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DENT,AL TECHNOLOGY SECTION EDITOR KENNETH D. RUDD Microwave polymerization of acrylic resins used in dental prostheses J. P. De Clerck, L.S.D. Brussels, Rel$um P oly(methy1 methacrylate) (PMMA) is the resin most commonly used to manufacture dental prostheses. The polymerization of PMMA is an additive reaction requir- ing the activation of an initiator (benzoyl peroxide), creating the first free radicals that start the polymeriza- tion chain reaction by opening the double bonds of the methyl methacrylate. The thermal reaction at above a temperature 60” C (140” F) generates free radicals and the exothermic polymerization reaction (12.9 kcal/gm molecule) has a tendency to be rapid as the temperature is increased. At 100.8“ C (213.4” F), the methyl methacrylate monomer boils and creates porosity in the resin that polymerizes at this temperature. This critical boiling temperature is more easily reached when the self-generated heat cannot escape. The amount of heat evacuated through a wall is given by the formula: in which A is the amount of evacuated heat, C is the thermic conductivity of the investing materials, S is the surface area of the wall, T is the time, G is the thermic gradient between the hot and the cold side of the wall, and t is the thickness of the wall. The thermic gradient between the polymerizing resin and the outside room is important. Classically, the curing of the resin takes place in a hot-air oven or in water; therefore, the outside temperature is relatively hot, the thermic gradient is unfavorable, and the heat of polymerization cannot escape easily. To avoid porosity, especially in the thicker parts of dentures, polymeriza- tion must be conducted at relatively low temperature and slowly, approximately 8 to 10 hours in hot-air ovens and a minimum of 2 hours in water. The activation energy of the reaction reaches 16000 to 19000 Cal/gm molecule during the induction period and falls to 5000 to 8000 Cal/gm molecule during the propagation period of the polymerization reaction. Hypothesis If the heat required to break the benzoyl peroxide molecule into free radicals could be created inside the resin, the temperature outside the flask could remain cool. The polymerization heat could be dispersed more 650 efficiently and the polymerization could be rapid with less risk of porosity. In addition, this technique would eliminate the time needed to transfer the heat of the oven or the hot water, through the various structures, such as the flask, investment, and stone cast, to the resin itself. Use of microwaves Microwaves can be used to generate heat inside the resin. They are electromagnetic waves produced by a generator called a magnetron. Domestic microwave ovens use a frequency of 2450 megahertz (MHz) which gives a wave length of about 12 cm (5 inches). Methyl methacrylate molecules are able to orient themselves in the electromagnetic field of the microwaves and at a frequency of 2450 MHz their direction changes nearly 5 billion times a second. Consequently, numerous inter- molecular collisions occur and cause rapid heating. Because microwaves do not pass through metal, conven- tional metallic flasks cannot be used when heating the acrylic resin directly; thus it is necessary to use specially designed flasks. Rationale and development Tests were made to support the hypothesis and to answer seven questions. A 600-watt output domestic microwave oven with a total power of 1200 watts and a clear acrylic resin complying with International Stan- dards Organization specification No. 1567 for acrylic resin denture base material were used for the tests so that porosity and stress, made visible with polarized light, could be seen. The questions to be answered before the work could proceed and their answers were as follows: 1. Does the monomer react as a dipole and heat in a microwave field? In a glass test tube, 30 cc of monomer boiled in 3 minutes and 30 seconds in the oven. 2. Will the polymer heat in a microwave field? The polymer did not become hot in the microwave oven. 3. Does the mixed resin polymerize in the oven? Three gram portions of mixed polymer and monomer at the dough stage in separate cellophane bags, placed in the oven warmed in 2 minutes, began polymerization in 4 minutes, and were completely polymerized in 8 minutes. 4. If a small piece of metal is inserted into the resin, MAY 1987 VOLUME 57 NUMBER 5

Microwave polymerization of acrylic resins used in dental prostheses

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Page 1: Microwave polymerization of acrylic resins used in dental prostheses

