US 2573961 A
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Patented Nov. 6, 1951 UNIT STATES ATENT OF CE 2,5 3 LOW-RESISTANCE DRILLING FLUIDS No Drawing. Application June 5, 1950, Serial No. 166,298
LOW RESISTANCE OIL BASE DRrnLIN FLUIDS This invention relates to oil base drilling fluids, and in particular concerns oil base drilling fluids having low electrical resistance. It further relates to the specific process whereby such drilling fluids are obtained.
In drilling oil or gas wells by means of rotary drilling tools, a hollow drill pipe known as a drill stem having a bit attached to its lower end is extended downwardly through the well bore and rotated while the bit is pressed against the work ing face in the formation at the bottom of the hole. The actionof the rotating bit grinds away the formation as the drilling progresses. During the drilling, a fluid body known as a drilling fluid or mud is continuously circulated downwardly through the drill stem, through the bit and against the working face .of the hole, and then upwardly through the annular space between the drill stem and the wall of the bore hole. 'lhe drilling fluid serves a number of purposes, among which are cooling and lubricating the drill bit, suspending and removing cuttings from the bore hole, preventing the flow of liquids from the formations traversed by the bore into the same by applying hydrostatic pressure to such forma tions, and fulfilling other purposes.
In locations where the underground formations traversed and/or penetrated by the bore contain materials such as hydratable clays which swell and/or disintegrate in the presence of water, it has become customary to employ drill ing fluids which are substantially free of water in order to preclude the introduction of water into the bore by means of the drilling fluid. Such drilling fluids are termed oil base fluids since they usually comprise a mineral oil having dispersed or suspended therein minor proportions of various agents adapted to impart special prop erties to the composition. Among such agents, the most universally employed are: weighting agents, which are high density inert solids adapted to increase the apparent density of the base oil and thus increase the hydrostatic head provided by the drilling fluid within the bore; Wall-building agents, which are materials such as clay or asphalt adapted to coat or plaster the walls of the bore with an im ermeable layer which prevents the escape of the drilling fluid into permeable formations; and dispersing a ents which serve to maintain solid compnnents of the fluid uniformly dispersed therein. Oil-base drilling fluids may also comprise a variety of- 9 Claims. (01. z t-c) other agents such as gel strength improvement agents, viscosity modifiers, emulsifying agents, protective colloids, inorganic salts, etc.
Among the various methods for investigating and determining the nature: of the subsurface formations traversed by a well bore, i. e., well logging, those involving the measurement of one or more of the electrical characteristics of such formations enjoy wide application. Most of such methods require that the bore hole be filled with a liquid capable ofconducting relatively low voltage electric currents, and it is hence. highly desirable that the drilling fluid employed in drilling the well be adapted for such use, i. e., it should have low electrical resistance. In general, oilbase drilling fluids do not have the requisite low electrical resistance to adapt them for use during electric logging, and this deficiency has greatly restricted their general applicability despite their other highly desirable properties and characteristics. For example, the oil base drill-. ing fluids which are described and claimed in my copending application Serial No. 105,067, filed July 15, 1949, now Patent Number 2,542,020, and which essentially comprise a mineral oil dispersion of a hydratable clay stabilized by certain soaps of modified rosin, are possessed of a combination of physical and chemical properties which render them highly satisfactory'as far as the drilling operation itself is concerned. However, their electrical resistivity is of the order of 1 10 ohm-crns. which is 'far'too high to permit their use in electric logging operations.
It is accordingly an obiect of the present in: vention to provide oil-base drilling fluids suitable for use in electric well logging operations.
Another object is to provide oil base drilling fluids having relatively low electrical resistance.
A ur he e t i $9 idl? E O T creasing the electrical resistance of drilling fluids of the type specifically referred to above without d actin rom other de r b e cha act ri cs of such fluids.
Qther objects will be apparent from the following detailed description of the invention, and various advantages not specifically referred to herein will occur to those skilled in the art upon employment of the invention in practice.
I have now found that the above and related objects, may be realized. in drilling fluids pre: pared by a special technique involving the dis: Per an vin b s o l o ar u ly cnt ollc amounts of a hydrated clay, certain saponified rosin p oducts a a lia i e arth meta a e water, and an alkali-metal silicate or a mixture of an alkali-metal silicate and an alkali-metal carbonate or bicarbonate. Such drilling fluids are characterized by low fluid loss values, high gel strength, good viscosity characteristics, and other desirable physical and chemical properties, and in addition have electrical resistivity values of the order of 1 10 ohms-ems, which is approximately the same as that of ordinary surface waters. The invention thus consists in drilling fluids essentially comprising the aforementioned components and prepared as hereinafter more fully explained.
