table i reports a group of approximate glass compositions , expressed in terms of parts by weight on the oxide basis , demonstrating the operable parameters of the instant invention . since the sum of the constituents totals or approximately totals 100 , for all practical purposes the reported values can be deemed to be expressed in weight percent . the actual batch ingredients can comprise any materials , either an oxide or other compound , which , when melted together with the other batch materials , will be converted into the desired oxide in the proper proportions . inasmuch as it is not known with which cation ( s ) the fluoride is combined , it is merely recorded in terms of the batch ingredient utilized . as 2 o 3 was included in a number of the recited compositions to perform its conventional function as a fining agent . the total iron oxide content is tabulated as feo . the batch ingredients were compounded , mixed together in a ball mill to aid in attaining a homogeneous melt , and the mixture deposited into platinum crucibles . the crucibles were placed into a gas - fired furnace , melted at 1500 °- 1650 ° c . for about 5 - 16 hours , the melts cast into 6 × 6 × 1 / 2 inches steel molds or poured onto a steel plate to produce rounded slabs having a diameter of about 10 - 12 inches , and then immediately transferred to annealers operating at about 400 °- 650 ° c . table ia reports wet chemical iron oxide analyses in weight percent performed upon several of the exemplary glasses of table i . table i______________________________________ 1 2 3 4 5 6 7______________________________________sio . sub . 2 65 . 4 63 . 7 62 . 2 61 . 6 60 . 3 59 . 0 60 . 8al . sub . 2 o . sub . 3 17 . 4 16 . 9 16 . 5 16 . 2 16 . 2 16 . 2 16 . 4b . sub . 2 o . sub . 3 3 . 0 2 . 9 2 . 9 1 . 4 2 . 8 4 . 1 2 . 8li . sub . 2 o 4 . 8 4 . 7 4 . 6 4 . 5 4 . 5 4 . 4 4 . 5feo 6 . 2 8 . 6 10 . 8 12 . 9 12 . 8 12 . 8 13 . 0tio . sub . 2 1 . 3 1 . 3 1 . 2 1 . 4 1 . 4 1 . 4 1 . 0zro . sub . 2 1 . 3 1 . 3 1 . 3 1 . 5 1 . 5 1 . 5 1 . 0as . sub . 2 o . sub . 3 0 . 6 0 . 6 0 . 6 0 . 6 0 . 6 0 . 6 0 . 6 8 9 10 11 12 13 14______________________________________sio . sub . 2 57 . 0 59 . 4 59 . 4 58 . 9 58 . 8 57 . 7 57 . 7al . sub . 2 o . sub . 3 18 . 0 16 . 1 16 . 0 15 . 8 15 . 7 15 . 5 15 . 5b . sub . 2 o . sub . 3 3 . 1 2 . 7 2 . 7 2 . 7 2 . 7 2 . 7 2 . 7li . sub . 2 o 4 . 9 3 . 5 3 . 5 3 . 5 4 . 3 4 . 3 4 . 3feo 14 . 3 12 . 7 12 . 7 12 . 6 16 . 6 10 . 9 10 . 9tio . sub . 2 1 . 1 1 . 4 1 . 4 1 . 4 1 . 4 1 . 4 1 . 4zro . sub . 2 1 . 1 1 . 5 1 . 5 1 . 4 1 . 4 1 . 4 1 . 4as . sub . 2 o . sub . 3 0 . 7 0 . 6 0 . 6 0 . 6 0 . 6 0 . 6 0 . 6cao -- 1 . 7 -- -- -- -- -- zno -- -- 2 . 4 -- -- -- -- la . sub . 2 o . sub . 3 -- -- -- 3 . 2 -- -- -- coo -- -- -- -- -- 5 . 7 -- nio -- -- -- -- -- -- 5 . 7 15 16 17 18 19 20 21______________________________________sio . sub . 2 54 . 1 62 . 0 55 . 2 48 . 0 57 . 1 54 . 8 62 . 5al . sub . 2 o . sub . 3 18 . 2 14 . 0 14 . 4 14 . 4 5 . 2 -- 2 . 1b . sub . 2 o . sub . 3 2 . 5 -- -- 2 . 4 15 . 6 9 . 9 12 . 5li . sub . 2 o 2 . 4 3 . 0 2 . 4 2 . 4 -- -- -- feo 20 . 5 18 . 0 21 . 6 25 . 9 16 . 8 20 . 2 18 . 5tio . sub . 2 0 . 9 -- 2 . 0 2 . 0 -- -- -- zro . sub . 2 0 . 9 -- 2 . 0 2 . 0 -- 5 . 0 -- as . sub . 2 o . sub . 3 0 . 5 -- -- -- -- -- -- mgo -- 1 . 0 -- -- -- -- -- na . sub . 2 o -- -- -- -- 5 . 2 5 . 0 4 . 2cao -- -- -- -- -- 4 . 9 -- 22 23 24 25 26 27 28______________________________________sio . sub . 2 34 . 8 35 . 8 35 . 8 41 . 9 44 . 6 52 . 4 54 . 9b . sub . 2 o . sub . 3 10 . 2 10 . 4 15 . 6 15 . 7 14 . 9 20 . 9 14 . 8al . sub . 2 o . sub . 3 29 . 4 30 . 2 25 . 0 10 . 5 5 . 0 -- -- feo 16 . 2 18 . 7 18 . 7 21 . 3 20 . 2 21 . 3 20 . 1na . sub . 2 o -- -- -- 10 . 5 5 . 0 5 . 2 5 . 0naf 4 . 1 4 . 2 4 . 2 -- -- -- -- cao -- -- -- -- 5 . 0 -- -- tio . sub . 2 3 . 0 -- -- -- -- -- -- zro . sub . 2 -- -- -- -- 5 . 0 -- 5 . 0alf 0 . 5 0 . 5 0 . 5 -- -- -- -- 29 30 31 32 33______________________________________sio . sub . 