DENT,AL TECHNOLOGY SECTION EDITOR

KENNETH D. RUDD

Microwave polymerization of acrylic resins used in dental prostheses

J. P. De Clerck, L.S.D. Brussels, Rel$um

P oly(methy1 methacrylate) (PMMA) is the resin most commonly used to manufacture dental prostheses. The polymerization of PMMA is an additive reaction requir- ing the activation of an initiator (benzoyl peroxide), creating the first free radicals that start the polymeriza- tion chain reaction by opening the double bonds of the methyl methacrylate. The thermal reaction at above a temperature 60” C (140” F) generates free radicals and the exothermic polymerization reaction (12.9 kcal/gm molecule) has a tendency to be rapid as the temperature is increased. At 100.8“ C (213.4” F), the methyl methacrylate monomer boils and creates porosity in the resin that polymerizes at this temperature. This critical boiling temperature is more easily reached when the self-generated heat cannot escape.

The amount of heat evacuated through a wall is given by the formula:

in which A is the amount of evacuated heat, C is the thermic conductivity of the investing materials, S is the surface area of the wall, T is the time, G is the thermic gradient between the hot and the cold side of the wall, and t is the thickness of the wall.

The thermic gradient between the polymerizing resin and the outside room is important. Classically, the curing of the resin takes place in a hot-air oven or in water; therefore, the outside temperature is relatively hot, the thermic gradient is unfavorable, and the heat of polymerization cannot escape easily. To avoid porosity, especially in the thicker parts of dentures, polymeriza- tion must be conducted at relatively low temperature and slowly, approximately 8 to 10 hours in hot-air ovens and a minimum of 2 hours in water.

The activation energy of the reaction reaches 16000 to 19000 Cal/gm molecule during the induction period and falls to 5000 to 8000 Cal/gm molecule during the propagation period of the polymerization reaction.

Hypothesis

If the heat required to break the benzoyl peroxide molecule into free radicals could be created inside the resin, the temperature outside the flask could remain cool. The polymerization heat could be dispersed more

650

efficiently and the polymerization could be rapid with less risk of porosity. In addition, this technique would eliminate the time needed to transfer the heat of the oven or the hot water, through the various structures, such as the flask, investment, and stone cast, to the resin itself.

Use of microwaves

Microwaves can be used to generate heat inside the resin. They are electromagnetic waves produced by a generator called a magnetron. Domestic microwave ovens use a frequency of 2450 megahertz (MHz) which gives a wave length of about 12 cm (5 inches). Methyl methacrylate molecules are able to orient themselves in the electromagnetic field of the microwaves and at a frequency of 2450 MHz their direction changes nearly 5 billion times a second. Consequently, numerous inter- molecular collisions occur and cause rapid heating. Because microwaves do not pass through metal, conven- tional metallic flasks cannot be used when heating the acrylic resin directly; thus it is necessary to use specially designed flasks.

Rationale and development

Tests were made to support the hypothesis and to answer seven questions. A 600-watt output domestic microwave oven with a total power of 1200 watts and a clear acrylic resin complying with International Stan- dards Organization specification No. 1567 for acrylic resin denture base material were used for the tests so that porosity and stress, made visible with polarized light, could be seen. The questions to be answered before the work could proceed and their answers were as follows:

1. Does the monomer react as a dipole and heat in a microwave field? In a glass test tube, 30 cc of monomer boiled in 3 minutes and 30 seconds in the oven.

2. Will the polymer heat in a microwave field? The polymer did not become hot in the microwave oven.

3. Does the mixed resin polymerize in the oven? Three gram portions of mixed polymer and monomer at the dough stage in separate cellophane bags, placed in the oven warmed in 2 minutes, began polymerization in 4 minutes, and were completely polymerized in 8 minutes.

4. If a small piece of metal is inserted into the resin,

MAY 1987 VOLUME 57 NUMBER 5

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MICROWAVE POLYMERIZATION OF ACRYLIC RESINS

Fig. 1. Cross section of a porcelain tooth and acrylic resin backing. Tooth is held firmly in microwave oven- cured resin.

Fig. 3. Polyester thermoplastic resin denture flask in its special stand. Flask is positively locked (bolted) and does not require a compress.

