Production of silver surfaces on dental casts

PRODUCTION

OF SILVER

SURFACES

ON DENTAL

CASTS

DAVID C. HOPPENSTAND* Ohio

State

University,

College

of

Dentistry,

Columbus,

Ohio

HE MAIN ADVANTAGESOF~A METAL-COATEDDIE areitshardness and its marginal detail despite considerable handling. These advantages are accentuated during the fitting of a cast-gold inlay, because deformation of the die will not occur and an inaccurate casting becomes immediately apparent. Inasmuch as a lubricant between the silver surface of the die and the wax pattern is unnecessary, another possible source of difficulty is eliminated. Phillips and Schnelll concluded that metal-coated dies are less accurate than those produced from dental stone. Nevertheless, they indicated that plating of polysulfide rubber impressions produced casts of clinical acceptability. They used the Bureau of Standards’ MOD type of die as a basis for comparison and observed a slight convexity of the pulpal floor of the die produced from it. In general, difficulty in removal of the plated cast from the polysulfide rubber impression was not encountered. However, the faithful reproduction of rough surfaces could cause it to be difficult to separate the cast or die from a rubber impression.

T

PLATING

EQUIPMENT

Suitable commercial apparatus are available for simultaneously plating several rubber impressions. However, some difficulty has been experienced in adjusting the current density to appropriate values for a single-quadrant impression because of the low sensitivity of the electrical meter. An inexpensive apparatus can be easily constructed from parts obtainable from any supplier of radio components. The basic features of the wiring circuit were suggested by Bliss2 (Fig. 1). The total cost of the apparatus that I constructed was about $12.00. The apparatus is built into an aluminum chassis with plug-in leads for the anode and cathode connections (Fig. 2). A plastic dish of the sort used in domestic refrigerators contains the plating bath. The plastic cover of this dish is drilled to accommodate sockets, and alligator clamps are soldered in place on the underside of the cover to hold the anode metal and the object being plated. EXPENDABLE

The quart of cyanides avoided. *U.

MATERIALS

plating solution distilled water. are extremely The plating unit

S. Public

Health

consists of 15 ounces of silver cyanide dissolved in 1 The solution should be aged 24 hours before use.3 The toxic, and possible contamination with acids must be should be operated in a well-ventilated room.

Service

Post-Sophomore

Fellow. 733

734

J. Pros. July-August,

HOPPENSTAND

Den. 1960

The silver content of the solution is maintained by dissolution of the anode. Therefore, the original volume of solution should not change except through evaporation of water. An appropriate mark on the tank should indicate the original volume, which can be preserved by addition of distilled water. Minor amounts of sludge that accumulate on the bottom of the tank must be removed by filtering the solution through paper. The surface of the anode should be approximately twice that of the object being plated. A tine silver powder is required in addition to the piece of silver which serves as the anode. The powder establishes an electrical conducting surface on the nonconducting impression material. PROCEDURE

If no contaminating substances are present, the impression is cleaned and rinsed with water and dried. If blood or saliva is present, the impression is washed with a mixture of 50 per cent denatured alcohol and 50 per cent hydrogen peroxide using a camel’s hair inlay brush.z A commercial debubblizing solution also may be used.

S

1

CATHODE

R

ANODE

T, the transformer: 6.3 volts, 2 Fig. L-A wiring diagram for a simple plating apparatus. B-plate selenium (estimated cost, $1.50); R, the amp. (estimated cost, $3.00); S, the rectifier: potentiometer: 100 ohms (estimated cost, $2.00); M, the milliammeter: zero to 100 Ma. (estimated cost, $2.00); B, the switch: off-on, a-pole (estimated cost, $.60).

A masking substance is applied to the surfaces of the tray or band that are not to be plated. The adhesive used to coat the plastic trays before making a rubberbase impression is a suitable masking material. Next, the fine silver powder is dusted into the impression. All surfaces to be plated must be completely covered. The impression is attached to the cathode. A piece of 2%gauge bare copper wire is wrapped around the impression to produce good electrical contact with the silver powder. The wire must not distort the impression. The fine wire is attached to a piece of stiff wire which is held in the alligator clamp (Fig. 3). Then, the impression is filled with solution from the tank by means of an eye dropper. If air bubbles are present, the solution is removed from the impression and replaced.

Volume Number

10 4

PRODUCTION

OF

SILVER

SURFACES

ON

DENTAL

735

CASTS

The impression is carefully placed into the solution and completely submerged. The concavities in the impression should face upward to prevent entrapment of air and to allow inspection during the plating process. The plating unit is connected and the current adjusted by means of the potentiometer knob to 10 Ma. for a band impression or 60 Ma. for a tray impression.

Fig.

Fig.

%-The

Z.-A

impression

Fig.

4.-The

silver

plating

apparatus

and

tank

for

the

is attached for plating. The portions of have been dusted with silver powder.

silver-plated

die

is

completed

and

ready

cyanide

the

for

bath.

impression

sectioning.

to

be

plated

HOPPENSTAND

736

J. Pros. Den. July-August, 1960

The flash coating is deposited within 1 or 2 minutes. If the current is too great, the solution will bubble and the silver deposit will have a granular appearance. After 1 hour, the impression is removed from the tank and inspected. Surfaces that have not received the flash coating are apparent and should be redusted with silver powder after the impression has dried. The impression is replaced in the solution, and the current is raised to 50 Ma. for each band impression or to 100 Ma. for each tray impression. The plating should be continued overnight if a thick coating on the die is desired. If plated for 4 to 6 hours, the impression will have a silver thickness of about 0.004 inch. The silver coating, of course, does not alter the dimensions of the final cast. Dowel pins are inserted in the usual manner, and the cast is poured in artificial stone. After the stone has set, the die is removed from the impression (Fig. 4) and sectioned. SUMMARY

A simple procedure and an inexpensive apparatus for making silver-plated dental casts from rubber-base impressions has been described. The accuracy of such casts is clinically acceptable under most circumstances, and the hard, silver surface has qualities superior to those of artificial stone. My appreciation is expressed to Dr. Duncan McConnell for technical and editorial and to Dr. Donald W. Dickson for general guidance on operative techniques.

guidance

REFERENCES 1. Phillips,

R. W., and Schnell, R. J.: Electroformed Dies From Thiokol and Silicone Impressions, J. PROS. DEN. 8:992-1002, 1958. 2. Bliss, Stanley: Personal communication, Sept. 10, 1958. 3. Directions for Plating Permlastic Impressions Using a Silver Cyanide Bath (Brochure), Kerr Manufacturing Company, Detroit, Mich. OHIO STATE UNIVERSITY COLLEGE OF DENTISTRY 305 WEST TWELFTH AVE. COLUMBUS 10, OHIO