the term vermiculite , as it is used herein , refers to the group of rock forming mineral species characterized by a layer latticed structure in which the silicate layer units have a thickness of approximately 10 angstroms . the main elements present in the layer are magnesium , aluminum , silica , iron and oxygen with the layers being separated by one or two sheets of water molecules associated with cations such as magnesium , calcium , sodium and hydrogen . the layers have considerable lateral extent relative to the thickness of the basic 10 angstrom unit layer . the term vermiculite as used herein therefore includes minerals consisting wholly or largely of vermiculite , or minerals of a mixed - layer type containing vermiculite layers as an important constituent , such as hydrobiotites , and chlorite - vermiculite , but does not include minerals of the montomorillonite group . heretofore , the degree to which the expansion temperature is lowered by ion exchanging the vermiculite with a given salt solution has been attributed primarily to the type of cation present in the solution . the anion component of the salt has been given less consideration , and has conventionally been chloride . unexpectedly , it has been found by the inventors that the expansion temperature and characteristics of the cation exchanged vermiculite can be determined by the combination of cation and anion used . in particular , it has been found that the use of the nitrate salt of potassium provides substantial improvements in both the yield of expanded vermiculite and the expansion characteristics , as opposed to that obtained with other potassium salts and other anionic components . thus , as shown in example 1 , potassium nitrate exchanged vermiculite has , after expansion at a given temperature , a significantly lower bulk density , and thus higher yield , than ammonium nitrate or potassium chloride exchanged vermiculite . example 1 also shows that superior yields can be obtained when potassium nitrate exchanged vermiculite is expanded at temperatures as low as 475 ° f ., whereas higher temperatures are required to obtain equivalent yields with ammonium nitrate or potassium chloride exchanged vermiculite . generally , when potassium nitrate exchanged vermiculite is expanded at 475 ° f . or above the resulting bulk density is less than 30 lbs / ft 3 . the use of a potassium nitrate solution is also desirable from the standpoint of disposal , as this solution can be recycled as a fertilizer without risk to the environment . the potassium ion exchange generally takes place by subjecting the vermiculite ore to an aqueous solution of potassium nitrate . the concentration of the potassium nitrate solution is preferably from 0 . 1 to 5 molar , and most preferably from 0 . 5 to 2 . 5 molar . when ammonium ion exchange is performed , the optimal concentration is generally 5 molar , making potassium nitrate significantly more cost effective . the optimum duration of ion exchange is directly dependent on the concentration and temperature of the potassium nitrate solution . generally , the duration of the ion exchange is from 0 . 5 to 72 hours , and preferably from 10 to 36 hours . the preferred exchange conditions for a 1 molar solution are 24 hours at room temperature . however , shorter exchange times at higher temperatures may be used , e . g . 4 hours at 200 ° f . when weaker solutions are used longer exchange times may be necessary to achieve the same yield . the weight ratio of vermiculite to salt solution also influences the expansion characteristics of the ion exchanged vermiculite . preferred ratios are from about 0 . 5 : 1 . 5 to about 1 . 5 : 0 . 5 , with the most preferred ratio generally being about 1 : 1 . it is generally preferred that the excess solution be decanted and the vermiculite be rinsed subsequent to ion exchange in order to remove any excess salt . the vermiculite may be rinsed with distilled water , and it is preferred that the vermiculite be rinsed several times . it is also preferred that the vermiculite be dried prior to expansion . generally the vermiculite is allowed to air dry at room temperature . the cation exchanged vermiculite of the invention may then be expanded using conventional techniques and equipment . generally , the expansion temperature of vermiculite which has been cation exchanged with potassium nitrate is lower than the expansion temperatures of similar vermiculite ore which has been treated under similar conditions with other salt solutions . for example , initial expansion temperatures of intumescent sheets containing vermiculite which had been cation - exchanged under similar conditions with potassium nitrate , ammonium nitrate , potassium chloride , and ammonium chloride , as measured using thermal mechanical analysis , were approximately 465 ° f ., 490 ° f ., 520 ° f . and 520 ° f . respectively . for advantageous yields it is preferred to expand the vermiculite of the invention at a temperature of at least 475 ° f ., and most preferably in the range of from 475 ° f . to 600 ° f . the minimum expansion temperature of the cation exchanged vermiculite , and the preferred temperature for full