also to the thick-walled finely-crystalline platinum crucibles); such repeated rehammering and reannealing always improves the thick-walled single-crystal crucibles. Since the metal of such single-crystal crucibles is extremely ductile, slight changes in the shape of such crucibles are corrected very easily.- The metal of the single-crystal crucible is transformed into the highest state of passivity; therefore such crucibles are chemically less corroded than the finely crystalline crucibles. At extremely high temperatures (1200 to 1700 C.) the metal of the single-crystal crucibles is slightly less volatile in atmospheres which contain oxygen, than the finely crystalline crucibles.

Taking-into consideration all the facts mentioned before concerning the advantages of the heavy-walled platinum crucibles (.020 to .060" wall thickness, especially those which'have a bottom thickness of .025") and the changes resulting from the transformation of such iinely crystalline crucibles into single-crystal crucibles, it is most evident that the single-crystal crucible with a heavy wall constitutes the most perfect type of platinum crucible. In this type of crucible, with the use of the least possible amount of platinum, the following advantages are combined:

1. It is the most perfect kind of a single-crystal crucible. All the forces which could cause defects through recrystallization are absolutely killed.

l2. Greatest mechanical stability; the crucibles do not sag under the influences of alkali carbon-- ate melts; there is never any necessity for reinforcing the crucibles at the rim 'or for reshaping them.

d polishing powders. Natural, clean surface with high briilia'ncy.

The principles outlined above for making an .bottom of the crucible. V/

improved platininn crucible can be applied to any shape of the platinum crucible used in the chemical laboratory, to the standard shape of the common platinum crucible (these crucibles have the shape of a truncated cone. the smaller end of which is used as the bottom)to the many variations between this shape and the simple, cy lindrically shaped crucible, as well as to the recently introduced shapes which are known as Interchangeable and combination crucibles.V

Fig. 3 of the accompanying drawings illustrates a crucible made from iinely crystalline platinum and embodying my invention, while Fig. 4 shows a similar crucible made from single-crystal material.

The term "platinum as used in the appended claims is to be interpreted as including the vari` ous kinds of platinum which I have stated to be suitable for use in connection with my present invention, that is to say, so-called crucible platinum, as well as C. P. (chemically pure) platinum and platinum-rhodium alloys of the character explained above.

Various modifications may be made without departing from the nature of my invention as defined in the appended claims.

I claim:

1. A platinum crucible made of platinum in the completely unstrained condition and having a wall composed of uniform large crystals at 30 least a majority of which are of a thickness substantially equal to that of the crucible wall and of a length and width at least three times the thickness of the crucible wall.

2. A platinum crucible made of platinum in the completely unrestrained condition and having a wall composed of uniform large crystals at least a majority of which are oi'. a thickness substantially equal to that of the crucible wall,

such wall thickness being at least .020" at the 40 bottom. y 3. A platinum crucible made of platinum in the completely unrestrained condition and having a wall composed of uniform large crystals at least a majority of which are of a thickness substantially equal to that of the crucible wall, such wall thickness being .025" at the bottom.

4. A platinum crucible made from C. P. platinum in the completely unrestrained condition and having a wall composed of. uniform large crystals at least a majority of which are of s.

thickness substantiallyequal to that of the crucible wa such wall thickness being .025" at the 5. A platinum crucible made of platinum the completely unrestrained condition and having a wall composed of uniform large crystals at least a majority of which are of a thickness substantially equal to that of the crucible wall. such wall thickness being within the range of from .060" at the bottom.

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