Components The base oil which forms a maior component of the new drilling fluid compositions is preferably of mineral origin and may be crude petroleum or a distillate :or residuum material. Heavier materials such asfli ht tars, cracked residua. heavy extracts and the like are especially well suited, particularly when blended with a light distillate such as gas oil, diesel fuel, etc. A
highly satisfactory mixed base of this type comprises a ma or proportion, e. g., 66-95 percent,
of a relatively heavy oil such as a light residual oil having a specific gravity of about 13l5 API- and a viscosity of about 30-40 seconds SSE at 122 F. and containing substantial amounts of asphaltenes, polymeric bodies and the like, and a minor proportion, e. g., 5-40 per cent, of a light distillate such as a diesel fuel having a specific resin acids with analkali-metal alkali, e. g.,
sodium hydroxide, sodium carbonate, potassium hydroxide, etc., in such manner that the saponification reaction is only partially complete and the saponified product contains from about 1 to about per cent of free unsaponified resin acids. The heat-treatment of wood rosin, i. e., rosin which has been extracted from pine wood stumps, whereby the resin acids thereof are isomerized and/or otherwise modified is well known in the naval stores art, and may be effected in various ways to obtain modified rosin products which vary somewhat in their physical and chemical properties depending upon the nature and extent of the heat-treatment. Thus, any of the various color grades of refined wood rosin may be heated under non-oxidizing conditions at temperatures between about250 C. and about 350 C. for a length of time sufficient to raisethe specific rotation of the rosintfrom its original negative .value to a value between about +5 and about +15". The resulting rosin product closely resembles the original rosin in appearance, ease of saponification, etc., but is considerably altered chemically as evidenced by its increased specific rotation, increased dehydroabietic acid content, lower iodine number, etc. By carrying out the heat-treatment at somewhat 'higher temperatures and/or over longer periods of time, the
specific. rotation may be raised further. e. g., to
+ or even-higher, and the degree of QlQfiIl Q unsaturation further decreases. Also, under such conditions decarboxylation takes place with the formation of unsaponifiable bodies which are usually referred to as rosin oils. The heat-treatment of wood rosin to secure the desired modification of the resin acids as indicated by increase in specific rotation to a value above about +5 may also be effected in the presence of catalysts at relatively low temperatures as described in U. S. Patent 2,154,629. The catalysts employed are of the hydrogenation type, e. g.,
-metallic platinum or palladium, although the treatment is carried out in the absence of added hydrogen. The reaction which takes place is termed disproportionation since it involves the simultaneous hydrogenation and dehydrogena tion of abietic-type acids with the consequent formation of'dihydro'abietic and dehydroabietic acids and their analogues; and the resulting product is referred to as disproportionated wood rosin. Similarly, the product obtained by heattreating wood rosin under conditions sufficiently drastic that carboxyl groups are removed from' the rosin acids is termed decarboxylated wood, rosin, and the product obtained by heat-treating wood rosin under less drastic conditions so that the change effected is substantially onlyone of molecular rearrangement is referred to as "isomerized wood rosin. All of these modified rosin products are characterized by having been prepared by heat-treating wood rosin under. conditions of time and temperature, and in the res:
once or absence of a hydrogenation catalyst but in the absence of added hydrogen, sufiicient to raise the specific rotation of the rosin to a value above about +5".
Any of the above described modified rosin prod ucts may be used to obtain the saponification products employed in preparingthe drilling fluid. compositions of the present invention. Proced-. ure for carrying out the saponification reaction. is well known in the art, and in general consists merely of adding the modified rosin in the solid or molten state to a hot aqueous solution of the desired alkali-metal alkali and thereafter, heat: ing the mixture until the reaction. is complete and the product contains the desired amount of water. The amount of alkali employed is somewhat less than that required for the complete saponification of the resin acids in order that the saponification product may contain the requisite The concentration of the aqueous alkali is usually. so;
amount of free unsapcnified resin acids.
adjusted that the product obtained takes the form of a viscous liquid or thick paste containing 60-85 per cent solids. The physicalform of the product also depends somewhat upon the type of modified rosin employed. The saponification product obtained from decarboxylated wood rosin containing a substantial amount of rosin oils, for example, is a relatively fluid liquid even though it may'contain only 5-10 per cent of water.