Fig. 2. Jacket flask made of PMMA held together with three bolts..

will it disturb the polymerization? The procedure described in No. 3 was followed except that a small piece of metal was partially embedded in each piece of acrylic resin, which was cured for 8 minutes. The well- hardened resin held the metal firmly, indicating that microwave--curved resin will hold porcelain teeth (Fig. I\

5. What influence do various amounts (weights) of resin have on the duration of curing? In culinary baking, if the weight of food is doubled the baking time increases 80%. The test was made by curing a 20-gram mix of resin, which is approximately the amount in a denture base. It polymerized in 8 minutes. Forty grams of resin also completely polymerized in 8 minutes. The weight of the resin had no effect on the curing time.

6. Will a new resin mix bind to polymerized resin when cured in a microwave oven? The purpose of this test was to determine whether plastic teeth will be held in microwa.ve oven-cured resin and whether it is feasible

Fig. 4. Components of denture flask. Top row, left to

right, Lower part of flask, upper part of flask surrounded by three conical stainless steel centering bolts and corresponding nuts and washers, and special stand for deflasking without injury to flask or investment. Center, Tubular spanner wrench that fits all nuts and bolts, and ejecting puck. Bottom, Caps for lower and upper parts of flask, and cap-tightening bolts and washers. Upper left

and center, Corresponding nuts are sealed in holes in lower and upper parts of flask.

to repair dentures in a microwave oven. A resin central incisor was placed with its ridge lap in contact with a mix of resin. After 8 minutes of curing, the tooth was held firmly in the resin. The only precaution taken was to degrease the tooth. The ridge lap was not ground. (Note: For the previous tests, because the resin was not cured under pressure it was porous, but the preliminary results justified proceeding with the remainder of the experiment.)

7. How long does it take to boil water? Water (30 cc) was placed in a test tube and the procedure was completed as in No. 1. The water boiled approximately

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Fig. 5. Decobloc Exactornat block of microwave trans- lucent material is in position to reduce amount of flasking gypsum.

Fig. 6. Clear plastic incisor cured in 10 minutes in jacket flask (Fig. 2).

seven times sooner than the monomer. However, because the acrylic resin for a denture must be flasked and surrounded with investment, even though it contains water, the curing of the resin will take longer than the curing of the unflasked test samples.

Flasks

The flasks cannot be made of metal because the microwaves will reflect from the surface and have no effect on the resin. They must be made of microwave translucent material such as common resin, high resis- tance ceramic (neoceramic), or unbreakable glass. Proto- type flasks for microwave curing have unconventional designs (Figs. 2 through 4).

Denture investing material

In the traditional technique, gypsum is used to invest the cast and the waxed denture in a flask. This method

Fig. 7. Blocks of clear acrylic resin processed to show (left) external porosity and (right) internal porosity.

Fig. 8. Polished samples of acrylic resin photographed on graph paper showing that clear plastic is clear (leff), and pink plastic is translucent (right). Presence of a metal wire clamp (center) does not create a curing problem.

may be used for microwave curing; however, it is wise to desiccate the gypsum and reduce its volume. The desic- cation of the gypsum can be done in the microwave oven. The volume may be further reduced by using a volume reducer such as the Decobloc Exactornat spacing block made of microwave transluent material (Fig. 5).

It is also possible to use completely different materials for investing, such as heavy-bodied silicone impression paste for the model as well as for the investing. Investing in silicone is not accurate enough for dentures but does permit mass production of smaller objects where high dimensional accuracy is not required. Fig. 6 shows an incisor tooth made by investing the model in silicone and processing it in clear PMMA.

The microwave generator

Theoretically, any domestic microwave oven could be used to cure dental prostheses in plastic flasks. Practical-

652 MAY 1987 VOLUME 57 NUMBER 5

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MICROWAVE POLYMERIZATION OF ACRYLIC RESINS

RADIATING ANTEN\NA + MAGNETRON I:__ :

1 F

THERMKZ SENSOR \ND THERMIC REFERENCE IESIN BODY

FLASK 1

Fig. 9. Schematic drawing of feedback control system showing placement of thermic sensor in relation to thermic reference resin body and radiating antenna.

Fig. 10. Coating lower part of flask with lubricant. Fig. 11. Cover to lower part of flask bolted in place.

ly, however, it is preferable to use a specifically designed oven because domestic microwave ovens are generally not continuously adjustable and cannot be set for a curing program to control the rate of cure.