2 49 . 8 51 . 9 47 . 3 46 . 5 46 . 2al . sub . 2 o . sub . 3 16 . 7 14 . 0 19 . 1 15 . 8 12 . 4b . sub . 2 o . sub . 3 2 . 3 2 . 4 2 . 2 2 . 7 2 . 1li . sub . 2 o 3 . 7 3 . 1 2 . 1 2 . 6 2 . 7feo 23 . 6 24 . 6 26 . 9 27 . 8 32 . 8tio . sub . 2 2 . 0 2 . 1 1 . 2 2 . 3 1 . 8zro . sub . 2 2 . 0 2 . 1 1 . 3 2 . 4 1 . 9______________________________________ table ia______________________________________ total iron ferrous ironexample as feo as feo ferrous iron / total iron______________________________________7 12 . 9 % 6 . 5 % ˜ 50 % 10 12 . 6 % 6 . 4 % ˜ 51 % 18 26 . 6 % 12 . 7 % ˜ 48 % 31 26 . 0 % 13 . 4 % ˜ 52 % 32 26 . 9 % 11 . 5 % ˜ 43 % fe . sub . 3 o . sub . 4 ( theoretical ) ˜ 33 % ______________________________________ the following calculations can be made if one wishes to express the total iron oxide content as feo + fe 2 o 3 . thus , using example 18 from table ia , the feo content is 12 . 7 %. the fe 2 o 3 content is calculated as : therefore , feo + fe 2 o 3 = 12 . 7 % + 15 . 4 % = 28 . 1 % the ratio presented in the final column of table ia with respect to example 18 illustrates that ˜ 48 % of the iron ions are in the ferrous ( fe + 2 ) state and ˜ 52 % are in the ferric ( fe + 3 ) state . table ii records a visual description of the glass bodies , identification of the cyrstal phases present where determined via x - ray diffraction analysis , the coefficient of expansion over the range of 0 °- 300 ° c . where measured (× 10 - 7 /° c . ), a qualitative determination of the sample magnetism , and the temperature achieved after 1 , 2 , 3 , or 5 minutes heating in an induction coil . samples about 1 × 1 × 0 . 5 inches were employed in the heating trials . the coil was a cylindrical winding about 6 . 25 × 5 inches composed of eight turns of 0 . 625 inches diameter conduction wire . within the coil was a concentric ceramic coil with a 0 . 25 inch wall thickness and additional asbestos packing for thermal insulation . the sample was mounted on an asbestos pad atop a ceramic pedestal and was situated at the geometric center of the cylinder . the unit operated at 10 kilowatts and 10 kilohertz to provide an oscillating magnetic field having a peak of about 800 oersteds . the temperature of the sample was monitored by a chromel - alumel thermocouple in contact with the sample surface and a potentiometer . because the samples are not electrically conductive in bulk , it is thought that the heating effect is caused mainly by energy dissipated in the sample during the rapid magnetization - demagnetization process , frequently defined as the hysteresis loss . table ii records temperatures reached after 1 , 2 , 3 , or 5 minutes continuous heating . in the majority of cases , the temperature achieved after a 5 - minute exposure was close to the maximum temperature demonstrated by the sample after heating for 6 - 7 minutes . all the glass samples had glassy black interiors which exhibited typical conchoidal fracture patterns . table ii__________________________________________________________________________ coef . temperature afterexample no . appearance crystal phases exp . 1 min . 2 min . 3 min . 5 min . magnetism__________________________________________________________________________1 dark olive little hematite , -- -- -- -- -- weak trace magnetite2 orange brown hematite , trace 43 . 3 46 69 92 135 weak magnetite3 orange magnetite , trace 45 . 6 90 173 262 402 moderate hematite4 dark orange - &# 34 ; -- 84 141 205 332 strong brown5 &# 34 ; &# 34 ; 52 . 3 89 172 274 397 &# 34 ; 6 dark violet - &# 34 ; -- 97 186 288 400 &# 34 ; brown7 dark brown &# 34 ; -- 210 332 391 422 very strong8 &# 34 ; &# 34 ; -- 208 327 383 416 &# 34 ; 9 dark red - brown &# 34 ; -- 107 189 268 340 strong10 dark yellow - weak magnetite , -- 137 200 232 257 moderate brown weak hematite11 &# 34 ; &# 34 ; -- 77 124 170 256 weak12 dark gray - magnetite 54 . 