U.S. Coin Minting Process History & Mint Error Creation
April 2nd, 1792 Congress authorized the production of U.S. coins. Each coin will have their year minting (Date), “UNITED STATES OF AMERICA”, “LIBERTY” and the Denomination.
1792 – 1836: Hand Engraved Dies:
Design sketches were approved by U.S. Mint and design engraved directly into Master Die. Inscriptions, Dates, Stars, etc. were added to the Working Dies later and not on Master Dies. Edge dies (reeding) added to discourage shaving of coins for their metal content.
1836 – 1867: Hubbing Process Modified and French Portrait Lathe Introduced:
New Hubbing process involved the machining away of the disturbed metal being pushed to the periphery of the die during Hubbing. Main change was the incorporation of the French Portrait Lathe used to copy/reduce the master form into a Master Hub the final size of the coin.
Master Hubs used to created Master Die which would have the remaining design elements (stars, lettering, etc) and is further refined and touched-up by the engraver. The Master Die was used to Create Working Hubs which in turn were used to create Working Dies which then had the Date and Mint Mark punched into the Working Die.
1867 – 1907: Hill Reducing Lathe Introduced:
Better quality Master Hubs were produced. Some coins had the edge lettering struck into the Master Die until around 1886 after when they were into to the Master Hub. Coins of this era did not have the Date and Mint Mark punched into the Master Die so that it could be reused for years. There is mention that the reducing lathe could create a Master Die from a Model which is either relief (raised) or intaglio (recessed). Normally the Master Hub is made from the Galvano (relief image) in the reducing lathe.
1907-2008: Janvier Reducing Lathe Introduced:
Janvier lathe replaced the Hill lathe allowing for larger original models and working with varying heights of relief. In addition, the date was no longer punched into the Working Dies rather, only the first two digits (19 for example) were sculpted into the original model.
The Final two digits were added to the Master Dies and the Mint Marks added to the Working Dies. This changed starting in the early 1980’s when all four digits of the date were added to the Galvanos and new Master Hubs created for each year.
2008-Present: The US Mint completely transitioned to digital technology; using the Computer Numerically Controlled die-cutting. The conversion to CNC digital technology made the die cutting process more efficient and more precise than ever before.
*Note - Prior to 1907 the four digits of the date were hand punched into the Working Dies. Beginning in 1908 the first two digits were sculpted directly to the original clay model with the last two digits hand punched into the Master Dies. Beginning in the mid 1980’s to date a new Master Hub is created every year from a Galvano with all four digits of the date.
Proof Coin Dies:
Mirror Finish - 1817-1906 Hand picked working dies with the greatest detail chosen, cleaned and polished to Mirror Like finish. Planchet also polished and coins struck 2 or more times.
Matte Finish - 1907-1916 Same die & planchet preparation as for Brilliant Mirror Proof but rough granular features obtained by dipping the stuck coin in weak acid or sandblasting it.
Satin Finish - No Proof coins from 1917-1935. Resumed Proof coin production with Satin Finish. Same process as Brilliant Proof coins but without the mirror finish. Brilliant Proof created again from 1936-1942 with no proof created from 1943-1949, and no proof sets 1965-1967, but Special Mint Sets (SMS) or SP coins. Satin finish from 2005-2010 in Mint SP (Special Proof) sets.
Cameo - In 1950’s Proof Coin production resumed. Working Dies with sharpest details were selected, cleaned & polished. The Die was then immersed in a solution of nitric acid and alcohol to produce a light frosting over the entire die surface (call Cameo affect). Surrounding areas would be polished leaving a mirror finish on the inset area of the design elements in between the letters, numbers and main design features. Deep Mirror Proof Like (DMPL) were often the result of this process and DMPL examples are most rare from 1950-1969 for proof sets.
*Note - In the 1970’s additional improvements were made to how Proof Dies were created; Dies were first cleaned, then sandblasted for Cameo affect instead of dipped. The surrounding fields were then highly buffed into a ultra-high polish and further treated with a diamond compound and finally chrome plated. Most proof cons after 1970 are Cameo.
1838 heralded the beginning of mint marks being used on U.S. Coinage. It has continued until to this day, The Department of Engraving & Design's Die Manufacturing Division, located within the Philadelphia Mint Facility, has produced nearly all Engravings/Galvanos, Master Hubs, Working Hubs, Master Dies and Working Dies used to produce all U.S coins (In 1996 the Denver Mint also began acting as an auxiliary die production facility when demand requires).
Until the mid 1980's all working dies produced at the Philadelphia Mint had the Mint Mark hand punched into each one individually and sent to the various other Mint Branches. The mints included: Philadelphia (No mint mark or a "P"), Denver (D), San Francisco (S), New Orleans (O), Carson City (CC), Dahlonega ("D" only used for gold coins 1838-1861), Charlotte ("C" only on various gold coinage), and West Point (W) respectively.