While any of the alkali-metal alkali saponification products of wood rosin which has been heat-treated to raise its specific rotation to a value. above about +5 may be employed in .pre- .paring the present drilling fluids, I. have found that superior results, particularly with respect to the fluid loss value of the drilling fluid, are attained by employing either of two specific products of this type. The first, of such preferred saponification products is an alkali-metal alkali saponificationproduct of wood rosin which has been. heat-treated at temperatures between about 250? i Q- and ent 350 Q the absen f a c t ly t amass:
t'c'suchan extent-that-it contains only about 50-60" per cent of free resin acids, 30-40 per cent, of unsaponifiable oils, and small 'amounts of phenolic materials, water, and-products of unknown constitution. A particularly preferred product of this. type is the potassium hydroxide saponification product ofsuch heat-treated rosin containing about 45-55'per cent potassium resin acid soaps, about 30-35 per cent unsaponifiable materials; about 5-l0-per cent free resin acids, and about 5-10 per cent water. class of saponification products in the product obtained by heating wood rosin at a temperature of about 225 -300" C.,f,or about -60 minutes in contact with a palladium catalyst, distilling the-.lresulting product. and collecting a fraction distilling at about 2109-2750. under about 5-10 mm. pressure, and thereafter saponifying such. fractionwith. aqueous sodium hydroxide in the knownmanner. Such product is available commercially under the trade name Dresinate 731.
The alkaline-earth metal base employed in preparing the new drilling fluids may be any oxide, hydroxide or basic salt of any of the alkaline-earth metals, e. g., calcium oxide, calcium hydroxide, barium hydroxide, strontium hydroxide, calcium acetate, etc. Calcium hydroxide, ee-g ordinary hydrated lime, and calcium oxide aropreferred for reasons of low cost and'genera-l availability.
'Thehy'dratable clay component is preferably a high quality material such as bentonite, montmorillonite, or kaolinite, but may be common clay such as is available in almost any locality. Bentonite is preferred. Similarly, the water component should also be relatively pure or fresh. Water containing substantial amounts of dissolved inorganic salts, e. g., salt brines and The second of the preferred the like, is not suitable since such salts have an adverse effect'on the physical and electrical properties ofthe drilling fluid.
' The alkali-metal. silicate component is preferablysodiumsilicate, although potassium, lithium or ammonium silicate may be employed. In accordance with usual practice, the. ammonium radical is herein considered equivalent to the alkali-metals, and the latter term should be understood as including the ammonium radical. Thea'lkali-metal silicate is conveniently employed in its commercially available form, i. e., as an aqueous solution of about 20-60 per cent concentration. Ordinary per cent aqueous sodium silicate or water glass" has proved very satisfactory.
The alkali-metal carbonate or bicarbonate which may be employed in conjunction with the alkali-metal silicate is preferably sodiumorammonium carbonate or bicarbonate, although the carbonates or bicarbonates of other alkali-metals maybe employed if desired.
addition to the foregoing essential components, the drilling fluids oi the invention may optionally comprise a number of other components. Usually, it will be desired that the fluid contain a weighting agent to increaseitsapparent density. Suitable weighting agents include finely-divided whiting, barytes, iron oxides, lead dusts, fullers earth, calcined clay. calcium carbonate, and other high density inert solid materials, and such agent may be employed in amounts sufiicient to provide a composition having an apparent density from abouti65 lbs/cu, ft. to as high as 1 30 lbs/cu. it. .Other optional components include carbohydrate or proteinaceous colloids, viscosity modifiers oi. cond tioners, etc.