1 128 213 289 348 moderate brown13 black cobalt ferrite -- 143 211 240 255 weak14 black - brown nickel ferrite -- -- -- -- -- weak15 brown - gray magnetite 46 . 9 216 305 344 364 strong16 black magnetite -- -- -- -- -- strong17 &# 34 ; &# 34 ; -- -- -- -- -- &# 34 ; 18 &# 34 ; &# 34 ; -- -- -- -- -- &# 34 ; 19 &# 34 ; &# 34 ; 61 -- -- -- -- very strong20 red - brown &# 34 ; 55 -- -- -- -- &# 34 ; 21 &# 34 ; &# 34 ; 71 -- -- -- -- &# 34 ; 22 &# 34 ; &# 34 ; 45 -- -- -- -- &# 34 ; 23 &# 34 ; &# 34 ; -- -- -- -- -- &# 34 ; 24 &# 34 ; &# 34 ; -- -- -- -- -- strong25 red - brown magnetite -- -- -- -- -- very strong26 &# 34 ; &# 34 ; -- -- -- -- -- &# 34 ; 27 &# 34 ; &# 34 ; -- -- -- -- -- &# 34 ; 28 &# 34 ; &# 34 ; -- 138 234 303 -- &# 34 ; 29 dark brown magnetite , trace -- -- -- -- -- &# 34 ; hematite30 light brown &# 34 ; -- -- -- -- -- &# 34 ; 31 gray - brown -- -- -- -- -- -- &# 34 ; 32 dark brown -- -- 170 307 373 -- &# 34 ; 33 light brown -- -- -- -- -- -- &# 34 ; __________________________________________________________________________ a comparison of tables i and ii points up several general observations . first , although the quantity of magnetite precipitated in the glass upon cooling from the melt and , consequently , the inductive heating effect , appears to increase with increasing feo levels in the glass , an inspection of the two tables indicates that as little as 10 % feo can give rise to useful heating effects and that quite efficient heating is achievable in compositions containing 12 % feo . hence , as can be seen , several such compositions approach or exceed 400 ° c . second , these superior compositions demonstrate two factors which are important to attain efficient heating performance at low iron concentrations : ( 1 ) relatively high boron contents ( examples 5 and 6 ); and ( 2 ) low amounts of the tio 2 and / or zro 2 nucleating agents ( examples 7 and 8 ). both of these features tend to promote spontaneous magnetite crystallization in the glasses . at low iron concentrations , viz ., less than about 10 % by weight , and normal redox melting conditions , i . e ., melting in an air environment , the non - magnetic crystal phase hematite tends to develop preferentially . at about 12 % feo , magnetite becomes the predominant phase , this again illustrating the need for glasses with relatively high iron levels . examples 10 , 11 , 13 , and 14 demonstrate that the preferred glasses will consist essentially solely of the recited base ingredients . hence , the inclusion of extraneous components customarily results in a reduction of the amount of magnetite crystallized . examples 13 and 14 indicate that glasses which crystallize a ferrite phase exhibit less efficient inductive heating than the comparable feo - containing glass ( example 15 ) which contains magnetite as the predominant crystal phase . table iii sets forth heat treatment schedules a - i which were applied to exemplary precursor glasses 1 - 33 utilizing an air atmosphere in an electrically - heated furnace . customarily , the schedules produced a body having a dull surface appearance , although in several instances a lustrous appearance resulted . in general , the interior microstructure of the glass - ceramics was highly crystalline with the crystals , themselves , being very fine - grained , i . e ., less than about 5 microns in diameter with the majority being less than one micron in diameter . the na 2 o and / or k 2 o - containing glasses free from or containing less than about 15 % al 2 o 3 did not exhibit internal crystallization . rather , the articles developed cristobalite crystals in the surface only . at the conclusion of each crystallization treatment , the articles were cooled to room temperature at furnace rate , i . e ., the electric powder was extinguished and the furnace allowed to cool with the articles retained therein . this cooling rate was estimated to vary between about 3 °- 5 ° c ./ minute . table iii______________________________________schedule a : heat at 300 ° c ./ hour to 800 ° c . hold thereat for five hoursschedule b : heat at 300 ° c ./ hour to 900 ° c . hold thereat for five hoursschedule c : heat at 300 ° c . to 750 ° c . hold thereat for four hours heat at 200 ° c ./ hour to 900 ° c . hold thereat for four hoursschedule d : heat at 300 ° c ./ hour to 1000 ° c . hold thereat for 5 hoursschedule e : heat at 300 ° c ./ hour to 700 ° c . no hold heat at 200 ° c ./ hour to 1000 ° c . hold thereat for six hoursschedule f : heat at 300 ° c ./ hour to 800 ° c . hold thereat for 1 . 