Our discussion and analysis of our financial condition and results of operations are based upon our consolidated financial  statements, which have been prepared in accordance with U.S. generally accepted accounting principles. The preparation of these  financial statements requires us to make estimates and judgments that affect the reported amounts of assets, liabilities, revenues and  expenses, and related disclosure of contingent assets and liabilities.  On an ongoing basis, we evaluate our estimates and assumptions, including those related to revenue recognition, allowance for  doubtful accounts, valuation of inventories, valuation of long-term assets, goodwill and other intangible assets, pension plan  assumptions, income taxes, asset retirement obligations, income tax valuation allowances, stock-based compensation, and litigation  and environmental liabilities. We base our estimates on historical experience and on other assumptions that we believe to be  reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and  liabilities that cannot readily be determined from other sources. There can be no assurance that actual results will not differ from  those estimates.  We believe the following critical accounting policies require us to make significant judgments and estimates in the preparation of  our consolidated financial statements:  29  Critical Accounting Policies x Revenue recognition: Revenue from sale of products is recognized at the time the goods are shipped and title  passes to the customer. In most of our PCC contracts, the price per ton is based upon the total number of tons  sold to the customer during the year. Under those contracts, the price billed to the customer for shipments  during the year is based on periodic estimates of the total annual volume that will be sold to the customer. Revenues are adjusted at the end of each year to reflect the actual volume sold. There were no significant  revenue adjustments in the fourth quarter of 2013 and 2012, respectively. We have consignment arrangements  with certain customers in our Refractories segment. Revenues for these transactions are recorded when the  consigned products are consumed by the customer. Revenues from sales of equipment are recorded upon  completion of installation and receipt of customer acceptance. Revenues from services are recorded when the  services are performed.  x Allowance for doubtful accounts: Substantially all of our accounts receivable are due from companies in the  paper, construction and steel industries. Accounts receivable are reduced by an allowance for amounts that  may become uncollectible in the future. Such allowance is established through a charge to the provision for  bad debt expenses. We recorded bad debt expenses of $0.6 million, $1.0 million and $0.9 million in 2013,  2012 and 2011, respectively. In addition to specific allowances established for bankrupt customers, we also  analyze the collection history and financial condition of our other customers considering current industry  conditions and determine whether an allowance needs to be established or adjusted.  • Significant under-performance relative to historical or projected future operating results;  • Significant changes in the manner of use of the acquired assets or the strategy for the overall business;  • Significant negative industry or economic trends;  • Market capitalization below invested capital.  x Property, plant and equipment and other long-lived assets: Property, plant and equipment are depreciated over  their useful lives. Useful lives are based on management’s estimates of the period that the assets can generate  revenue, which does not necessarily coincide with the remaining term of a customer’s contractual obligation to  purchase products made using those assets. Our sales of PCC are predominately pursuant to long-term  evergreen contracts, initially ten years in length, with paper mills at which we operate satellite PCC plants. The terms of many of these agreements have been extended, often in connection with an expansion of the  satellite PCC plant. Failure of a PCC customer to renew an agreement or continue to purchase PCC from our  facility could result in an impairment of assets or accelerated depreciation at such facility.  x Valuation of long-lived assets, goodwill and other intangible assets: We assess the possible impairment of  long-lived assets and identifiable amortizable intangibles whenever events or changes in circumstances  indicate that the carrying value may not be recoverable. Goodwill is reviewed for impairment at least annually.  Factors we consider important that could trigger an impairment review include the following: 