Proportions While the proportions in which the'various es sential components are employed in preparing the new compositions may be varied between certain limits depending upon the identity of such components and the specific properties desired in the composition, such proportions are relatively criticalwith respect to any given group of components and/orset ofproperties. Ordinarily, however, the partially saponified modified rosin containing from about 1 to about 15 per cent of free resin acids is employed-in an amount representing from about Ito-about 10, preferably between about 4 and about 8 per cent by weight of the entire composition. The alkaline-earth metal base is employed in an amount representing from about 60 to about per cent of the amount chemically equivalent to the saponified rosin product. The latter-amount ofalkaline-earth metal base is the quantity required to-rnetathesize the alkali-metal soaps contained in the saponified rosin product plu'sthe quantity required to neutralize the 1 to 15 per cent of free resin acids present in such prod not. When ordinary hydrated lime is employed as the'alkaline-earth metal base and the saponified rosin product is one or the other of the preferred products previously described, the lime is usually employed in an amount representing between about 0:2 and 0.3-per cent by Weight of the entire composition. The hydratable clay is employed in anamount representing between about 0;l and about 5, preferablybetween about 0.4 and about 1-2, per cent by weight of the entire composition. The alkali-metal, silicate or mixture of alkali-metal silicate and alkali-metal carbonateor bicarbonateis employed in an amount (dry basis) representingbetween about 0.3 and 0.6 per centv by weight of the entire composition. The mixture will usually comprise between about '75 and about per cent by weight of the aqueous alkali metal silicate and between about 5 and about 25 per cent by weight of the alkali-metal carbonate or bicarbonate. A preferred mixture of thistypeconsists of about 50 per cent by weight of 40per cent aqueous sodium silicate, 2.5 per cent by weight of ammonium carbonate and 47.5 per cent by Weight of water. The water is provided in suchamount that the composition contains between about'1.5 and 5 per cent by Weight of free water. Inasmuch as the saponified rosin product usually contains some free water and since the alkali-metal silicate is employed in the form 01 an aqueous solution, the amount of free water actually added during preparation of the initial composition will be adjusted accordingly so that the total free water content of the final composition will be within the aforementioned limits.
Procedure The exact manner and order in which the aponified rosin product, alkaline-earth metal base, hydratable clay and water are dispersed in the base oil are not of primary importance. However, it is essential that the alkali-metal silicate or mixture of alkali-metal silicate and alkali-metal carbonate or bicarbonate not be Q mbined with the other components until sub- Stantially all of the alkaline-earth metal base has. reacted by metathesis with the saponified rosin product to form the corresponding alkalineearth metal rosin, soap. Accordingly, the procedure employed in p p i g the present omp tion Consists essentially in dispersing the requisite amounts or the saponified rosin product, ali'r'aline-earth metal base, hydratable clay and water in the base oil, allowing the resulting mixture to stand until substantially all of the alkaline-earth metal base has reacted with the saponifled rosin product, and thereafter adding the requisite amount of alkali-metal silicate or mixture of alkali-metal silicate and alkali-metal carbonate or bicarbonate. In order to insure a uniform and stable composition, it is desirable that the composition be subjected toviolent agitation, preferably homogenization under a pressure of 1000 lbs. or more, during or after the addition of the alkali-metal silicate.
According to one mode of procedure, the alkaline-earth metal base is suspended in a portion of the base oil, after which the partially saponifled modified rosin product is added with vigorous stirring. The hydratable clay and part of the water are then added, either separately or together, and stirring is continued to obtain a homogeneous concentrate composition. Such composition may be allowed to stand until the metathesis reaction between the saponifled rosin product and the alkaline-earth metal base is substantially complete, and then diluted with the remainder of the base oil, followed by the addition of the aqueous alkali-metal silicate, or it may be diluted with the remainder of the base oil before being allowed to stand. When the base oil comprises a mixture of a light and a heavy oil as previously described, it is preferred that the light oil component be employed informing the initial concentrate composition. According to an alternative mode of procedure, the saponifled rosin product is dispersed in a mixture of the light oil and a portion of the heavy oil, after which the hydratable clay and water are added with vigorous stirring. The alkaline-earth metal base is then added, after which the remainder of the heavy oil is added and the resulting composition is allowed to stand for to days. The alkali-metal silicate or mixture of alkali-metal silicate and alkali-metal carbonate or bicarbonate is then added, and the mixture is homogenized for about one minute under a pressure of 1000-2000 lbs. Other methods of combining the various components may be employed, it being essential only to insure that substantially all of the alkaline-earth metal base be reacted with the saponifled rosin product prior to addition of the alkalimetal silicate.