5 hours heat at 125 ° c ./ hour to 1050 ° c . hold thereat for one hourschedule g : heat at 300 ° c ./ hour to 750 ° c . no hold heat at 30 ° c ./ hour to 850 ° c . no hold heat at 200 ° c ./ hour to 1100 ° c . hold thereat for two hoursschedule h : heat at 300 ° c ./ hour to 800 ° c . hold thereat for four hours heat at 300 ° c ./ hour to 900 ° c . hold thereat for four hoursschedule i : heat at 300 ° c ./ hour to 700 ° c . hold thereat for four hours heat at 300 ° c ./ hour to 950 ° c . hold thereat for four hours______________________________________ obviously , other modifications in heating and cooling rates , as well as dwell periods , are possible . such are , however , deemed to be well within the technical ingenuity of the ordinary worker in the glass - ceramic art . table iv reports the crystallization heat treatments applied to the glass articles of table i along with a visual description of the resultant body , the internal crystallization developed in situ , as identified through x - ray diffraction analyses ( s . s . represents solid solution ), temperatures achieved after 1 , 2 , 3 , or 5 minutes continuous heating utilizing the same induction coil apparatus described above with respect to the glass samples , and the coefficient of thermal expansion over the range of 0 °- 300 ° c . where measured . table iv__________________________________________________________________________ coef . temperatures afterexample no . heat treatment appearance crystal phases exp . 1 min . 2 min . 3 min . 5 min . __________________________________________________________________________1 a glossy brown -- 6 . 7 50 72 93 1272 a glossy violet - -- 9 . 8 108 182 237 300 brown3 a dull violet -- 11 . 1 128 211 269 3354 b dull violet - -- -- 144 246 303 349 gray5 a violet - gray beta - spodumene 11 . 8 124 206 271 336 luster s . s ., magnetite hematite5 b dull gray -- -- 145 235 291 3436 b dull slate beta - spodumene -- 139 223 272 322 s . s ., magnetite hematite7 b dull gray beta - spodumene -- 152 239 287 332 s . s ., magnetite , trace hematite8 a gray luster -- -- 160 256 308 3508 b dull gray -- -- 157 238 282 3239 b dull violet - -- -- 125 208 271 330 gray10 b dull gray beta - spodumene -- 161 252 300 338 s . s ., magnetite , gahnite , trace cristobalite , trace hematite10 d slate -- -- 176 265 308 31111 b dull slate -- -- 135 213 261 30512 a dull gray beta - spodumene 14 . 8 138 220 270 315 s . s ., magnetite , hematite12 c &# 34 ; &# 34 ; -- 190 276 317 34312 e &# 34 ; &# 34 ; -- 132 202 245 29013 a charcoal -- -- 113 197 227 24113 c &# 34 ; -- -- 101 183 224 25013 e charcoal beta - spodumene , -- 143 211 240 255 s . s ., cobalt ferrite14 e sooty brown beta - spodumene -- 93 131 155 178 s . s ., nickel ferrite15 a dark violet - alpha - quartz s . s ., 91 . 2 220 327 362 378 gray beta - spodumene s . s ., magnetite , zrtio . sub . 4 s . s . 15 f dull violet - -- -- 172 240 263 275 gray15 g &# 34 ; -- -- 185 242 264 27816 h dull red - brown -- -- -- -- -- -- 17 h &# 34 ; -- -- -- -- -- -- 18 h &# 34 ; beta - spodumene 25 -- -- 370 -- s . s ., beta - quartz s . s ., magnetite19 i cristobalite in surface , magnetite in interior20 i &# 34 ; 21 i &# 34 ; 22 i mullite , magnetite23 i &# 34 ; 24 i &# 34 ; 25 i cristobalite in surface , magnetite in interior26 i &# 34 ; 27 i &# 34 ; 28 i &# 34 ; 29 a dark gray -- 22 . 