Sometimes impressing the MM into the working dies required multiple strikes and if there was movement or misalignment in between strikes a RPM (Repunched Mint Mark) could be created and passed on from the Working Die to all coins it was struck by. Beginning around 1965 the Mint Marks were hand punched into the Working Hubs instead of the Working Dies which cut down on the number of RPMs but they were still possible.
Beginning in 1985/86 for Proof Coins and the early 90s for normal Business Strikes the Mint Marks we no longer stamped into the working dies by hand, but engraved directly into the original design/plaster mold for all denominations. Due to this RPMs are not found on coins after about 1991, when all denominations were finally assimilated into this part of the mint mark process. (If you do think you have found an RPM after 1991 then it's possible a Doubled Die Mint Error, but not a possible RPM).
Philadelphia, Pennsylvania (1793-Date, "P" Mint Mark or None)
Since inception this has been the main U.S. Mint facility coins struck in Philadelphia bear no Mint Mark with the exception of Wartime Nickels from 1942-54, Susan B. Anthony Dollars and all Philadelphia coins struck since 1979 besides the cent. The Philadelphia minted One Cent still does not have a MM.
Denver, Colorado (1906-Date, "D" Mint Mark) Created after major gold and silver discoveries in Colorado. Don't confuse the Denver "D" Mint Mark on coins from 1906 to date with the "D" mint mark on gold coins of 1838-61 struck at the Dahlonega, Georgia Mint.
West Point, New York (1984-Date, "W" Mint Mark)
West Point struck coins and commemorative medals beginning in 1974 although no mint marks appeared until 1983. From 1973 through 1986, the West Point Mint produced Lincoln Cents bearing no Mint Mark making them indistinguishable from those struck at the Philadelphia Mint. From 1977 to 1979 Bicentennial and Washington Quarters were also produced there. Beginning in 1983 a "W" mint mark was first used on the 1984 Olympic $10 gold eagles making it the legal tender U.S. gold coin since 1933.
In 1986 American Gold Eagle Bullions were produced in solely in West Point again with no mintmarks. Today all American Eagle series proof and uncirculated bullion coins in gold, silver and platinum are produced at West Point, along with all gold commemorative, some silver commemoratives and beginning in 2006 the American Buffalo Gold Bullion coins. All commemoratives from West Point are struck with the "W" mint mark. The West Point Bullion Depository was made an Official U.S. Mint Branch in 1988.
San Francisco, California (1854-Date, "S" Mint Mark)
Created after major California gold discoveries. No coins were minted in San Francisco from 1956-67. In 1968, it took over most proof coinage production from the Philadelphia Mint, but continued striking a circulating coinage from 1968 through 1974 as needed. Since 1975, the San Francisco Mint has been used only for proof coinage, with the exception of the Susan B. Anthony from 1979-81 and a portion of the cents minted in the early 1980s. The dollars bear a "S", but the cents are otherwise indistinguishable from those minted at Philadelphia which have no mintmarks, unlike those years' proof cents from San Francisco and circulation cents from Denver.
Carson City, Nevada (1870-1893, "CC" Mint Mark)
Created after the discovery of The Comstock Lode. This Nevada mint was a short-lived facility and produced some of the scarcest coins in numismatics.
New Orleans, Louisiana (1838-1861 & 1879-1909, "O" Mint Mark)
After its closure in 1909, the building was used as an assay office until 1942, and was renovated as a museum in 1979.
New Orleans, Louisiana (1838-1861 & 1879-1909, "O" Mint Mark)
After its closure in 1909, the building was used as an assay office until 1942, and was renovated as a museum in 1979.
Charlotte, North Carolina (1838-1861, "C" Mint Mark)
Only gold coins were struck at the Charlotte Mint 1861 when it was taken by the Confederacy and never re-opened.
Dahlonega, Georgia (1838-1861, "D" Mint Mark)
Also only struck gold coins until it too was seized by the Confederate Forces in 1861 and never re-opened.
Planchet (Blanks) Errors
Blank - Type I Planchet Planchet - Type II Planchet (blank after being "upset")
Struck Blank: Coin unintentionally struck on a blank (Type I Planchet) rather than a "upset" Type II planchet.
Partial Plated: Zinc core Copper plated planchets (cents 1982-Date) with only a portion of the outer copper plating present.
Unplated: Zinc core Copper plated planchets (1982-Date) with the outer copper plating missing completely.
Improper Alloy Mix: Use of incorrect metal for planchets produces coins of different colors and/or with streaks, peeling or flaking.
Defective Planchet: "Broken" Planchet containing foreign substance or bubbles due to improper allow mix. Can resemble a Ragged Clip.