expansion , will depend on the type and grade of vermiculite used , due to differences in the mineral and moisture contents of the ores . the expansion process proceeds very quickly ; for example within 10 minutes full expansion is generally obtained . the flexible intumescent sheet material according to the invention comprises unexpanded vermiculite prepared according to the invention , one or more inorganic fibrous materials , and one or more binders . preferably , the intumescent sheet of the invention comprises from about 40 % to 65 % potassium nitrate exchanged vermiculite , from about 25 % to 50 % of the fibrous material , and from about 5 % to 15 % of the binder . inorganic fibrous materials which may be used include glass fibers , slag , rock and ceramic fibers . a preferred fibrous material is ceramic fibers . suitable binders can include various polymers and organic elastomers in latex form , as for example , natural rubber latexes , styrene - butadiene latexes , butadiene - acrylonitrile latexes , latexes of acrylate and methacrylate polymers and copolymers and the like . the sheet material may be formed by standard papermaking techniques , either hand laid or machine laid , taking suitable precautions to attain substantially uniform distribution of particles throughout the web . the sheet material may be provided with or temporarily laminated to a backing sheet of kraft paper , plastic film , non - woven synthetic fiber web or the like as desired . generally , the potassium nitrate treated vermiculite is combined in a large volume of water with the fibrous material and binder . small amounts of surfactants , foaming agents and flocculating agents may also be added before forming the sheet . the sheet is then formed by standard papermaking techniques either in a hand - sheet former or fourdrinier screen . the resulting green sheet is compressed to give a dry weight density of generally at least 0 . 35 g / ml , and dried at relatively low temperature , generally less than about 90 ° c ., to form a handleable , flexible sheet material . after drying , the intumescent sheet may be employed in any application where it is desired to employ a heat expandable , heat resistant sealing or filling material . a preferred use for the intumescent sheets of the invention is as a packing for mounting and positioning automotive catalytic converter monoliths . the intumescent sheet , when used in this application , is wrapped around a catalytic converter core , and the core and sheet are placed inside an outer metal canister . the resulting assembly is then usually mounted in an automobile in standard fashion and heated , generally by the engine exhaust gases , to expand the intumescent sheet . using the intumescent sheet of the invention , the sheet will generally expand upon reaching a temperature of approximately 475 ° f . intumescent sheets produced according to the invention generally have initial expansion temperatures in the range of from 400 ° f . to 600 ° f ., have a maximum negative expansion of about 5 % at about 425 ° f ., and return to at least original thickness at about 480 ° f . these properties are better than those obtained with intumescent sheets produced with vermiculite which has been treated with other salt solutions , as measured by thermal mechanical analysis . the following examples are intended to be illustrative only and not of limiting effect . four samples of a vermiculite ore were cation exchanged for 24 hours at room temperature with 1 molar solutions of potassium nitrate , ammonium nitrate , potassium chloride and ammonium chloride , respectively . the cation exchanged vermiculites were then each rinsed five times with distilled water after the excess salt solution was decanted , and then allowed to air dry at room temperature until substantially dry . each vermiculite sample was then separated into three batches , one of which was expanded at 475 ° f ., one at 530 ° f ., and one at 600 ° f . all samples were expanded in a conventional expansion oven . after expansion , the samples were tested for bulk density , with results as shown in table 1 . the data of table 1 show that the expanded vermiculite which had been cation exchanged with potassium nitrate had significantly lower bulk density , and thus was more fully expanded , after expansion at a given temperature than the vermiculite which had been cation exchanged with other salts . further , the bulk density obtained for the potassium nitrate treated vermiculite which was expanded at 475 ° f . was much lower than that of the other samples which were expanded at the same temperature , and equivalent to the bulk density obtained for the other samples which were expanded at 530 ° f ., indicating a lower initial expansion temperature and more complete expansion at the low expansion temperature . table 1______________________________________ bulk density ( lb / ft . sup . 3 ) when expanded atsample treated with 475 ° f . 530 ° f . 600 ° f . ______________________________________1 kno . sub . 3 24 . 3 14 . 8 11 . 02 nh . sub . 4 no . sub . 3 44 . 9 24 . 3 17 . 13 kcl 62 . 3 24 . 0 17 . 04 nh . sub . 4 cl 61 . 8 45 . 1 22 . 