Curing program and residual monomer ratio

Conventional curing of acrylic resin can be done in one of two ways: (1) a long-duration program at constant temperature in a hot air oven or water for 8 to 10 hours at 70” C (160” F), or (2) a short-duration program at rising temperature in water for 1 to 1.5 hours at 70” C (160” F) plus 30 minutes to 1 hour at 100” C (212” F). In both programs, the goal is to completely polymerize the acrylic resin without porosity in the denture. Pol.y- merization depends on the monomer being attached to the polymer free radical. As the amount of monomer is reduced as a result of the polymerization, it becomes more difficult to bring the monomer and the free radical together because the available heat stabilizes. The more the temperature rises, the faster the molecules move and the more complete is the polymerization reaction. With the conventional method, the temperature rises at the end of the curing cycle and some free monomer is left in the resin. Microwaves act only on the monomer, which

THE JOURNAL OF PROSTHETIC DENTISTRY

Fig. 12. Separating medium being painted on gyp- sum.

decreases in the same proportion as the polymerization degree increases. Therefore, the same amount of energy is absorbed by less and less monomer, making the molecules increasingly active. This is important because a form of self-regulation of the curing program takes place and leads to complete polymerization of the resin.

It seems that the difference of the residual monomer

653

Page 5: Microwave polymerization of acrylic resins used in dental prostheses

Fig. 13. Upper part of flask bolted to lower part. Fig. 16. Both parts of flasks with caps removed in microwave oven for 5 minutes to dry investment.

Fig. 14. Top cap bolted in place after flask is filled with gypsum.

Fig. 17. Sodium alginate applied to gypsum with a brush.

Fig. 15. Wax is removed from flasked denture by using boiling water and detergent.

Fig. 18. A bowl of water in oven with flask prevents overheating magnetron.

ratio between the classical and the microwave curing method lies in the way the monomer molecules are moved into the network of the polymer molecules. In the conventional method, the monomer molecules are moved by thermic shocks they receive from other molecules; they are thus passively moved and their movements are only the consequence of the outside heat. In the micro- wave method, the monomer molecules are positively

moved by a high-frequency electromagnetic field; their movements are the cause of the internal heat and the heat is only the consequence of their movements.

Feedback control

The use of microwaves does not automatically heat the monomer to the correct temperature of 100.8” C (213.4” F), which is easily surpassed during a microwave curing.

654 MAY 1987 VOLUME 57 NUMBER 5

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MICROWAVIE POLYMERIZATION OF ACRYLIC RESINS

Fig. 19. Flask, with cap removed, in deflasker with ejecting puck in position ready for tapping investment for removal from flask.

Fig. 20. A, Top part of flask removed. B, Lower part of flask repositioned in deflasker for removal of invest- ment.

This temperature must be controlled accurately and the timing must be correct. An output of a few additional watts at the wrong time will cause porosity in the resin. Fig. 7 shows two samples of resin; one has absorbed too much energy in the beginning of the curing, causing external porosity, and the other has absorbed too much energy at the end of the curing, causing internal porosi-

ty. Correctly cured samples are clear and show that the

presence of a metal clamp during curing does not create a problem by deflecting the waves (Fig. 8). Microwave

Fig. 21. Investment and denture recovered in one piece.

Fig. 22. Microwave ovens can be used for processing tooth-colored veneers. A, Jacket crown with palatal face made of clear PMMA. 8, Plastic processed on removable partial denture framework.

curing is affected by the volume of the investing gypsum, the amount of water contained in the gypsum, the powder/liquid ratio of the resin, the thermic conductivi- ty of the flask, and the microwave translucency of the flask material.

To assure regularity of curing and prevent overheat- ing, the oven power output must be under feedback control. This regularity can be accomplished with a microprocessor-regulated control system fed by data from a thermic sensor integrated with a time and temperature program.

THE JOURNAL OF PROSTHETIC DENTISTRY 655

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1

Fig. 23. A, Radiating antenna; B, thermic sensor; C, flask; D, magnetron; E, microproces- sor. Shaft of arrow is in a half-sphere-shaped depression formed by a special cap during investing procedure to reduce volume of gypsum. An identical sized half-sphere made of microwave translucent material containing A and B fits into depression and acts as a centering device, wave dispensor, and data collector.