In determining the electrical resistivity of the drilling fluids provided by the invention, a pair of electrodes comprising two l-inch square nickel plates spaced about one inch apart is immersed in a sample of the fluid maintained at the desired temperature. A low voltage alternating current is applied across the electrodes, and the current flowing between the electrodes is determined by a milliammeter inserted in the circuit. By calibrating the electrode assembly against a liquid of known resistivity and by properly shunting the milliammeter, the resistivity of the fluid in ohmcms. may be read directly from the milliammeter. Other properties of the fluids referred to hereinafter are determined by standard methods well known in the art. Detailed procedure for determining the fluid loss value is set forth in Recommended Practice on Field Procedure for Testing Drilling Fluids, API Code No. 29, July 1942. The viscosity is usually determined with the standard Marsh funnel viscosimeter.
The following examples will illustrate several ways in which the principle of the invention has been applied, but are not to be construed aslimiting the same. parts by weight.
' Example I Three drums (1383 lbs.) of a domestic diesel fuel having the following specifications:
Gravity, API 31.0 Viscosity, SUS at F 40 Flash point, 9F 180 Ash, per cent by wt 1 0.01; Sulfur, per cent by wt 0.9" Boiling range, "F 400-700,
and 10 bbls. (3325 lbs.) of light domestic fuel'oil having the following specifications:
Gravity, API 14.5
Viscosity, SSF at 122 F 36 Flash point, F 1'70 Ash, per cent by wt 0.06 Sulfur, per cent by wt 2.5-
were placed in a large mixing tank, and 2 drums (930 lbs.) of "Dresinate 731 were added with vigorous stirring. 'The latter product is the sodi-- um hydroxide saponification product of distilled disproportionated wood rosin hereinbefore -described. There were then added 250 lbs. of -Wyoming bentonite, bbl. lbs.) of water, and 50 lbs. of hydrated lime.
after which '45 bbls. (14,962 lbs.) of the fuel oil were added with continued stirring; The fluid so:
prepared was then placed in a tank and'allowed to stand at atmospheric temperature for 10 days,
during whichtime substantially all of the lime reacted with the rosin soap. There were thenadded 204 lbs. of N Grade 40 per cent aqueous sodium silicate, 204 lbs. of water, and 10 lbs. of, ammonium carbonate. The resulting mixture was then homogenized at a pressure of about 1200 lbs. to obtain the finished drilling fluid. A sample of this fluid had a resistivity oflxlO' ohm-ems. at F., a fluid loss value of 10 mL/hr. at 200 F., and a Marsh viscosity of 154 at 115 F. Prior to addition of the sodium sillcate, water and ammonium carbonate, the fluid had a resistivity of 1x10 ohm-ems, a fluid loss value of 25 ml./hr., and a Marsh viscosity of 325.
The proportions in which the various components were employed in preparing the fluid were as follows:
Per cent by weight treated wood rosin, 0.4 part of hydrated lime, 0.6 part of water, and 0.6 part of bentonite were dis persed in a mixed oil base consisting of 4.0 parts of the light diesel fuel and 90.3 parts of the light fuel oil described in Example I.
The saponified rosin employed was the partial potassium hydroxide saponification product of wood rosin which had been heat-treated at about 280-320 C. for2-4 hours. -It wasa dark-colored highly viscous fluid, and comprised about 50 per cent of the potassium soaps of modified -rosin acids, about 7 per cent of unsaponified rosin acids;
All proportions are given in:
The resulting concen-. trate composition was stirred for 20 minutes,f
asset 33 per cent or unsaponifiable rosin oilsand uct was found to have a resistivity of about I 8 -10 ohm cms.
Example-III -A- drilling mud concentrate was prepared by dispersing 6750 parts of the saponifiedrosin product described in Example I, 1400 parts of water, and 2180 parts of bentonite in 4780 parts of the light diesel fuel and 2880 parts of the light fuel foil described in Ekai'nple I. Approximately 3700 parts of this concentrate were then diluted with about 33,000 parts of the fuel oil, and to the diluted composition there were then added 76 parts of hydrated lime and 185 parts of 40pm cent aqueous sodium silicate. The resulting composition was then allowed to stand for about 5 days. The various components were employed in the following proportions:
In order to demonstrate the eifect of varying the amount of sodium silicate in the composition, additional sodium silicate was added to each of four samples of the above composition. After addition of the sodium silicate each sample was homogenized for 2 minutes at 180 F. and under 2500 lbs. pressure, and the resistivity and fluid loss values were determined. The data obtained are tabulated as follows:
7 ti e R a iica e 8515 1V1 y, a ue, sample Content, Ohm-ems. Ml./hr. at
Per Cent 200 F.