6 205 366 426 -- 29 h dark gray beta - spodumene 24 . 5 223 330 376 -- s . s ., magnetite30 h dull slate beta - spodumene 27 . 4 240 355 394 -- s . s ., magnetite , hematite31 a gray - violet -- 53 . 0 296 437 459 -- 32 a gray -- 51 . 0 170 365 431 -- 32 h dull gray beta - spodumene 34 . 6 275 391 418 -- s . s ., magnetite33 a gray -- 38 . 0 195 395 437 -- __________________________________________________________________________ table iva reports wet chemical iron oxide analyses in weight percent performed upon two of the exemplary glass - ceramics of table iv . table iva______________________________________ ferrous heat total iron iron ferrous iron / example treatment as feo as feo total iron______________________________________7 b 12 . 8 6 . 2 ˜ 48 % 10 b 12 . 5 6 . 4 ˜ 51 % fe . sub . 2 o . sub . 3 ( theoretical ) ˜ 33 % ______________________________________ as can be seen , within experimental error the analyses are essentially the same as those for the glasses reported in table ia . a comparison of tables ii and iv clearly illustrates that the crystallization in situ heat treatment substantively affects induction heating efficiency ; thus , both increased and decreased efficiencies can be observed . x - ray diffraction analyses confirm that the marked increases in efficiency observed in the glass - ceramic articles vis - a - vis the parent or precursor glasses are effected through increases in the amount of magnetite grown during the crystallization heat treatment . although less obvious , the decreased efficiency of example 6 with heat schedule b , as an illustration , may be associated with increased hematite development during the heat treatment , presumably at the expense of magnetite . in any event , it is apparent that several exemplary compositions containing 8 - 16 % by weight feo exceed the arbitrary inductive heating rate of 250 ° c . after 3 minutes . the presence of relatively high boron levels and low concentrations of tio 2 and / or zro 2 is important to the formation of satisfactory glass - ceramic articles . b 2 o 3 contents above about 2 % by weight inhibit surface spalling and cracking during the crystallization heat treatment . low tio 2 and / or zro 2 nucleant levels are operable because the high iron levels in the compositions assist in the nucleation process . it is believed that iron in the form of fe 2 o 3 aids nucleation . where the concentration of iron is totally reduced to feo , as by melting in an atmosphere of forming gas , hydrogen , or other reducing environment , only surface crystallization is produced . moreover , glasses which contain comparable levels of tio 2 ( equivalent to tio 2 + zro 2 on a molar basis ) will not develop fine - grained glass - ceramics in these composition systems . also , tio 2 tends to cause the formation of pseudobrookite ( fe 2 tio 5 ), ilmenite ( fetio 3 ), or ulvospinel ( fe 2 tio 4 ) solid solution in competition with the desired magnetite phase . in contrast , the presence of tio 2 + zro 2 tends to lead to the preferential crystallization of zrtio 4 , thereby removing one potential impediment to efficient magnetite development . table iv makes clear that glass - ceramics in the li 2 0 - b 2 o 3 - al 2 o 3 - feo - sio 2 composition system can have coefficients of thermal expansion sufficiently low to be suitable for use as top - of - stove cooking ware . example 15 , containing high feo and low li 2 o contents , exhibits an undesirably high coefficient of thermal expansion resulting from the presence of a substantial quantity of alpha - quartz s . s . crystallization . that failing is readily correctable with an adjustment in the al 2 o 3 / li 2 o ratio as illustrated in the other examples . examples 19 - 21 and 25 - 28 demonstrate the need for the presence of substantial quantities of al 2 o 3 in the na 2 o and / or k 2 o - b 2 o 3 - sio 2 compositions , viz ., at least 15 %, in order to achieve body crystallization in situ .