Lamination Error: Planchet containing foreign substance or bubbles due to improper allow mix causes peeling or flaking of outer layer.
Split / Broken Planchet: Planchets either split/broken in two separate pieces or cracked down the middle like a clamshell.
Split Planchet, Before Strike: Planchet splits before being struck and each half is thin normally with weak design and/or lines on Obv & Rev.
Split Planchet, After Strike: Planchet splits after being struck. Each half is thin with normal features on one side and weak/no design details on the other.
Split Planchet, Hinged: Coin struck normally but ready to split and being held together by a small portion. Often known as "Clamshells"
Incomplete Planchets: Planchets missing a portion prior to being struck. Referred to as Clips or Clipped Planchets.
Rim Clip: Very small curved or straight portion of the rim/edge missing.
Incomplete Clip: Long incuse curves on Obv. & Rev. due to an incomplete blank punch overlapping a previous punch. Considered Rare.
Straight Clip: Similar to a Rim Clip only with a larger portion of the edge (2-3%+) missing along a straight or slightly bowed out line.
Corner Clip: Coin struck on planchet created by a blank punched from the corner of metal strip. Very rare as only four even possible per roll of metal.
Curved Clip: Similar to a Rim Clip only with a larger portion of the edge (2-3%+) missing along a inward curved line.
Ragged Clip: Jagged fairly straight portion of the planchet edge missing. Considered a Defective Planchet if not straight and runs deep into planchet.
Elliptical Clip: Oval shaped coin due to a complete blank punch overlapping a previous punch. Normally 'Elliptical' & 'Crescent' Clips created together.
Disk Clip: Similar to a Rim Clip only even smaller and along the rim's edge. Seen on the Edge as an tiny indent or dip along the coin's copper core.
Assay Clip: Blank/Planchet with a portion missing as if cut out. Very Rare.
Crescent Clip: Similar to a curved clip only with 50% of the coin by wt. missing in an inward curve. Normally the other smaller half of an 'Elliptical Clip'.
Incomplete Cladding: Copper-Nickel Clad coins with outer nickel layer missing from large portions of the Obv. And/or Rev. revealing copper core
Sintered Planchets: Planchets are sometimes cleaned/bathed in a "dirty" solution having been used to previously clean coins of a different metal composition causing trace amounts of the metal from the previously cleaned planchets to adhere to the coins surface causing discoloration of the entire coin.
Fragments: Coins fully struck on Obv and Rev. on a abnormal shaped piece/scrap of metal. If only struck on one side considered a Lamination Errors.
Bow-Tie: A punched bow-tie shaped blank that is pressed into a coin by die pressure. The Bow-Tie fragment is often the surrounding metal from when the round blanks were cut out of the metal sheet, and accidently makes it into the die chamber. Very Rare.
Wrong Stock: Coins of a given denomination struck on a planchet made from blanks rolled to the thickness of different denomination. Normally round.
Wrong Metal: Irregular shaped coin due to striking of a planchet meant for a different denomination or foreign coin. *Can be considered Strike Errors.
Thick / Thin Planchets: Planchets made from blanks which were not rolled properly and are thicker or thinner than normal confirmed by weight.
CUD or Heavy Die Break: Large break in a die causing raised metal lump normally running from the rim inwards onto the design field.
Split Die Strike: Type of Die break but rather than going inwards from the rim a crack/split down the middle produces a raised line across a struck coin.
Die Chip: A small chip in a die leaving a raised area on a struck coin.
Die Gouge: A damaged Die, will show as various shaped raised areas on coins struck after the die has been damaged.
Die Crack: Small cracks in a die can result in thin raised lines on a struck coin and can develop into CUDs.
Broken Collar Die Break / Collar CUD: A break in the containing collar can result in a lump or extra metal along a struck coins edge.
Rotated Die: Misaligned or loose Dies can result in a rotation between the Obv. And Rev. images. Greater that 12-15 degree rotations are most desirable.
Die Polish Errors: Mint polishing of dies to polish or remove/modify features can leave think raised polish lines and weak features if over polished.
Die Clash, Clashed Die or Counter Clash: Depending on the pressure two dies impacting each other without a planchet in place will cause features of each die to impress on the other. Those clashed features can be transferred to future coins struck with those dies.
Die Varieties: Errors that occur during the Hubbing or hand punching process that can result in a separate variety classifications.
Re-punched Mint Mark (RPM): Prior to 1990 (Circulation) / 1985 (Proof) Mint Marks were punched into working dies by hand. If not centered exactly re-punching could cause separation or doubling of the MM.
Dual Mint Mark (DMM): Working Die punched with Mint Marks from two or more different mints completely separate from each other, no overlap.
Over Mint Mark (OMM): Working Die punched with Mint Marks from two or more different mints overlapping each other.