O. 4 2x10 18 0. 6 2x10 50 1. 1X10 68 1. 8 x10 75 2. 4 5X10 80 It will be noted that while the resistivity remained relatively constant with increasing sodium silicate content, the fluid loss value increased. Accordingly, it is desirable that the alkali-metal silicate be employed only in the amount required to provide the desired low electrical resistance.
Example IV The eifect of water content upon the resistivity of the fluid was determined by adding aqueous sodium silicate of varying concentration to the fluid prepared in Example III. In each case sufl'icient sodium silicate was added to bring the sodium silicate content of the fluid up to 1.0 per cent. Each sample was homogenized as in Example III, after which its resistivity was deter- 10 jined. The data obtained are tabulated as r01- lows:
Water Con- I 1 Resistivity, Sample No. tcitnlzer Ohmcms.
1.0- 1 l0 2.5 8 10 3.5 1X10 5; 5 5X10 6.5 5X10 It will be noted that amounts of water above about 5 per cent adversely affect the resistivity.
Various modifications within the scope 'of the invention will be apparent to those skilled in the art. Such modifications may include the useof specific types of hydratable clays, various types of weighting agents, viscosity modifiers, gel
strength improvement agents, and various types and blends of base oils, as well as different alkeill'n'e-eaith metal bases, alkali-metal silicates, and saponified rosin products obtained from different types of heat-treated wood resins and different alkali-metal alkalies. The essence of the invention lies in the combination of the various componentsin the proportions herein specified. Since the desirableproperties' of the new compositions provided by the invention are dependent upon the manner in which the compositions are prepared, and certain of the components are mutually reactive, the new compositions are defined in the following claims in terms of their method of preparation rather than by their uncertain chemical composition.
Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the materials or procedure employed, provided the compositions stated by any of the following claims, or the equivalent of such stated composition, be obtained.
I, therefore, particularly point out and distinctly claim as my invention:
1. An oil base drilling fluid composition characterized by relatively low electrical resistivity and prepared by (l) dispersing in mineral oil between about 0.1 and about 5 per cent by Weight of a hydratable clay; between about 1 and about 10 per cent by Weight of a saponified rosin product obtained by saponifying with an aqueous alkali-metal alkali a wood rosin whichhas been heat-treated at a temperature between about 250 and about 350 C. for a period of time sufiicient to raise its specific rotation to a value above about +5, said saponified rosin product containing between about 1 and about 15 per cent by weight of free resin acids; an alkaline-earth metal base in an amount representing between about 60 and about per cent of the amount chemically equivalent to the saponified rosin product; and water in an amount such that the total water content of the finished composition is between about 1.5 and about 5 per cent by weight; (2) allowing the resulting dispersion to stand until substantially all of the alkaline-earth metal base has reacted with the saponified rosin product; and (3) thereafter adding to the dispersion an aqueous solution essentially comprising an alkali-metal silicate in an amount such that the alkali-metal silicate content of the finished composition is between about 0.3 and about 0.6 per cent by weight.
2. A composition as defined by claim 1 wherein the saponified rosin product is the potassium hydroxide saponification product of wood rosin which has been heated at a temperature between about 250 C. and about 350 C. to raise its specific rotation to a value above about and comprises between about 45 and about 55 per cent of potassium resin acid soaps, between about 30 and about 35 per cent of unsaponifiable materials, between about 5 and about per cent of free resin acids, and between about 5 and about 10 per cent of water.
3. A composition as defined by claim 1 wherein the saponified rosin product is the sodium hydroxide saponification product of distilled disproportionated wood rosin.
4; A composition as defined by claim 1 wherein the alkaline-earth metal base is selected from the class consisting of calcium hydroxide and calcium oxide.
5. A composition as defined by claim 1 wherein the alkali-metal silicate is sodium silicate.
6. A composition as defined by claim 1 wherein the alkali-metal silicate is employed in admixture with an alkali-metal carbonate.
7. A composition as defined by claim 1 wherein the alkali-metal silicate comprises sodium silicate in admixture with between about 5 and about 25 per cent by weight of ammonium carbonate.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name r Date 2,316,967 Miller Apr. 20, 1943 2,350,154 Dawson May 30, 1944 2,356,776 Miller Aug. 29, 1944 2,363,499 Campbell Nov. 28, 1944 2,430,039 Anderson Nov. 4, 1947 2,461,483 Self Feb.'8, 1949