Because a microwave oven has almost no thermic inertia, the temperature control is effective and the response is almost instantaneous. This control may be of the ON/OFF type, but continuous adjusting of the power output is preferable. Ideally, the thermic sensor would be placed in the resin of the denture or in a thermic-reference resin body. However, it is much more convenient and practical to measure the temperature of the investing gypsum or the flask wall (Fig. 9).

More is at risk than with the classic technique, because the lack of the dampening effect of the external temperature of a dry oven or water method, the chances of overheating without accurate control of the power output are great and porosity will develop. The use of a low power output avoids this problem, but the advantage of short-time curing is lost.

Physical and chemical tests

Impact strength tests show that there is no significant difference between a classically cured resin and one that is cured in a microwave oven, and there appears to be little internal stress in a microwave oven-cured resin. Chemical tests show that a microwave oven-cured resin has an exceptionally low residual of monomer.

Dimensional accuracy hypothesis

Heat-cured resins have a small amount of residual monomer and relatively poor dimensional accuracy. Autocuring resins have good dimensional accuracy but contain a relatively large amount of residual monomer. In a microwave oven there is almost no thermic inertia,

656

which permits good control over the resin temperature and allows for curing at a stictly controlled low temper- ature. Curing at a low temperature results in a resin with little residual monomer and good dimensional accuracy. However, some of the time-saving advantage of the method is lost.

PROCEDURE

1. Coat the inner parts of the flask bolts and nuts with petroleum jelly (Fig. 10).

2. Bolt the cover to the lower part of the flask (Fig. 11). Invest the cast and the waxed denture in the lower part of the flask, and apply a separating medium to the gypsum (Fig. 12).

3. Bolt the upper part of the flask to the lower part (Fig. 13) and fill it with gypsum.

4. Before the gypsum sets, bolt the upper cap on the flask (Fig. 14).

5. Place the flask in the microwave oven at full power for 30 seconds to soften the wax. Remove and open the flask, remove the bulk of the wax from the flask, then clean the denture thoroughly in boiling water with a detergent (Fig. 15).

6. Place the opened flask, with caps removed, in the oven for 5 minutes at full power to dry the gypsum (Fig. 16).

7. Coat the gypsum with tinfoil substitute (sodium alginate) (Fig. 17).

8. Pack the acrylic resin as usual. After the last trial pack, bolt the flask securely and place it in the microwave oven. If the oven is not equipped with a

MAY 1987 VOLUME 57 NUMBER 5

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MICROWAVE POLYMERIZATION OF ACRYLIC RESINS

FLASKwl FLASKn-2 FLASKn~ FLASKret

yj DISTRIBUTOR )

MICROPROCESSOR -l

Fig. 24. Schematic drawing of regulation system of multiple-flask oven.

dummy load or other security device to protect the magnetron from overheating, it is wise to put a bowl of water in the oven (Fig. 18).

9. After 15 to 20 minutes of curing, remove the flask and let it cool.

10. Remove the bolts from the upper part of the flask and separate it by using the ejector and a small hammer (Fig. 19).

11. Remove the remaining bolts, turn the flask over in the ejector, and tap the remaining part of the flask free from the gypsum (Fig. 20).

12. The investment-coated denture will separate in one piece (Fig. 21)), and the denture can be recovered and finished in the usual manner.

Acrylic resin veneer crowns and plastic on removable partial dentures may be cured in a microwave oven if the curing time is reduced to 10 minutes (Fig. 22).

DISCUSSION

Although domestic microwave ovens adapted to the particular requirement for microwave curing and plastic copies of classical metal flasks can be used, specially designed equipment is superior. Because common plas- tics are weak materials and even fiber-reinforced plastics are relatively resilient, the flask wall must be made thicker to resist distortion under packing pressure, which may make the flask too large to fit into a flask press.