Over Dates: One Date is punched into a Die and a different Date is re-punched over top. It can be the entire date or one or more numbers.
Re-Punched Date (RPD): Similar to RPM normally the last two digits of a date were punched by hand and if not aligned properly would leave separation or doubling of those digits of the date.
Misplaced Date (MPD): One or more digits of a date can be seen. Can occur almost anywhere on a coin and often found in the denticles below the official date.
Double Dies: See next Page for more details on Classes of Double Dies.
Class I: Die being re-hubbed is rotated near the center of the die from the position it was in during the original Hubbing.
Class II: Design features on the Die/Hub become distorted or miss-shaped during the annealing/tempering process causing the images to no longer align properly when being re-hubbed.
Class III: Happens when working hubs with different designs features are used to Hub a Die.
Class IV: Happens when a Die is moved off center in the Hubbing Press from the position it was in during the initial Hubbing.
Class V: Happens when a Die is pivoted around a point near the rim during the rehubbing process.
Class VI: Happens when a Hub is overused and the design features become thickened or flattened creating thick design features on the Working Dies.
Class VII: Happens when a Die being Hubbed is impressed with a normal hub and a then re-hubbed using a hub that has had its features changed or removed
Class VIII: Happens when the top of a Die being Hubbed is not parallel to that of the Hub.
Strike (Machine) Errors
Die Trial Strike, Die Adjustment or Low-Pressure Strikes: Coins struck with weak features due to test strikes when calibrating proper pressure.
Filled / Overused Die Strikes: Dies can become overused or clogged with grease or other substances causing weak, filler or missing design features (Dropped Numbers/Letters) on a struck coin.
Strike Through: Coin struck through foreign substance/debris leaving impression of whatever the foreign substance was.
Indent: Depression or Indent in a struck coins surface due to an overlapping second planchet present during striking. Indent will show no design features.
Brockage: Same as an Indent but instead of a planchet an already struck coin overlaps. Depression will show design features of the overlapping coin.
Counter Brockage: A Brockage Coin created by another Brockage Coin.
Die Cap (Strike Through): A struck coin can become attached to the Happen Die and after subsequent strikes can begin to roll up over the Die creating what resembles a bottle cap. Inner portion usually shows deign features which outer portion does not.
Capped Die Strike (Strike Through): Considered a late stage Brockage where a capped die is created on the upper Hammer Die.
Broad Strike: Coin struck without the retaining collar will result in a round (center struck) or slightly elliptical (off-center) coin normally larger than the intended size with the design elements all still visible.
Off-Center Strike or Miss-aligned Strike: When a planchet is not aligned properly and rests off the retaining collar it will result in a coin struck with design features missing from the Obv & Rev along one edge (Denticles missing); 40-80% of features Obv. & Rev. missing to be most desirable.
Double / Multiple Strikes: A coin that is struck two or more times resulting in strike or machine doubling or design features.
Double Struck, In Collar: Coin already struck is put back in the hopper and re-struck at a later time or coin not released properly. Will show some rotation between strikes.
Double Struck, Out of Collar: A coin first struck normally is not released properly is stuck again partially out of the retaining collar.
Double / Multiple Strikes, Flip-Over: Same as above only the coin is flipped in between strikes either in or out of collar.
Double Denomination Mule: Occurs when a denomination is struck on a coin that was already struck with a different denomination.
Emergency / Intentional Mule: Due to lack or dies/resources a Rev or Obv. Die for a different denomination is paired with a Die from another denomination creating an intentional Mule.
Error Mule: A coin where two paired Dies are used from different denominations or types unintentionally (Not always unintentionally).
Missing Clad Layer, Split Before Strike: Coin display normal (maybe weak) strike features but is thinner and normally Copper on one side and Nickel on the other.
Missing Clad Layer, Split After Strike: Thin coin Copper on one side and Nickel on the other with full details on one side and none on the other. After splitting each half will show full details/features on one side and lines or striations on the other side where the split occurred.
Missing Clad Nickel Layer: If struck before splitting will show as all Nickel with one side normal and the other showing striations. If struck after splitting will be very thin and show weak features on both sides.
Edge Strike: When a coin sits in the collar on its edge rather than flat and is released after being struck before folding over.
Fold-over strike: Similar to an Edge Strike but instead of getting released immediately and folded over when struck.
Partial Collar Strike: Happens when the bottom Anvil does not come sufficiently above the planchet causing the diameter of the portion above the collar to increase.
Saddle Strike: Produced in Dual or Quad Presses where an off-center strike can cause the coin to bend down the middle in the shape of a Saddle.
Mated Pairs: Two coins struck together creating a bonded pair.
Waffled Coin: U.S Mint intentionally defaces Coins found to have Errors using a machine which presses the coin into a Waffle pattern. The Mint's intent is to send the waffled coins to be melted; their metal recycled.