8______________________________________ a 2 kilogram batch of vermiculite ore was separated into two 1 kilogram samples , which were exchanged for 24 hours at room temperature with 1 molar solutions of potassium nitrate and ammonium chloride , respectively . the solutions were then decanted from the samples and each sample was dried overnight at 140 ° f . next , two intumescent sheets were identically prepared using the procedure described below , one using the potassium nitrate exchanged vermiculite , and the other using the ammonium chloride exchanged vermiculite . in a blender , 15 . 4 grams of alumina - silica ceramic fibers ( washed fiberfrax available from the carborundum co .) were added to 1000 ml of water and agitated . this mixture was then transferred to a beaker and 10 ml of a 0 . 1 % solution of sodium acrylate / acrylamide copolymer surfactant ( commercially available from allied colloids under the tradename percol 166 ) and 10 ml of a 0 . 1 % solution of a quaternary acrylate salt / acrylamide copolymer surfactant ( percol 292 ) were added . the mixture was then stirred with a mechanical stirrer at low speed . to this mixture , 28 grams of vermiculite ( see above ), and 8 grams hycar ® 1562x103 ( a 40 % solution of butadiene - acrylonitrile latex , commercially available from b . f . goodrich chemical co .) were added , and the mixture was again mixed at low speed . the mixture was then returned to the blender and flocculated with a 10 % aluminum sulfate solution ( sufficient to reduce the ph to a range of 4 . 5 - 5 ). the resulting slurry was poured into approximately 0 . 5 inch of water in an 8 inch diameter hand sheet former ( available from valley iron works , appleton , wis .) and the water drawn off . the wet intumescent paper thus produced was then dried on a steam table . the relative expansion of the two papers was tested as follows . the initial thickness of the intumescent paper ( t 0 ) was measured using an ames adi - 71 thickness gauge with the spring removed and an external 28 gram weight added . the paper was then expanded at a given temperature ( see table 2 ) in a conventional forced air oven for 60 minutes . the expanded thickness ( t e ) was then measured using the procedure described above . the relative expansion ( t e / t 0 ) was then calculated for each sample , with results as shown in table 2 . the data of table 2 indicates that while the relative expansions of the two papers are similar when expanded at 350 ° f ., the papers containing the potassium nitrate expanded vermiculite expanded significantly more than those containing ammonium chloride exchanged vermiculite at both 400 ° f . and 450 ° f . table 2______________________________________intumescent sheet t . sub . e / t . sub . 0containing 350 ° f . 400 ° f . 450 ° f . ______________________________________kno . sub . 3 exchanged vermiculite 1 . 14 1 . 94 2 . 71nh . sub . 4 cl exchanged vermiculite 1 . 18 1 . 25 1 . 57______________________________________ four different batches of vermiculite ore were provided . each vermiculite batch was separated into two samples ( samples 1 - 8 in table 3 ). samples 1 , 3 , and 5 were cation exchanged for 24 hours at room temperature with a 1 molar solution of potassium nitrate , samples 2 , 4 , and 6 were cation exchanged for 24 hours at room temperature with a 1 molar solution of ammonium chloride , and samples 7 and 8 were cation exchanged for 24 hours with 0 . 5 molar solutions of potassium nitrate and ammonium chloride , respectively . the cation exchanged vermiculites were rinsed and then partially dried . each vermiculite sample was then separated into two portions , one of which was dried at 140 ° f . for 20 hours ( d lt ), and the other of which was dried at 1625 ° f . for 2 minutes ( d ht ). the low temperature drying volatilized the loosely bound or adsorbed volatiles in the vermiculite , while the high temperature drying volatilized both loosely bound / adsorbed and tightly bound volatiles . the difference in weight loss obtained using the two drying processes ( d ht - d lt ), indicates the amount of tightly bound interlayer volatiles contained in the vermiculite . the data of table 3 shows that for a given batch of ore , the tightly bound interlayer volatile level ( percent interlayer volatile , ( d ht - d lt ) is greater for potassium nitrate exchanged vermiculite than for ammonium chloride exchanged vermiculite . the percent interlayer volatile of vermiculite which has been cation exchanged in a given solution can vary from one batch of vermiculite ore to another , as seen in table 3 , due to minerological , morphological , compositional and other differences between ores . table 3______________________________________ exchange d . sub . ht d . sub . lt d . sub . ht - d . sub . ltsample solution batch (%) (%) (%) ______________________________________1 kno . sub . 3 1 17 . 2 3 . 2 14 . 02 nh . sub . 4 cl 1 16 . 9 7 . 9 9 . 03 kno . sub . 3 2 10 . 8 0 . 4 10 . 44 nh . sub . 4 cl 2 7 . 7 0 . 4 7 . 35 kno . sub . 3 3 15 . 8 7 . 4 8 . 46 nh . sub . 4 cl 3 6 . 0 0 . 4 5 . 67 kno . sub . 3 4 16 . 0 1 . 3 14 . 78 nh . sub . 4 cl 4 7 . 8 1 . 1 6 . 7______________________________________