Metallic flasks may be used if the flask and the microwave generator are altered (Fig. 23). A metallic flask may be used if the top cap is replaced with a cap having a half sphere on its under side. After the investment sets, the false cap is removed to expose a rounded spherical depression in the investment. An oven equipped with an equivalent sphere to act as flask cap, which also contains a radiating antenna (static wave spreader) and a thermic sensor, will cure the resin inside the metal flask. When the equivalent sphere is seated in the top of the flask, the antenna and sensor are posi- tioned correctly and the oven flask cap is clamped in position. The cap also reduces the amount of investment in the flask (Fig. 23). The energy distribution within a microwave oven is not uniform so a rotating antenna or a rotating food tray in the oven serve to distribute the energy evenly through the food. Because neither of these methods will work in a metal flask, hot and cold spots will develop. This problem may be overcome by using modulated microwaves of a shorter wavelength than the 12 cm (5 inch) waves used in a domestic microwave oven.

Multiple flask oven It may be desirable to cure many dentures at one time.

It is possible to cure acrylic resin in many flasks simultaneously. This can be done with a microprocessor equipped with a computer to control the total power

THE JOURNAL OF PROSTHETIC DENTISTRY 657

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IXCLERCK

output of the magnetron, the individual power output required by each flask, the time each flask will be irradiated, and the distribution of the microwaves. The power output of a single magnetron is distributed to the several wave guides and antennas by a wave distributor working on the electromagnetic field deflection principle. The distribution is controlled by the microprocessor, which computes and distributes the duration of irradia- tion needed by each flask to maintain a correct curing temperature.

Sensors in each flask collect data such as the number of flasks being handled, their location in the oven, an’d the internal temperature of each flask, and feed it to the microprocessor, which regulates the curing cycle and shuts off the waves to individual flasks when curing is complete (Fig. 24). Microwave curing can be even more advantageous when injection molding of the acrylic resin is used.

CONCLUSIONS

Microwave processing has a potential for saving a great amount of time and money in processing dentures. Microwave oven-cured resin has a lower residual mono- mer ratio and the same physical properties as conven- tionally cured resin. It requires at least special flasks and a programmable microwave oven, but specially designed equipment will give the best results. The method described here is patented.*

*Information a’bout the method and the patent is available from thr author.

Kc?,iml req2lest.c lo- DR. J. P. DE CLEKCK

277 BLVD DE SMET DE NAEYER

1090 BRUSSELS

BELGlUhl

A reline procedure for prostheses supported by ramus frame and mandibular staple implants

Enrique J. Rodriquez, D.M.D.,* Keki R. Kotwal, B.D.S., D.M.D., M.S.,** and Jack B. Meyer, D.M.D.*** U.S. Army DENTAC at William Beaumont Army Medical Center, El Paso, Tex.

lh e ramus frame implant was developed by Roberts and Roberts’ in 1970 and described by Nelson2 in 1974. The mandibular staple implant was described by Small and Kobernick’ in 1964, by Metz4 in 1974, and by Small5 in 1975. Both of these implants, intended to support mandibular complete dentures, present a chal- lenge to the dentist when a reline procedure becomes necessary.

Available elastomeric impression materials and hydrocolloids can be trapped between the ramus frame and the residual alveolar ridge or between the ridge and the superstructure connecting the posts of the mandibu- lar staple bone implant. Trapping will often occur even when the space between the ridge and the implant is

*Major, DC, LJ.S. Army; Resident, Prosthodontics. **Colonel, DC, U.S. Army; Director, Prosthodontic Residency Pro-

gram. ***Ma,jor, DC, U.S. Army; Prosthodontist.

blocked. Forces during the removal of the impression can jeopardize the stability of the implant and, in extreme situations, may even require sectioning of the trapped denture to remove it. In addition, blocking the space between the ridge and the implants often creates a distorted surface below and adjacent to the frame and superstructure.

This article describes a technique to make a complete denture reline impression of a mandibular ridge contain- ing a ramus frame or a mandibular staple implant. A modification of the fluid wax impression pchnique described by Applegate in 1955 is ideal for this pur- pose.

PROCEDURE

1. Relieve the mandibular complete denture to allow space for the impression material and remove the attachment parts imbedded in it.

2. Use a brush and paint melted Korecta Wax No. 4

658 MAY 1987 VOLUME 57 NUMBER 5